Building and Environment: Mosha Zhao, Schew-Ram Mehra, Hartwig M. Künzel

Building and Environment 217 (2022) 109106
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Building and Environment

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Energy-saving potential of deeply retrofitting building enclosures of

traditional courtyard houses – A case study in the Chinese
Hot-Summer-Cold-Winter zone
Mosha Zhao a, *, Schew-Ram Mehra a, Hartwig M. Künzel b
a
Institute for Acoustics and Building Physics, University of Stuttgart, Pfaffenwaldring 7, 70569, Stuttgart, Germany
b
Department of Hygrothermics, Fraunhofer Institute for Building Physics IBP, Fraunhoferstraße 10, 83626, Valley, Germany
A R T I C L E I N F O A B S T R A C T
Keywords: It is increasingly believed that energy retrofits are a protection tool to preserve cultural heritage for future
Heritage preservation generations. However, few studies are available regarding the deep retrofits of traditional courtyard houses in
Energy retrofit the Chinese Hot-Summer-Cold-Winter zone, although the winter conditions in these buildings are very uncom
Traditional building enclosures
fortable based on existing studies. Besides, few studies analyzed the actual energy performance of still-occupied
Multi-zone airflow modeling
Hygrothermal simulation
traditional buildings in this region. Therefore, this study investigated the energy-saving potential of deeply
Courtyard houses retrofitting the building enclosures of a traditional courtyard house in Tongren in southern China. The investi
gation was based on calibrated hygrothermal simulation with WUFI®Plus applying the multi-zone airflow
modeling. For the calibration, short-term monitoring (two weeks) was conducted in winter and measured the air
temperature, relative humidity, and heating energy consumption. The calibration showed satisfactory results by
assessing the Normalized-Mean-Bias-Error, the Coefficient of Variation of Root-Mean-Square Error, and the
Coefficient of determination. In contrast, the application of Goodness of Fit shows poorer performance and re
quires further discussion for short monitoring periods. According to this study, improving the U-values of
building envelopes to meet at least the current Chinese energy-efficiency standard can reduce more than 56%
energy demand for heating and cooling, with an estimated payback time of 2.5 years for the material cost. With
growing expectations of building occupants on indoor thermal comfort, the improvement of the energy per
formance of traditional buildings will become more critical for their long-term preservation and should be
implemented in China’s energy efficiency policies and initiatives in the future.
1. Introduction and demolished [6,7].

Many studies have been conducted regarding the energy retrofits in
In recent years, energy efficiency and thermal comfort in historic historic and traditional buildings applying different criteria, analysis
buildings have attracted great attention [1], which can be improved methods, and decision-making processes, as indicated by Webb [5]. This
through a lot of measures of energy retrofits [2], such as adding wall or study conducted an extensive literature review of existing work for
roof insulation, upgrading windows, air-sealing, replacing lamps and historic and traditional buildings. It grouped the applied criteria in their
ballasts, upgrading heating, ventilating and air conditioning (HVAC) energy retrofits into four categories: global environment primarily
equipment, and changing operational schedules [3,4]. However, energy concerning energy aspects (energy consumption, energy production and
retrofits of historic and traditional buildings are restricted due to their supply, climate change vulnerability, and embodied energy), building
physical characteristics and the applied conservation principles and fabric regarding their conservation and hygrothermal behavior, indoor
practices [5]. Still, it is increasingly believed that retrofitting buildings environment for occupants and collections, and economic costs. In order
to current energy efficiency and thermal comfort standards is a protec to assess these criteria for historic and traditional buildings, the methods
tion tool to preserve cultural heritage for future generations. It ensures of field measurements, laboratory testing, building performance simu
that these retrofitted buildings will be kept in use rather than neglected lation, and damage functions were commonly applied [5]. However, the
* Corresponding author.
E-mail addresses: zhaoms_53@hotmail.com (M. Zhao), mehra@iabp.uni-stuttgart.de (S.-R. Mehra), hartwig.kuenzel@ibp.fraunhofer.de (H.M. Künzel).
https://doi.org/10.1016/j.buildenv.2022.109106
Received 28 September 2021; Received in revised form 13 March 2022; Accepted 13 April 2022
Available online 18 April 2022
0360-1323/© 2022 Elsevier Ltd. All rights reserved.
M. Zhao et al. Building and Environment 217 (2022) 109106
studies about the energy efficiency and thermal comfort in historic 2. Hygrothermal modeling
buildings have a noticeable diversity in their quantity in different re
gions, according to another review study Martínez-Molina et al. [1]. This Simulation tools are necessary for the field of historic buildings since
study indicates that the research from Europe, particularly Italy, is they can help analyze the possible degradation risks to building mate
dominant making up about 40% of the reviewed studies published be rials, estimate some phenomena that cannot be easily evaluated (such as
tween 1978 and 2014, followed by 11% from the UK and 6% from Spain, the occurrence of surface condensation), and efficiently assess the
while only 4% is from China [1]. impact of multiple energy retrofit scenarios [32]. Many simulation
In China, the courtyard house was imbedded in society as a basic products are currently available. In this study, the software WUFI®Plus
building form from long ago until modern times [8], and many tradi [33] was applied. WUFI®Plus has been developed by the Fraunhofer
tional courtyard houses are still remained and occupied today all over Institute for Building Physics in Germany. It allows for a transient
the country [9]. This building form is believed to originate from the assessment of the interaction between the building envelope, building
north of China and was brought to the south due to the migration and equipment, and building utility with the exterior climate [34]. Its
commercial activities of people [9]. It is characterized by similar spatial mathematical and physical models are based on the work of Künzel [35]
compositions, enclosed layouts, symmetry along the middle axis, and a and are also presented in Ref. [36]. WUFI’s calculation method complies
clear distinction between the primary (occupied by the head of a family with EN 15026 [37], and its validation has been conducted, for example,
– normally the eldest ones) and the secondary (occupied by other family by Refs. [36,38–40]. The validations showed good results of WUFI®Plus
members or guests), as well as outside and inside space [10]. Existing simulation in predicting heat and moisture transfer in building compo
studies of such traditional Chinese courtyard houses mainly focus on nents and indoor thermal environment.
their historical and architectural aspects such as spatial design, cultural This part introduces the incorporated multi-zone airflow modeling in
context inheritance, and heritage conservation [8–12], while increasing WUFI®Plus and the applied statistical indices for its calibration within
attention is being paid to their environmental and energy-saving aspects this study.
[13–18]. However, the objectives regarding the environmental and
energy-saving aspects are restricted to the interaction between the 2.1. Multi-zone airflow modeling
outdoor climate, the adaptive behaviors of occupants such as in
Ref. [15], the building design [19] (including orientation [20], the WUFI®Plus, and many other building energy simulation tools, is a
window-to-wall ratio [16], spatial design [21]) and the traditional multi-zone building model defining each zone consisting of one or more
building envelopes with few thermal improvements [18,22–24]. Only a rooms with the same inner climate. The calculation of room air tem
small number of researchers explored the impact brought by deeply perature or room air humidity is based on the heat and moisture balance
retrofitting traditional buildings (such as through the widely applied between defined zones. The outdoor environment is defined as a zone as
measures like adding wall or roof insulation and upgrading windows in well. Building components such as walls, ceilings, and floors form the
some European countries) [25,26]. zone boundaries and connect the zones with each other. Weather data
Furthermore, the conflict between the poor thermal performance of for the zone of the outdoor environment is required as inputs such as air
traditional building enclosures and the growing requirements of build temperature, air humidity, solar radiation, wind velocity, wind direc
ing occupants on indoor thermal comfort in winter has been overlooked tion, and precipitation and are not influenced by indoor climate [41]. In
in southern China with hot summers and cold winters (HSCW zone). such multi-zone airflow modeling, a constant air change rate is
According to many studies [20,22–24,26–31], traditional courtyard commonly defined in each zone for the calculation of the heat and
houses in this region have a satisfactory indoor thermal environment in moisture transfer due to natural ventilation. However, this value would
summer but very poor conditions in winter. However, these studies did be difficult to set in traditional courtyard houses with few measurement
not look into the actual energy performance of traditional courtyard data regarding the airtightness of building enclosures and the unpre
houses. Their investigated objects were either under free-running con dictable occupants’ behaviors, i.e., the opening and closing of windows
ditions (without heating and cooling) [22–24,28,29], in an unoccupied and doors. The dynamic calculation of air change rate considering these
state without any user behavior-dependent indoor climates [20,30], or features of natural ventilation in traditional courtyard houses would be
with no specific information about their use [18,31]. Some other studies possible when applying multi-zone airflow modeling.
