OTTV

Building Fabric
Learning Outcomes
◉ To identify the elements of Thermal Properties and

Conductivity of Building Material
◉ To explain the characteristic of the Individual Structural

Elements for Building Fabric
◉ To demonstrate the calculation of the Overall Thermal

Transfer Value (OTTV) of the Building Fabric
1 Introduction
Building Envelope
“ the external portions of a building

through which thermal energy is
transferred”
and
“ this thermal transfer is the major

factor affecting interior comfort level
and energy usage”
Coleman Street Building
◉ To prevent heat loss in cold climates and cut
off heat gain and prevent loss of cool in hot
climate.
◉ A building loses its thermal balance through

either heat loss or heat gain depending upon
the climatic conditions.
México Fortius Office Building

Thermal Balance
Heat Gain Heat Loss
Fabric heat Ventilation heat

losses losses
Conduction Convection Radiation

Energy Usage In Buildings
“ Wasteful use of energy is affecting

our planet and our environment. If
we design, build and manage our
buildings so the need for energy is
reduced, only then our effort will
make a real difference”
Reuse, Reduce, Recycle

Thermal Properties of Building
2
Materials
Thermal Conductivity
◉ In heat transfer, the Thermal

Conductivity (k) of a material is
its ability to conduct heat.
◉ Heat moves from a warm place

to a cold place through three
different mechanism: conduction,
convection & radiation.
Conduction
◉ The heat transfer process that takes place through a
temperature gradient between the molecules which form
the solid parts of a building fabric.
◉ Dense materials, where molecules are more closely packed

– good conductors as compared to less dense material as the
molecules are widely spread.
Convection
◉ Heat transfer through a fluid (liquid, gases, etc)
◉ Air surrounding a surface that is hotter than

itself would also experience a rise in temperature.
◉ Cold air dense, therefore, it settles down but

warmer air rises up.
◉ The warm air may come in contact with a

surface which is cooler than itself and in that case,
the heat will be transferred from the air to the
surface.
Radiation
◉ Heat transfer due to emission of
electromagnetic waves is known as
thermal radiation. Heat
transfer through radiation takes place
in form of electromagnetic waves
mainly in the infrared region.
◉ Sunlight is part of thermal radiation

generated by the hot plasma of the Sun.
◉Temperature of the surfaces and

emissivity/ absorptivity of the
source/receiving surfaces are the
primary factors that radiation depends
on.
Thermal Conductivity
◉ Examples of thermal conductivity, k
Material k (in W/mK)

Mineral wool insulation 0.039
Mineral fibreboard 0.053
Plasterboard 0.160
Common clay brick 0.950
Glass 3mm thick 1.050
Concrete 2.160
Thermal Resistance
◉ Heat energy flows from a hot object to a

cooler object
◉ Whenever there is a temperature

gradient, heat transfer will always occur.
◉ It can never be stopped, and it can only

be slowed.
Thermal Resistance
◉ It is the magnitude of the ability of the thickness of material to resist the
conduction of heat through it.
◉ The characteristic is dependent on the thermal conductivity & the

thickness of the material.
◉ The formula:
R = t/k
t: thickness of material (m)

k: thermal conductivity (W/m.K)
◉ Increasing the thickness of the material will increase its thermal resistance.
In a composite wall construction, the total R is
Rtotal = R1 + R2 + R3 +….Rn
Thermal Transmittance
(U-Value)
◉ U- Value: Overall heat transfer

coefficient, states how well a building
element conducts heat.
◉ It measures the rate of heat transfer

through a building element over a given
area, under standardized conditions.
◉ U is the inverse of R (Thermal

Resistance) with SI units of W/ (m²K)
Thermal Transmittance
(U-Value)
◉ Thermal transmittance is a measure of the speed with which

heat is lost through 1 square metre of the element with 1K
temperature difference across its faces.
◉ The lower the U-value, the better insulation a material can

provide against thermal losses.
U - Value
Albedo & SRI
◉ The albedo of an object or a surface is the extent to

which it reflects light from the sun.
◉ Generally, a light coloured surface has a higher albedo.

It is shown as SRI, & ranges from 0 to 1.
Eg. Material SRI

Black arcrylic paint 0.05
New asphalt 0.40
Concrete w Portland cement 0.70-0.80
White acrylic paint 0.80
Cooling roofs by increasing SRI
• A dark colored roof absorbs most sunlight
• Increasing solar reflectance – reduces solar heat gain,

lowers roof temperature
• High thermal emittance – facilitates radiative cooling,

helps keep roof temperature low
• Lowering roof temperature – can reduce electricity

required to cool building down ambient air temperature.
Overall Thermal Transfer Value
(OTTV)
◉ OTTV aims at achieving the design of building envelope to cut down
external heat gain and hence reduce the cooling load of the air-conditioning
system.
◉The OTTV should not exceed 50 W/m²
◉ OTTV formula:
OTTV = (A01 x OTTV1) + (A02 x OTTV2)… + (A0n x OTTVn)