for traditional courtyard dwellings with occasional heating or cooling The principle of the multi-zone approach for the ventilation calcu
also did not quantify their energy performance [27] or were only based lation is similar to that of the building energy simulation. It assumes a
on simulation results [26]. In these houses, an improvement of the in uniform distribution of airflow in a confined space or zone. Several
door thermal comfort is mainly achieved by adaptive behaviors of their validating studies [42–45] indicate that multi-zone models show rela
occupants such as adjusting their clothes and energy- and tively high accuracy with acceptable computational costs. CONTAM is
resource-consuming activities such as fire-making, or by other active currently the most popular software employing the multi-zone method
measures such as electric heating devices [27]. In short, the influence of [46]. WUFI®Plus has implemented a similar model to that of CONTAM,
deeply retrofitting the building enclosures of these traditional courtyard and it couples the airflow modeling with hygrothermal simulation,
houses was not taken into account. which was introduced by Pazold et al. [47,48]. According to these
Therefore, this study investigated the hygrothermal performance and studies, defined zones (rooms or spaces) in WUFI®Plus are inter
energy-saving potential of deeply retrofitting the building enclosures of connected by airflow paths. These airflow paths represent different
one chosen traditional courtyard house in China’s HSCW zone, consid leakage paths through building components, i.e., opened windows or
ering the actual interaction between the traditional building envelopes, doors, joints, ventilation slots and shafts, mechanical ventilation sys
the heating and cooling energy demands of occupants, and the outdoor tems, and other openings. Mass flow through such airflow paths can be
climate. This chosen courtyard house, which is located in Tongren in calculated using pressure differences between adjacent zones and
southern China, is occupied and consumes heating energy in winter. Its airflow parameters of each type of airflow path. Therefore, an air mass
hygrothermal performance and energy-saving potential were predicted balance is added besides the heat and moisture balance of the hygro
by WUFI®Plus applying the multi-zone airflow modeling, which was thermal simulation. All three balances require an iterative solution. The
calibrated based on on-site monitoring of two weeks within this study. air mass flows induce heat and moisture transport between exterior and
The results of this study can provide quantified information and new interior climate as well as inside the building, which leads to the fact
perspectives for decision-makers and planners to rethink their methods that three interdependent balances get converged for each time step
of heritage preservation by improving the energy efficiency of tradi [41].
tional buildings in southern China. Air mass flow ṁ [kg/s] through each airflow path can be calculated
2
using equation (1). The function f() represents calculation methods If the neutral height is within the height of the opening, the bidi
depending on the type of airflow paths, while ΔP [Pa] is the pressure rectional airflows (ṁa [kg/s], which is higher than the neutral height
difference between two adjacent zones. The building airflow model and ṁb [kg/s], which is lower than the neutral height) can be calculated
strongly relies on empirically derived equations and flow coefficients of as follows:
defined airflow paths in the building [41]. According to the law of mass √̅̅̅̅
(7)
3
conservation, air mass flow entering and leaving each zone should be ṁa = − G ρj |H− Zn |2
equal (equation (2)).
√̅̅̅̅
(8)
3
ṁb = G ρi |Zn |2
ṁ = f (ΔP) (1)
∑˙
m=0 (2) ṁa Air mass flow higher than the neutral height [kg/s]
ṁb Air mass flow lower than the neutral height [kg/s]
ρi,j Air density of zone i or j [kg/s]
ṁ Air mass flow [kg/s] H Height of the opening [m]
ΔP Pressure difference between two adjacent zones [Pa] Zn Neutral height [m]
In traditional buildings, ventilation will mostly take place through With respect to joints or gaps, the length and width of joints and gaps
opened windows and doors, gaps or cracks around windows and doors, are required to calculate the mass flow using equations (9) and (10): L
unsealed penetrations for cables as well as leaky walls, floors and ceil [m] and H [m] represent the length and width of existing joints or gaps.
ings. The mass flow rate ṁ [kg/s] through big openings such as opened
windows and doors can be calculated by the orifice equation (equation ṁ = 0.0097 (0.0092)n ρL (ΔP)n (9)
(3)). It combines the effect of stack, wind and mechanical ventilation
across the opening [49]. (10)
− H
n = 0.5 + 0.5 e 2
√̅̅̅̅̅̅̅̅̅̅̅̅
ṁ = CD A 2ρΔP (3)
L Length of the opening (joints or gaps) [m]
H Width of the opening (joints or gaps) [m]
ṁ Air mass flow [kg/s]
ΔP Pressure difference between two adjacent zones [Pa] For walls and ceilings, which are not airtight, the air leakage can be
CD Discharge coefficient [− ] calculated with flow coefficients, which are normalized by component
A Opening area [m2] area CA [dm3/(s m2 Pan)] or by component length CL [dm3/(s m Pan)]. n
ρ Air density [kg/m3] [− ] is the flow component.
In big openings, it can happen that the direction of airflow between ṁ = 0.001 A CA ρ (ΔP)n (11)
two zones becomes bidirectional due to the difference in air density
caused by temperature differences. In this case, the orifice equation ṁ = 0.001 L CL ρ (ΔP)n (12)
cannot be applied, since it only considers one-directional airflow. In the absence of any reliable data of flow coefficients, AIVC [50,51]
Another airflow model is applied in WUFI®Plus which defines a neutral provides some numerical leakage data for both whole building and
height Zn [m] as follows: component leakages, which includes numerical data i.e. for leakages
ΔP through windows, doors, component/wall interface, wall construction,
Zn = (4) ceilings and floors, ceiling/wall/floor interface, wall/wall interfaces,
gΔρ
penetrations, roofing, fireplaces, ventilators and vents. They were
assembled and derived for a wide variety of sources and International
Zn Neutral height [m] Energy Agency IEA projects [50,51].
ΔP Pressure difference between two adjacent zones [Pa] As introduced above, flow coefficients, flow components and the
g Gravity acceleration [m/s2] sizes of different types of airflow paths are required as inputs to conduct
Δρ Air density difference between two zones [kg/m3] multi-zone airflow modeling in the hygrothermal simulation of
WUFI®Plus. For the simulation of traditional courtyard houses within
It’s worth nothing that if the difference in air density Δρ between two this study, such inputs were collected and prepared through on-site
zones is zero, there will be no air exchange between these zones. surveys, which are further described in section 3.2.2.2.
Otherwise, the neutral height will decide the direction and amount of air
exchange. If the neutral height is higher or lower than the opening, the 2.2. Statistical indices
airflow will be one-directional and is calculated with the following
equations. In these equations, H [m] and W [m] represent the height and For the quantification of the agreement between simulation and
width of openings. monitoring results, four well-accepted statistical indicators are applied.
√̅̅̅ ⃒⃒ 3⃒
⃒ They are Normalized Mean Bias Error NMBE, Coefficient of Variation of
(5)
3
ṁ = G ρ ⃒|H− Zn |2 − |Zn |2 ⃒
Root-Mean-Square Error CV(RMSE), Goodness of Fit G, and Coefficient
of Determination R2. Some or all of these criteria were applied in studies
G=
2 1
W CD (2 g |Δρ|)2 (6) such as [20,38,52,53], other guidelines such as ASHRAE Guideline
3 14–2014 Measurement of Energy, Demand, and Water savings [54], and
International Performance Measurement and Verification Protocol
ṁ Air mass flow [kg/s] IPMVP [55]. These indices are defined as follows:
ρ Air density [kg/m3] ∑n ( )
tip − tim 1
Zn Neutral height [m] NMBE = i=1 ∗ ∗ 100 [%] (13)
n− 1 tm
H Height of the opening [m]
W Width of the opening [m]
Δρ Air density difference between two zones [kg/m3]
3
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
∑n ( )2
i=1 tip − tim 1
CV(RMSE) = ∗ ∗ 100 [%] (14)
n− 1 tm
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
∑n ( )2̅
i=1 tip − tim
G = 1 − √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
∑n ( ∑n )2 ∗ 100 [%] (15)
1
i=1 tim − n i=1 tim
⎛ ⎞2
∑n ( )
⎜ i=1 (tim − tim ) tip − tip ⎟
2
R = ⎝√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
)2 ⎠ [ − ] (16)
∑n 2 ∑n (
i=1 im(t − tim ) i=1 ip t − t ip
tip Simulated value of node i

tim Monitored value of node i
tm Arithmetic mean of a sample of n measured data
n Number of monitored data
The NMBE indicator measures the discrepancy between the simu

lated and the measured data: a positive value means that the simulated
Fig. 1. Geographical locations of Tongren City and the ancient Huizhou region
values are generally larger than the measured data and vice versa. The
(figure created based on the map of Chinese Ministry of Natural Re
CV(RMSE) indicates the relative ratio between these discrepancies and
sources [60]).
the average of all monitored data, which is always positive. According to
Ref. [56], systematic error or bias can be reflected by the NMBE value,
Huizhou regions are located in the Chinese HSCW zone, defined by the
while the CV(RMSE) indicator can quantify the simulation accuracy.
Chinese standard GB 50176-93 Thermal Design Code for Civil Building
ASHRAE Guideline 14–2014 recommends an NMBE of 5% and a CV
[58]. This national standard divides China into five main climate zones.
(RMSE) of 15% relative to monthly data and 10% and 30% to hourly
As primary zoning criteria, it applies the average temperatures in the
data, respectively, while IPMVP [55] recommends lower limit values of
coldest (January) and hottest (July) months of the year. Besides, the
5% for NMBE and 20% for CV(RMSE) for hourly data [52,55].
number of days on which the daily average temperature is below 5 ◦ C or
G correlates the measured and monitored data series and can eval
above 25 ◦ C is counted as the complementary indices [58]. For example,
uate the level of fluctuation [38]. [53] applied a predefined threshold
the HSCW zone has an average temperature in January and July be
value of 40% for room temperature and 35% for relative humidity, while
tween 0 and 10 ◦ C, and between 25 and 30 ◦ C, respectively. The tem
[38] used 80% for indoor temperature. With regard to R2, 0.75 is sug
perature in this zone is on less than 90 days colder than 5 ◦ C, while on
gested by ASHRAE Guideline 14–2014 [54] and IPMVP [55] as a
40–110 days the temperature is warmer than 25 ◦ C.
criterion.
The City of Tongren has a fully humid and hot summer and a
moderately cold winter. Fig. 2 illustrates the monthly average air tem
3. Analysis of hygrothermal performance
perature and relative humidity in black color. In addition, the hourly air
temperature is also shown in the blue color. Based on this figure, the
This section introduces the site information, the conducted on-site
average hourly air temperature in the coldest month of January can
monitoring of two weeks, the calibration of hygrothermal simulation,
reach about − 3 ◦ C, whereas the monthly average temperature is below
and the analysis of energy-saving potential. It is worth noting that the
10 ◦ C from November to February. The hottest months are June, July,
short monitoring period of two weeks mainly resulted from the concern
and August, with the highest average hourly temperature exceeding
of the householder about the privacy issues (further explained in section
35 ◦ C. Tongren has a very humid climate throughout the year, with high
3.2.1.1).
relative humidity exceeding 80% each month, especially in summer
with its high temperatures.
3.1. Site information
Tongren City is situated in the eastern Guizhou province of China,

which is inhabited by more than 20 different ethnic groups. The research
object is located in Tongren’s historic district, which is called the Dong
Shan complex of traditional architecture (in Chinese, “Dong Shan Gu
Jian Zhu Qun”). It has been recognized as a nationally protected his
torical and cultural site since 2006. Such a nationally protected site
lawfully holds the protection of the highest level, and it is prohibited to
be demolished [57]. The traditional architecture in this historical dis
trict was greatly influenced by the courtyard house form of the Han
Chinese. It shows a high degree of similarity with traditional courtyard
houses in the ancient Huizhou region, which is about 1100 km away
from Tongren. Several types of traditional buildings remain today in this
historical district of Tongren, such as temples, residential and com
mercial buildings, city walls, and harbors from the Qing Dynasty. Some
of these buildings are under modernization to attract tourists, whereas
others are still in a poor state and remain unoccupied.
Fig. 1 shows the geographical location of Tongren and the ancient
Huizhou region [9], from which the architecture of courtyard houses in Fig. 2. Dry bulb temperature and relative humidity of Tongren, weather data
Tongren originates. This figure also shows that both Tongren and from Meteonorm [61].
4
3.2. Hygrothermal performance of an occupied courtyard house On the first floor, three bedrooms are located at the backside of the
dwelling, away from the street. Each of them is connected by a living
The research object is a reconstructed and modernized courtyard room to the outer corridor. On the street side, there are three storage
house with two stories and is now operated half-commercially. Five rooms connected with a large activity room.
family members live in this dwelling: three middle-aged adults, one As shown by Fig. 4, the measurement points are located in all activity
teenager, and one older woman. It is worth noting that before this rooms (A2 to A4), the kitchen (A1), the main hall (A6), three bedrooms
dwelling was recognized as a major historical and cultural site protected (A7, A9, A10, B1, C1, and B3), two storage rooms (A8, B2 and A13), the
at the national level, it had already been renovated by its owner for rear courtyard (A5), and the middle courtyard at two different heights
further use. This is a practical dilemma in the preservation of historical, (A16: 2.5 m and A15: 5.5 m). The surface temperatures of bedrooms 1
especially historical domestic dwellings in China since some of these and 2 as well as of storage room 1 were also monitored. However, they
buildings need to be modernized for further use before they should be were not further considered in this study. The power consumption for
subject to some national preservation rules with delayed official heating was only measured in bedroom 1 of the grandmother, as the
recognition. However, the old layout and wooden structure of the other family members do not use a heating device in winter. The ground
investigated house are well maintained, while the space of both floors area of the house is about 232 m2 (courtyard: 55.3 m2). The area of
was enlarged by increasing their height. The wooden walls, ceilings, bedroom 1 is about 12 m2.
floor slabs, windows, and doors were replaced with similar materials to Different types of measurement instruments were utilized. The air
those in traditional buildings. Fig. 3 shows the position, the eastern temperature and relative humidity were measured with Testo 174H and
façade, some building components, and the middle courtyard of the HOBO UX100-003 applying different types of thermal couples (accuracy
investigated object in the historic district of Tongren. The (d) in this for air temperature between ± 0.21 ◦ C and ±0.7 ◦ C; accuracy for air
figure shows that the building enclosure is reconstructed with new humidity between ±3.5% and ±5%). Table 1 summarizes the informa
building components, and HVAC appliances are installed. tion about the applied measurement instruments.
For the monitoring of power consumption of the electric heating
3.2.1. On-site monitoring device, Voltcraft energy logger 4000 was used. It is a data-logger with a
socket measuring the working power of electric devices, as shown in
3.2.1.1. Measurement plan. The on-site monitoring was conducted in Fig. 5. The weather data during the monitoring period, except the solar
the introduced courtyard house from December 15th, 2019 to December radiation, were obtained from the local weather station in Tongren
29th, 2019. This short monitoring period was due to the requirement of (about 1.2 km away). The data of solar radiation were provided by
the householder for a minimum disturbance to their daily life. Besides, another weather station near the city boundary of Tongren. All mea
the householder requested to end the monitoring campaign before New surement instruments had an integrated data-logger and recorded the
Year’s Eve, after which they would begin to prepare for the Chinese New data at a time interval of 10 min except the Voltcraft energy logger 4000,
Year at the beginning of February. The measurement instruments could which recorded the data every minute. The total power consumption of
not be secured during the decoration work. the whole dwelling was also read for the monitoring period.
The air temperature and relative humidity were monitored at 15 Regarding the calibration of measurement instruments, the Testo
locations. Fig. 4 illustrates the layout of this house, the measurement and HOBO sensors were calibrated by the Academy of Metrology and
points, the measured parameters, and the measurement instruments in Quality Inspection in Chongqing in June 2019. The measurement ac
bedroom 2 and the middle courtyard. The dataloggers in courtyards curacy was in an acceptable range. The other instruments were new and
were protected from rain and solar radiation by putting them in plastic were checked for their functionality before the measurement.
bottles with openings on two bottoms, whose curved surfaces were
wrapped in aluminum foil. It can be seen from this figure that on the 3.2.1.2. Indoor hygrothermal environment. The measured air tempera
ground floor, there is a half-opened kitchen, three activity rooms for ture and relative humidity in activity room 2 (A3), bedroom 1 (A7),
guests, a courtyard, a toilet area, and a bedroom (for the grandmother). bedroom 2 (A9), in the middle courtyard (A15 and A16), and of the
Another two rooms on the street side are not the property of the family. weather station (outdoor) are illustrated in Fig. 6. The measured data at
Beneath the bedroom of the grandmother, there is a small storage room. other points are not further presented in this study. According to the
Fig. 3. The position (a) [62], the entrance (b) between two exterior shops, the glazed lattice doors in the first floor (c), the courtyard viewed from ground floor (d)
and roofs viewed from the first floor (e) of the investigated house in Tongren.
5
Fig. 4. Illustration of the locations and some measurement instruments of monitoring points and monitored parameters.
Table 1
Information about the applied measurement instruments.
Measurement instrument Measured Range of Accuracy Resolution Number
parameters measurement
HOBO UX100-003 Air temperature − 20 – 70 ◦ C ±0.21 ◦ C 0.024 ◦ C at 25 ◦ C 6

from 0 to 50 ◦ C
Air humidity 15–95% ±3.5% from 25% to 85%; 0.07% at 25 C and 30%
◦
±5% below 25% and above 85% RH

Testo 174H Air temperature − 20 – 70 ◦ C ±0.5 ◦ C from-20 ◦ C to 70 ◦ C 0.1 ◦ C 9
Air humidity 0–100% ±3% from 2 to +98% RH at 0.1%
+25 ◦ C;
±0.03%/K ±1 Digit
HOBO UX100-003 with HOBO thermocouple type Surface temperature − 260 – 400 ◦ C ±0.6 ◦ C 0.02 ◦ C 1
T
HOBO UX120-014 M with HOBO thermocouple Surface temperature − 260 – 400 ◦ C ±0.6 ◦ C 0.02 ◦ C 3
type T
HOBO UX120-014 M with HOBO thermocouple Surface temperature − 260 – 1370 ◦ C ±0.7 ◦ C 0.05 ◦ C 1
Type K
Voltcraft energy logger 4000 Power consumption 0.1–3500 W ±1% 0.1 W 1
figure, the outdoor temperature varied between 5 ◦ C and 10 ◦ C on most humidity, heating helped maintain a stable indoor relative humidity in
days. In the middle courtyard, the differences of the measured air tem bedroom 1, which had a daily fluctuation between 40% and 70% in the
perature and relative humidity between two heights (A15 and A16) were monitoring period, according to the right graph of Fig. 6. This is lower
not noticeable: the differences between their average values were about than that in the courtyard and of the weather station in Tongren, which
0.1 ◦ C and 1.3%, while the correlation values were about 1 and 0.99, varied between 45% and 100%.
respectively. Besides, the measured air temperatures were generally The measured electric heating energy consumed for heating for the
higher than the obtained air temperature from the weather station monitoring period was 150.3 kWh. The total power consumption during
(outdoor) by about 1 ◦ C–2.5 ◦ C. The reason could be the thermal pro the monitoring period was read from the electricity meter for the whole
tection effect of densely distributed courtyard houses in the historic dwelling, which accounted for about 696.2 kWh. This amount included
district and the fact that the measurement points in the investigated all electricity consumption, such as lighting, cooking, heating, and
courtyard were lower than that of the local weather station. The indoor entertainment of the dwelling.
environment in most rooms except bedroom 1 was very similar to the
climate in the middle courtyard, as no heating devices were operated in 3.2.2. Hygrothermal simulation with multi-zone airflow modeling
these rooms despite the cold temperature. An exception was observed in The conduct of hygrothermal simulation with WUFI®Plus requires
activity room 3 (A4), where an air conditioner with a heat function was the preparation of the simulation model, including defining the geom
operated on three days. In bedroom 1 (A7), an intermittent heating etry, the hygrothermal properties of building components, outdoor
behavior can be observed. During the heating time from afternoon to climate, and internal heat and thermal loads. In addition, information
midnight, the indoor air temperature varied between 15 ◦ C and 23 ◦ C, about airflow paths (flow component, flow coefficients, and size) is also
while it dropped to about 10 ◦ C in early mornings. This reflects that the necessary for the multi-zone airflow modeling, as described in section
elder occupant has a higher requirement on indoor thermal environ 2.1. This section outlines this procedure and presents the calibration
ment, while the other occupants accept low temperatures. Furthermore, results of the simulation model.
the thermal environment in the heated rooms was unstable with the use
of portable electric heating devices, which could result from the con 3.2.2.1. Calibration attempts. The hygrothermal simulation was cali
cerns of safety and cost-saving of the occupant. As for the relative brated based on the on-site monitoring from December 15th, 2019 at
6
Coefficient SHGC of windows, and the indoor air change rate) could be
quantified by conducting sensitivity analysis such as in Refs. [38,
63–65], yet is relatively complicated and time-consuming for practical
use. This study aims to validate the simulation quality of an
easy-to-conduct procedure by determining the necessary simulation
inputs based on on-site surveys, existing material databases, and the
simulation method of applying the multi-zone airflow modeling. The
calibration results should then show the feasibility of such a procedure
in practical use for the hygrothermal simulation of traditional courtyard
houses.
3.2.2.2. Preparation of simulation model

3.2.2.2.1. Geometry. Nineteen zones were defined for the simula
tion, including 12 simulated zones and seven attached zones. The
simulated zones include two courtyards, one hall, and nine interior
rooms, while the attached zones include the attic space and two shops on
the ground floor along the street, four activity rooms, and the semi-
opened kitchen.
For the calibration, the indoor climate in the attached zones was
defined according to measurement data and outdoor climate data. The
thermal environment in the semi-opened courtyards was simulated by
defining their openings as transparent surfaces. The simulation results
for the middle courtyard, bedroom 1, and bedroom 2 were compared
with measured data to check the quality of the two calibration attempts.
Bedrooms 1 and 2 were chosen to present unheated and heated rooms,
while the choice of the middle courtyard can help check the simulation
Fig. 5. Electric heater in bedroom 1, plugged into the measurement equipment quality of multi-zone airflow modeling for this buffering space between
for power consumption. indoor and outdoor environments. The simulation model of these zones
is illustrated in Fig. 7, showing the model of the whole building, the
00:00 to December 29th, 2019 at 00:00. Two calibration attempts were courtyard space, bedroom 1, and bedroom 2 from the left to the right
conducted with different sets of climate data. They are the obtained side of the figure.
weather data from local weather stations - calibration 1 and an 3.2.2.2.2. Thermal properties and airflow paths of building envelope.
improved data set replacing the air temperature and relative humidity of The thermal properties of the building envelope were defined based on
the local weather station with measured ones at A15 near to the top of the database of WUFI®Plus and the current state of the building con
the middle courtyard (hourly averaged based on data-recording of every struction investigated with on-site surveys. The wooden walls and doors
10 min) – calibration 2. have a thickness of 0.04 m with thermal conductivity of 0.13 W/(mK).
The reason for only considering the weather data is as follows. First, Their U-values were calculated to be about 2.09 W/(m2K). The thickness
there are considerable differences in the weather data (air temperature of the wooden ceiling and wooden floor slab is also 0.04 m. The ceilings
and relative humidity) between the locally measured and the obtained of the rooms on the first floor were constructed with wooden boards,
ones from the weather station. According to Ref. [38], choosing an creating attic space above the rooms. The pitched roofs are not visible
appropriate outdoor weather file at local sites play a significant role in from the inside. The exterior masonry walls between the outdoor envi
the calibration quality of historic buildings. Second, for the hygro ronment and indoor space are constructed with solid bricks (thickness:
thermal simulation of traditional buildings, many simulation inputs are 0.24 m), an air layer (thickness: 0.05 m), and a wooden board layer
difficult to collect due to restricted test possibilities, such as the hygro (thickness 0.04 m), which have a U-value of 0.8 W/(m2K). Other ma
thermal properties of building materials and components and the air sonry walls between the outdoor space and courtyards were simulated as
change rate of mainly naturally ventilated traditional buildings. The solid brick walls. The hygrothermal properties of the defined building
influence of such input uncertainty (like of the short-wave radiation components in the simulated zones are summarized in Table 2. The
absorption coefficient of material surfaces, the Solar Heat Gain ground temperature of storage room 1 below bedroom 1 was simulated
Fig. 6. The measured air temperature and relative humidity in activity room 2 (A3), bedroom 1 (A7), bedroom 2 (A9), in the middle courtyard (A15 and A16) and of
the weather station (outdoor).
7
Fig. 7. From left to right: geometry model for the simulation in WUFI®Plus, simulation model of the middle courtyard, bedroom 1 and bedroom 2.
with a sine curve using the average air temperature during the moni
Table 3
tored period (10 ◦ C) and an amplitude (4K) around it in WUFI®Plus.
Summary of the thermal properties of transparent boundary surfaces of the
This model was proved to provide the best accuracy, according to
research object.
Ref. [38] among many common models.
Thermal properties of transparent surfaces
In order to simulate the thermal environment in courtyards and the
air exchange between courtyards and indoor space, the semi-opened Building component U-value [W/ Frame SHGC
courtyards were defined as thermal zones in multi-zone airflow (m2K)] factor
modeling. The openings of the middle and rear courtyards were defined Windows 5.8 0.7 0.8
as openable windows with a frame factor of 1, an SHGC of 1, and a U- Transparent boundary surfaces of 9 1 1
courtyards
value of 9 W/(m2K) (set high enough to ensure the same temperature on
the external and internal sides of the imaginary boundary layer between
courtyards and outdoor environment). These settings allow the heat and windows and doors was considered by defining flow coefficients
solar energy to go through the boundary surfaces between the outdoor normalized by length. For the calculation of the flow coefficients, the
area and courtyards without much heat resistance. The U-values of width and length of the observed visible gaps around windows and doors
normal windows in the research object were assumed to be 5.8 W/(m2K) were measured by a crack width gauge and ruler. For simplification,
with a frame factor of 0.7 and an SHGC of 0.8, as summarized in Table 3. average values of the measured sizes were applied for all the windows
For multi-zone airflow modeling, the size and position of existing and doors in the simulation model. The average area of the gaps around
airflow paths in the research object were defined based on conducted on- each window and door was about 0.0028 m2 (width: 1.4 mm, length:
site surveys. The flow coefficients and flow exponents of walls, ceilings, 2024 mm) and 0.0186 m2 (width: 5 mm, length: 3720 mm). The airflow
and floor slabs were defined based on Orme [50], since no related data through these gaps was calculated with the orifice equation. This
of Chinese traditional wooden components are available. The flow co amount was subsequently set to equal the airflow calculated with the
efficients of windows and doors were estimated based on the measured power-law equation at the same pressure difference to calculate flow
size of visible air leaks around them, the orifice, and the power-law coefficients (equation (17)).
equations. The air leakage through edges, corners, and other connec √̅̅̅̅̅̅̅̅̅̅
tion areas was not considered for simplification. This exclusion is ΔP
(17)
2
Q = 3600 ∗ CD A 2 = CL L ΔP3
acceptable because visible openings and gaps in rooms would cause a ρ
predominant proportion of air leakage.
Fig. 8 shows some observed air leaks, such as a penetration hole for A [m2] is the area of gaps around windows or doors, CD is a constant
the air-conditioner and visible gaps around doors and windows in value of 0.6, CL [dm3/(s m Pan)] is the flow coefficient of these gaps
bedroom 1. The penetration holes were considered as small openings in normalized by length, and L [m] is the perimeter of windows or doors.
the simulation, whereas the air leakage through the visible gaps around The pressure difference ΔP was calculated as the average stack pressure
Table 2
Summary of the defined hygrothermal properties of building components in WUFI®Plus.
Thermal properties of building components
Building component (top: Density Specific thermal Thermal conductivity Thickness Porosity Water vapor diffusion Thermal transmittance
inside, bottom: outside) [kg/m3] capacity [W/(mK)] [m] [− ] resistance factor [− ] [W/(m2K)]
[J/(kgK)]
Wooden walls
Hardwood 650 1400 0.13 0.04 0.47 200 2.09
Masonry walls between outdoor and indoor space
Hardwood 650 1400 0.13 0.04 0.47 200 0.80
Air layer 1.3 1000 0.28 0.05 0.999 0.32
Brick 1042 1032 0.4 0.24 0.196 16
Masonry walls between courtyard and outdoor space
Brick 1042 1032 0.4 0.24 0.196 16 1.47
Ceiling/floor slab
Hardwood 650 1400 0.13 0.04 0.47 200 1.97
Ground of courtyard and storage room 1
Sandstone 2224 771 1.684 0.1 0.17 73 4.36
8
Fig. 8. Penetration for air-conditioning (left), gaps around doors (middle) and windows in bedroom 1 (right).
difference between bedroom 1 and the middle courtyard within the The defined flow coefficients and flow components are summarized in
monitoring period. This calculation was based on the assumption that Table 4.
the pressure difference between bedroom 1 and the courtyard only 3.2.2.2.3. Weather data and internal loads. Hourly weather data
resulted from the stack/buoyancy effect and that the neutral pressure were obtained from the local weather station in Tongren to define the
level was at the middle height of the room. This assumption is accept outdoor climate. Around the historical district, there are construction
able, as the air velocity in courtyard houses remains at a low level ac sites that are void of tall buildings. Therefore, the wind profile was
cording to existing studies Shi and Yu [28,29]. The pressure difference simulated with the “urban and suburban areas” setting in WUFI®Plus,
was calculated as follows: whose wind boundary layer thickness and the exponent of wind profile
( ) are based on [66].
Ti − To 1
Δps = ρo • g • • H (18) Bedroom 1 was occupied by the grandmother of the family. She
Ti 2
stayed primarily in her room and went outside only for short periods in
the daytime. The moisture and heat load produced by the grandmother
Where Δps [Pa] is the stack pressure difference between the outside and
in this room was defined as constant over time. A generic heat gain due
inside areas and g [m/s2] is the gravitational acceleration. ρo represents
to electric devices except heating devices was defined as 4.3 W/m2 based
outdoor air density assumed as 1.2 kg/m3, while Ti , To [K] are air tem
on [67]. Bedroom 2 was occupied by a teenager. On weekdays, he left
peratures monitored in indoor space and in the middle courtyard. H is
home at about 7:00 in the morning for school and returned at about
the height of bedroom 1 of 2.6 m.
13:00 for a short rest until 13:30. He then stayed at school until about
The average difference of the stack pressure was calculated to be
22:00. On weekends, the teenager rarely stayed in his room in the
about 6 Pa. The CL of the windows was calculated to be 0.34 dm3/(m s
daytime. For a simplification in the simulation, no distinction was made
Pa2/3), while the CL of the doors was about 1.97 dm3/(m s Pa2/3). The
between workdays and weekends. Besides, no generic internal heat gain
calculation of CL was only for observed visible gaps. The actual flow
was defined in bedroom 2. As the living rooms served only as a con
coefficients are likely to be higher due to other possible invisible cracks.
necting room between the bedroom and the outer corridor, no moisture
or heat gain was defined in all living rooms. Table 5 summarizes the heat
and moisture gains in bedrooms 1 to 3. Regarding other rooms such as
Table 4
the activity rooms (attached zones), measured indoor air temperature
Summary of the defined flow coefficients and flow components.
and relative humidity were used to define their indoor environment if
Building component Flow coefficient Flow exponent n available. Otherwise, the indoor air temperature and relative humidity
[− ]
were the same as the outdoor (attached zones) or were simulated
Wooden walls 0.52 [dm3/(s m2 0.67 without any indoor heat or moisture loads in the space (simulated
Pa^n)]
zones).
Masonry walls with interior wooden 0.18 [dm3/(s m2 0.72
board Pa^n)] It is worth noting that another heating radiator was applied in
Ceilings and floor slabs 0.15 [dm3/(s m2 0.74 bedroom 1 besides the monitored one during the two weeks since the
Pa^n)] room temperature heated by just one heating device was still too cold for
Windows 0.34 [dm3/(s m Pa^n)] 0.67 the grandmother. The power consumption of this second heating device
Doors 1.97 [dm3/(s m Pa^n)] 0.67
could not be monitored. However, the daily profile of the produced heat
gain was estimated based on an in-person interview after the
Table 5
Summary of heating independent heat gain and moisture load in bedrooms 1 to 3.
Heating independent heat gain and moisture load in bedrooms
Rooms Generic internal heat gain [W] Moisture gain [g/h] Convective heat gain [W] Radiant heat gain [W] CO2 [g/h]
Bedroom 1 Value 39 22 85 15 35
Time 0:00-24:00
Time - 0:00-7:00, 13:00-13:30,
22:00-24:00
Time - 0:00-8:00, 23:00-24:00
9
measurement period and the technical specification of the heating de A visual assessment of the calibration quality is shown in Fig. 9. This
vice. This device has a maximum heating power of 1500 W and five figure presents the hourly averaged measured and simulated air tem
heating levels. According to the interview, level 1 or 2 was usually perature and relative humidity of the heated bedroom 1, unheated
chosen, and this heating device was only switched off at night from bedroom 2, and the middle courtyard of calibration 2. The simulated air
about 23:00 to 9:00. In the simulation, an additional heat gain of 300 W temperature shows very good accuracy in all three investigated zones.
was defined for bedroom 1 from December 18th, 2019 to the end of the Only in some afternoons on certain days (i.e., from December 21st to
simulation between 9:00 and 23:00. December 27th), simulation under- or overestimated the air tempera
The door in bedroom 1 was only opened for short periods when the ture in bedroom 1 by a maximum of 2 ◦ C.
grandmother left the room or came back. In the simulation, the door was Concerning the relative humidity, the discrepancy between
defined as open for 6 min per hour from 15:00 to 18:00. In the remaining measured and predicted values could reach a maximal value of 10% in
time, the door was defined as closed. In bedroom 2, the window was bedrooms 1 and 2. The simulation tended to overestimate the relative
defined as closed in the simulation period, whereas the door was defined humidity in bedroom 1 in the first days of the monitoring period. The
as closed from 23:00 to 7:00 and opened from 7:00 to 23:00. reason could be that the door in bedroom 1 remained open more
frequently and longer than assumed on the first warmer days. In
3.2.2.3. Result of calibration. The statistical indices NMBE, CV(RMSE), bedroom 2, however, the relative humidity tended to be under
G and R2 were calculated for the two calibration attempts shown in estimated. This could result from the underestimation of moisture pro
Table 6 for bedroom 1, bedroom 2, and the middle courtyard (A15). duction of the young occupant or incorrect assumption of the opening
According to this table, the NMBE of calibration 1 cannot meet the time of the window, which was defined as always closed.
defined threshold value of 5% in bedroom 1 (6.9%) for the relative The consumed heating energy based on on-site monitoring and in
humidity, in bedroom 2 (− 13.1%), and the middle courtyard (− 11.8%) terviews was also compared with the calculated heating energy. In
for the air temperature. In calibration 2, all values are within the limit general, the predicted hourly values were accorded with the consumed
values except the relative humidity in bedroom 1, which only has a heating energy. As shown by Fig. 10, discrepancies were observed
slight reduction of about 0.4% from 6.9% to 6.5%. A similar improve mainly in the afternoons, when a fixed behavior of door opening—six
ment is obtained for the R2 of the relative humidity in bedroom 1 from minutes per hour from 15:00 to 18:00—was defined. This fixed venti
0.58 of calibration 1 to 0.61 of calibration 2, while other values of air lation behavior may not be able to reflect the actual situation. An
temperature and relative humidity show satisfactory results reaching opening duration of 6 min seems too long on specific days such as on
0.75. The CV(RMSE) shows satisfactory results in all three investigated 19th and 20th December. This resulted in an overestimation of hourly
zones achieving a value smaller than 20% for air temperature and heating power in the afternoon.
relative humidity. In contrast, the G value presents poor performance if Furthermore, the door could be also opened at other times, such as at
chosen a higher threshold value of 80% according to Ref. [38]: only the noon around December 12:00 on 19th and in the evening such as on
simulated values of the air temperature in bedroom 2 and the courtyard December 23rd and 24th, which is reflected by the underestimation of
are better than 80%. The other values are better than a lower threshold required heating power. With regard to the total heating energy con
value of air temperature (40%) and relative humidity (35%) according sumption, the consumed and simulated amounts were 196 kWh and
to Ref. [53] (underlined in Table 6) except the worst G-values of the 203.2 kWh, respectively. Therefore, the calculated heating energy de
relative humidity in bedroom 1 (bold in Table 6). mand can be regarded as satisfactory for the simulated period. Besides,
It can be concluded that the simulated results agreed well with the the consumed electric heating energy makes up about 28% of the
measured ones according to the statistical indices NMBE, CV(RMSE), household’s total consumed electricity (692.6 kWh). Considering the
and R2, especially the air temperature. However, the relative humidity area of bedroom 1 (12 m2) is only about 5% of the ground area of the
in the heated room (bedroom 1) could not be predicted well, and it did investigated dwelling, this proportion was relatively high.
not reach the defined threshold values of NMBE (5%) and R2 (0.75). The Therefore, since the focus of this study lies in the investigation of
G values of both calibration attempts show certain distances to the energy-saving potential, whereby the air temperature makes the main
higher threshold value of 80% to different degrees. It is worth noting contribution, the calibration quality of the conducted calibration 2 is
that even applying the measured air temperature and relative humidity regarded as satisfactory. The applied easy-to-conduct simulation pro
of A15 for the simulation, the G value of A15 only reaches 9.1% higher cedure based on on-site surveys, existing materials’ database, and the
than 80% for the air temperature and barely reaches 80% for the relative multi-zone airflow modeling could be further applied without adjusting
humidity. Therefore, the applicability of G value in the hygrothermal the inputs regarding indoor moisture gains.
simulation and the choice of a high threshold value for a short simula
tion period, such as two weeks within this study, is questionable. As a
reference, the study [45] conducted a long-term simulation period of 3.3. Energy-saving potential
one year for a historical museum and calculated the G value for water
vapor pressure instead of relative humidity. In order to investigate the energy-saving potential by deeply retro
fitting traditional building enclosures, the building energy balance of the
Table 6
Statistical indices of the two calibrations of the investigated zones.
Zones Parameter NMBE CV(RMSE) G R2
1 2 1 2 1 2 1 2
Bedroom 1 T − 4.5% 0.1% 10.5% 9.5% 48.6% 53.6% 0.80 0.80

RH 6.9% 6.5% 10.5% 9.9% 12.8% 17.3% 0.58 0.61
Bedroom 2 T ¡13.1% − 2.3% 14.2% 5.0% 43.5% 80.1% 0.95 0.97
RH − 0.3% − 3.1% 6.9% 6.5% 40.5% 43.9% 0.75 0.84
Courtyard (A15) T ¡11.8% − 0.4% 13.4% 3.3% 55.6% 89.1% 0.96 0.99
RH 2.3% − 0.1% 7.0% 3.6% 60.4% 79.8% 0.87 0.96
*Numbers with underline: NMBE not reaching a stricter threshold value of 5% ([55]) and G not reaching 80% ([38]).
Numbers in bold: NMBE not reaching a lower threshold value of 10% ([54]), G not reaching 40% for air temperature and 35% for relative humidity ([38]), and R2 not
reaching 0.75 according to [54,55].
10
Fig. 9. Comparison of hourly averaged measured and predicted air temperature and relative humidity in bedrooms 1, 2 and the middle courtyard (A15).
the heat gain during the monitoring period, which indicates a total
dependence on inner heat sources such as occupants and electric heating
devices. The contribution of solar energy is negligible.
It can be observed from the simulation results that even with the poor
airtightness of the building envelope, the current indoor air change rate
remained at a low level. Fig. 12 illustrates the calculated hourly air
change rate in bedroom 1 of the research object. According to this figure,
the maximal hourly air change rate of the investigated room varied from
2.0 to 4.1/h in the afternoons when the door was opened. When the door
was closed, the infiltration rate remained below 1/h. The indoor air
change rate mainly depended on the stack effect caused by the air
density difference between bedroom 1 and the outdoors. It dropped at
night when the heating devices were switched off and climbed in the
daytime when the indoor air warmed, according to Fig. 12. An average
Fig. 10. Comparison of the consumed and simulated hourly heating power.
air change rate was calculated to be about 0.9/h for the measurement
period, lower than the design air change rate of 1/h in the Chinese
heated bedroom 1 was then assessed. Fig. 11 illustrates the proportions standard JGJ 134 [67].
of total heat loss through opaque components (walls, ceiling, and floor), After evaluating the energy balance of bedroom 1, the reduction of
windows, and ventilation of bedroom 1. According to this figure, heat heat loss through its building envelope, especially the opaque parts,
loss due to ventilation accounts for only about 7% of the total heat loss. would play a determining role in reducing the heating energy demand.
In comparison, the amount through opaque areas is dominant, Therefore, the energy-saving potential of improving the U-values of the
contributing to about 81% of the total heat loss during the monitoring building envelope was investigated with further simulations calculating
period. Another 12% heat goes lost through windows. Fig. 11 also shows the heating and cooling energy demands for an entire year.
Fig. 11. Distribution of the heat loss (left) and heat gain (right) according to simulation results of the research object within the monitoring period.
11
Table 8
Information of the reference and the improvement cases to quantify the energy-
saving potential.
Abbreviation Description Weather Design
for case study data temperature
Case 0 Simulation model as The same as Heating: Measured

(reference described in section calibration 2 values in the
case) 3.2.2 for two weeks monitoring period
Case 1 Based on case 0 for an Meteonorm Heating: 15.4 ◦ C
entire year [61] (heating period:
November to
March)
Cooling: 26 ◦ C
(non-heating
period: April to
October)
U U-values of building – –
components are
Fig. 12. Simulated air change rate of bedroom 1. improved to reach the
Chinese standard JGJ
134
Table 7
U-values of the building envelope in the current state and in the improvement
case. Values in brackets are limit values according to Chinese standard but are
Table 9
higher than the defined values in the calibration case. In this case, no
Summary of energy demand with improved building envelope.
improvement of U-values was made.
U-value Monitoring One year
Overview of U-values
period
Building U-value in current state U-value according to Chinese
Heating energy Heating Cooling Total
component [W/(m2K)] standard (CN) [W/(m2K)]
demand [kWh] energy energy [kWh]
Wood walls 1.8 0.8 demand demand
Masonry walls 0.8 (1.0) [kWh] [kWh]
Ceiling 2.0 0.5
Current Case 0: 203.2 Case 1: 2087.5 Case 1: 409.2 Case 1:
Wood floor 1.5 1.0
state 2496.7
Windows 5.6 2.3
Chinese Case 0_U: 128.7 Case 1_U: Case 1_U: 51.5 Case 1_U:
Doors 1.7 (2.0)
standard 1041.4 1092.9
JGJ 134-
2010
For this investigation, energy-saving potential was evaluated for the Reduction − 37% − 50% − 87% − 56%
monitoring period (Case 0) and an entire year with yearly weather data
from Meteonorm [61] (Case 1). The improved U-values of the building
envelope were defined according to the Chinese standard [67], which for the monitored two weeks (from case 0 to case 0_U) and about 50% for
are listed in Table 7. For case 0, the required heating energy demand to the whole heating period—from about 2087.5 kWh of case 1 to 1041.4
maintain the measured air temperature was calculated for the monitored kWh of case 1_U. Furthermore, the cooling energy demand can drop by
two weeks. Concerning case 1, the heating period was defined to be from about 87%. It can be observed from the simulation results that the
November 1st, 2019 to March 31st, 2020 before and after applying the cooling energy demand only makes up about 19.6% (current state) and
improvement measure, while the rest of the time from April 1st, 2019 to 5.9% (after improvement) of the heating energy demand. This low
October 31st, 2019 was defined as a non-heating period with cooling proportion could result from the fact that bedroom 1 is well shaded by
energy demand. The limit value of indoor air temperature for heating the courtyard form.
was defined as 15.4 ◦ C, which was the average indoor air temperature of An economic analysis was conducted for bedroom 1. Table 10 shows
bedroom 1 during the monitored two weeks. For the calculation of
cooling energy demand, the upper limit value of indoor air temperature
was set as 26 ◦ C according to Ref. [67]. Besides, since the local in Table 10
habitants prefer to have their doors/windows opened as often as Material cost and payback time for the energy renovation of the building en
possible in traditional courtyard houses, the ventilation behavior and velope of bedroom 1 to reach the Chinese standard [67].
the operation of the air-conditioner in the simulated bedroom 1 were Components Area Thickness of Material cost Cost [¥]
defined as follows: the door of bedroom 1 is opened between 8:00 and [m2] XPS [m]
Price Unit
21:00 if the air temperature in the middle courtyard is between 18 ◦ C
Walls 14.25 0.021 600 [¥/m3] 179.6
and 26 ◦ C (design temperatures based on [67]). When both the air [68]
temperature in bedroom 1 and the middle courtyard exceed 26 ◦ C, the Floors 12.05 0.04 600 [¥/m ] 3
289.2
door will be closed, and the cooling energy demand will be calculated. [68]
3
Other boundary conditions, such as the internal heat and moisture loads Ceilings 12.04 0.013 600 [¥/m ] 93.9
[68]
in bedroom 1 as well as the ventilation behavior in the heating period,
Windows 4.8 – 302 [¥/m2] 1449.6
were the same as in the calibration case introduced previously. Table 8 (U = 2.3 [W/ [69]
summarizes the basic information about these cases. It is worth noting (m2K)])
that the simulations were conducted under ideal conditions. Total material cost [¥] 2012.3
After running the defined simulations in WUFI®Plus, the calculated Saved electricity [kWh] 1201.6
energy demands were exported and shown in Table 9. According to Electricity price [¥/kWh] 0.67
Table 9, improving the current U-values of the building envelope to the [68]
Saved electricity cost [¥] 664.6
Chinese standard can reduce the heating energy demand by about 37%
Payback time [Year] 2.5
12
the material cost for renovating the building envelope of bedroom 1 to simulated values even with G values ranging from 54% to 89%.
reach the Chinese standard. For simplification, the opaque components Hence, the following suggestions are recommended for further
were renovated by installing XPS boards, which are commonly applied investigation:
in China [68], and plasterboards on their interior side. The thicknesses First, measuring indoor air change rates would be necessary to
of XPS boards were calculated theoretically based on stipulated U-values evaluate the calculated air exchanges by multi-zone airflow modeling.
without comparing them to available sizes on the market. As for the The indoor air change rate can be obtained by the blower-door test,
saved electricity, the consumed electricity of the applied heating radi tracer-gas measuring method, or by measuring the CO2 concentration as
ators is assumed to be equal to the calculated heating energy demand described by Ref. [70]. Besides, information about the occupants’ be
since the electricity is completely converted to heat. Regarding the haviors should also be collected through additional measurement in
cooling condition, the electricity consumed by the air-conditioner was struments such as door/window sensors or interviews with occupants
calculated in a simplified manner by dividing the calculated cooling depending on available resources. In addition, it’s suggested to measure
energy demand by the energy efficiency ratio (2.3) of air-conditioners the weather data in the investigated district since weather data provided
given by Ref. [67]. Considering these saved amounts (Table 9) of a by weather stations may influence the simulation results negatively.
whole year and the electricity price of 0.67 ¥/kWh [68], the payback Second, a more extended measurement period covering an entire
time for the material cost is only about 2.5 years. Other expenses such as year is suggested to evaluate the current energy consumption for heating
labor costs were not considered in this table. Besides, this economic and cooling, which can help better evaluate the energy-saving potential
analysis was only made for bedroom 1, whereby the renovation could be for future scenarios. Besides, although the simulation results in this
conducted for the whole dwelling in practice. This also means that the paper indicate that the cooling energy demand accounts for only about
energy-saving potential could be higher if the building envelopes of 6–20% of the heating energy demand, the situation could be different in
more rooms get improved. other rooms where the solar radiation has a considerable influence (such
as rooms on the roof floor). Therefore, such rooms should be also
4. Discussion considered in the measurement plan to evaluate the economic efficiency
before and after energy renovations. It is worthy to note that the
This section discusses the main outcomes of this study and sugges acceptance of building occupants plays an essential role in the planning
tions for future research from two aspects: the application of numerical and conducting of measurement, which requires a good communication
simulation and data collection for other courtyard houses in the Chinese and organization process. Besides, in traditional buildings with portable
HSCW zone, which remain in a large amount, and the necessity to solve heating devices, the flexibility of using these devices should be consid
not only technical but also socio-cultural problems in preserving the ered in order to collect all necessary data regarding heating energy
courtyard housing form in southern China for further use. consumption. In the future, this data collection could be better obtained
with the government’s support. Since the government has purchased
4.1. Numerical simulation and data collection many traditional buildings from private persons for good protection of
historical heritages, initiatives such as renovation-for-low-costs-rent
Within this study, the indoor air temperature, relative humidity, and may attract private persons to share their indoor environment and en
energy consumption were measured only for a short period of two weeks ergy consumption data.
for the desired minimum interruption to the daily life of occupants.
Therefore, the obtained data were not enough to make a more gener 4.2. Combining technical and socio-cultural aspects
alized evaluation of the occupants’ behaviors in response to uncom
fortable indoor environments. However, these data could still reflect the This study suggests improving the U-values of the investigated
actual interaction between the building occupants, the traditional traditional building envelope, which shows considerable energy-saving
building enclosures, the outdoor environment, and the heating energy potential. This energy-saving potential could be higher than predicted
consumption in winter. Besides, the use of these data to calibrate within this study due to a growing requirement on indoor thermal
hygrothemal simulation also showed satisfactory results. It is worth comfort in the future. For the analysis of energy-saving potential, 15 ◦ C
noting that the applied procedure to simulate the hygrothermal per was set as the desired indoor air temperature to calculate heating en
formance of traditional courtyard houses with multi-zone airflow ergy. This is, however, lower than the design value of 18 ◦ C in the
modeling would also be valid for further investigations of summer Chinese standard JGJ 134–2010 [67] and the value of 20 ◦ C in other
conditions, whereby the ventilation behavior, the operation mode of air- international standards such as [71,72]. So, the acceptable indoor air
conditioners as well as the occupation of indoor rooms would be temperature is expected to climb in the future based on the existing
different. Discussions in this part could also provide instructions for the experience of other countries. For example, the experience of Germany
whole year’s simulation of traditional courtyard houses. shows that with the installation of central-heating devices in buildings,
The statistical indices NMBE, CV(RMSE), and R2 all show good the acceptability of low temperatures could grow dramatically within
quality of the simulated air temperature in both heated and unheated just one generation: from 15.5 ± 2 ◦ C in 1978 [73] to the standard value
zones as well as the semi-opened courtyard, even for a short simulation of 20 ◦ C nowadays [71].
period. This result means an easy-to-conduct simulation procedure Nevertheless, there are still open questions to be solved for energy
based on on-site surveys, existing materials’ database, and the multi- retrofits, both from the technical and the socio-cultural aspects.
zone airflow modeling can be applied to assess the energy perfor First, although the payback time for the material cost to renovate the
mance of such traditional courtyard houses in further research. building envelope is estimated to be less than three years, standards and
Regarding the relative humidity, it is believed to be highly sensitive to studies still need to be developed and conducted to ensure the integrity
the user behavior of ventilation and the setting of indoor moisture gains. of architectural preservation and energy retrofits. They should guide the
Therefore, a calibration for the relative humidity would require more choice of adequate insulation systems (like exterior or interior) and
information about these data. insulation materials for condensation- and fungi-free components in this
In contrast, the application of G value shows poorer performance. humid climate, the construction and installation by local artisans, the
Expanding the monitoring period may bring some improvements like necessary air-sealing concept for building envelope – which could
achieved in Ref. [38] (over 80%). Otherwise, lower threshold values become more critical with increased temperature differences between
could be considered, such as 40% for air temperature and 35% for indoor and outdoor spaces, and the production and preparation of
relative humidity, as used in Ref. [53], since the visual assessment building materials. In short, the energy renovation would require tech
within this study showed good agreement between the measured and nical solutions that combine the local socio-cultural conditions, such as
13
the gap between traditional carpentry techniques and contemporary development of old or lacking standards, the technical guidelines to
technologies. organize, decide and implement renovation measures, the training of
For example, for the construction of traditional Chinese buildings, local artisans, and the integration of individual historical buildings into
the Carpentry and Construction Techniques of Chinese Ancient Archi district energy plans may help ensure the success of energy retrofits. This
tecture [74] still recommends a maximal water content of 25% by mass means the current guiding principle for the conservation and restoration
for lumber in the construction phase, while current guidelines such as of important historical buildings and sites in China should be changed,
Wood Handbook [75] recommend an average moisture content of 12% which is “repairing the object to the state as it was” [77]. Furthermore,
for exterior wood items at the time of installation in damp and warm the historic and traditional buildings should be integrated into China’s
coastal areas in the U.S. Other standards such as DIN 68800-1 of Ger energy efficiency policies and initiatives in the future, such as already
many for wood preservation [76] stipulate a maximal water content of being implemented in the European standard EN 16883:2017 [78] and
20% to protect timber in service from wood-destroying fungi. The the new guideline entitled with “Energy Guideline for Historical Build
feasibility of the recommended high value for traditional Chinese ings” [79] in North America. More work is required to explore the
buildings may require further investigation in the preservation and possibility of energy retrofits for protection and further use of buildings
renovation of such buildings with the changed indoor hygrothermal as cultural heritages.
environment in the future.
Second, further energy-saving of traditional courtyard houses re Funding
quires attention from a higher level of district planning, including en
ergy use and energy supply. As Fig. 3 shows, the historical districts are This research did not receive any specific grant from funding
normally densely inhabited with closely connected courtyard houses. agencies in the public, commercial, or not-for-profit sectors.
Therefore, energy retrofits for the whole district may achieve better
effects than for individual houses, which require a detailed analysis of CRediT authorship contribution statement
the use of traditional buildings in the future, for example, whether they
are for residential or commercial uses or long-term or seasonal occu Mosha Zhao: Writing – review & editing, Writing – original draft,
pation. Such information can help develop a comprehensive renovation Visualization, Methodology, Investigation, Formal analysis, Data cura
plan, including the operation and the occupancy of traditional buildings, tion, Conceptualization. Schew-Ram Mehra: Supervision. Hartwig M.
the energy use, and the financial return of different measurements to Künzel: Writing – review & editing, Supervision.
improve their energy efficiency. Furthermore, new technologies such as
renewable energy (like geothermal energy combined with heat pumps
having a more stable energy supply and less intrusion into building Declaration of competing interest
fabric compared to photovoltaic) should also be considered, making the
traditional buildings more sustainable and attractive for further use. The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
5. Conclusion the work reported in this paper.
For the improvement of poor indoor conditions in winter, there is a Acknowledgement

lack of studies investigating the potential of deeply retrofitting tradi
tional building enclosures in southern China. Hence, this study analyzed We would thank Prof. Zhenjing Yang and Prof. Chi Feng in
the energy performance and energy-saving potential of traditional Chongqing University for their support in measurements and thank
courtyard houses on the example of one chosen traditional house in Tongren Urban Planning, Survey and Design Research Institute for their
Tongren, which is occupied and consumes heating energy. This is assistance in on-site surveys.
different from many other traditional buildings in existing studies,
which are unoccupied, under free-running conditions, or without any References
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16

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