A01 + A02….+ A0n
A1 : Gross exterior wall area for orientation1;

OTTV1: OTTV value for orientation 1
OTTV for the whole building ≤ 50 W/m²
Overall Thermal Transfer
Value (OTTV)
◉ The formula for the OTTV of any given wall orientation is as follows:
OTTVi = 15 α (1 – WWR) Uw + 6 (WWR) Ug + (194 x CF x WWR x SC)
Heat
Heat Solar heat gain
Conduction
OTTVi= Conduction + through + through = < 50 W/m²
through walls windows
windows
Value (OTTV)
◉ Heat conduction through walls
Heat
Conduction
OTTV= Conduction + through + through = < 50 W/m²
windows
Value (OTTV)
◉ Heat conduction through walls
15 α (1 – WWR) Uw
α = Solar Absorption = Colour of Walls

WWR = Window to wall ratio
Uw = U-Value of the wall
15 x α x Wall area ratio x U-Value of Wall

Value (OTTV)
◉ Heat Conduction through Windows
Heat
Conduction
windows
Value (OTTV)
◉ Heat conduction through windows
6 (WWR) Ug
6 x Window to wall ratio x U-value of Window
Glazing Type U-Values
Single glazed clear 5.7 to 6.2
Laminated clear/PVB/Clear 4.5 to 5.5
Double glazed clear/air/clear 2.6 to 2.9

Value (OTTV)
◉ The formula for the OTTV of any given wall orientation is as follows:
OTTVi = 15 α (1 – WWR) Uw + 6 (WWR) Uf + (194 x CF x WWR x SC)
Heat
Conduction
windows
Value (OTTV)
◉ Solar Heat Gain through window
194 x CF x WWR x SC
CF= Correction Factor

WWR = Window to wall ratio
SC = Shading Coefficient
Glass Shading Coefficient (SC)

Is the amount of solar energy that passes through the glass, relative
to a 3mm clear glass tested under similar conditions. A low value
means less heat passes through the glass.
OTTV Calculation: Menara UAC
3 Individual Structural Elements
Walls
◉ Heavyweight walls
- Usually made of masonry, concrete, etc
- Masonry cavity walls: extremely resilient towards external weather factors

& do not deteriorate with long periods of exposure to moisture as well.
- The cavity provides an insulation component to the wall.
- The width of the cavity is indirectly proportional to the U-Value, therefore, to

further lower the U-Value, the cavity width should be increased & insulants
which have low thermal conductance should be used.
- Have large thermal mass

Walls
◉ Lightweight Walls
- Lightweight walls are made of wooden frames, straw, grass, etc.
- Timber framed walls with insulants can achieve a U-value of 0.15

W/m²K or lower.
- Some solutions that can be used in conjunction with a timber framed

wall are limecrete, rammed earth, straw bale,frass, fiberglass & hemp
(hemp lime & hemcrete)
- Have low thermal mass

Windows
◉ Tinted Glass
Windows
◉ Coated Glass
- A polyester or metal
coating is applied to
improve the insulating as
well as acoustic of glazing
unit.
- Such coating give the

glazing a reflective
appearance.
Windows
◉ Low-Emissivity Glass
- A glazing unit with thin
coating (usually metal) in the
interior of the unit.
Other Transparent Materials
◉ Glass Reinforced Plastic
(Fibreglass)
- Fibreglass (usually made of

glass) are combined with a
fibre reinforced polymer
(epoxy, polyester,etc)
- Light, strong & robust

material, U-Value(2.0W/m²K)
- Primarily used in roofing &

door solutions
◉ Aerogel
- A gel (usually silica based) in which

the liquid component has been
replaced with gas, resulting in an
extremely low density solid
- In building application: as insulating

material for wall, flooring & roofing
- Thermal conductivity; as low as 0.03

W/m.K
◉ Structural Insulating Panels (SIP)
- Composed of a foam (polystyrene,

expanded polystyrene) which is
sandwiched between 2 layers of
structural board (sheet metal,
plywood sheathes)
- Used for foundation, flooring,

roofing & wall.
Floor
◉ Suspended/Raised Floor
-The spaces underneath the flooring are used to cool a

building through floor-mounted vents.
- The floor is raised on joists which are further provided with

an insulation barrier to close air gaps & create a tight fit.
Exercise
Thanks!
For your attention.

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Building Fabric

◉ To identify the elements of Thermal Properties and

◉ To explain the characteristic of the Individual Structural

◉ To demonstrate the calculation of the Overall Thermal

“ the external portions of a building

“ this thermal transfer is the major

◉ A building loses its thermal balance through

México Fortius Office Building

Heat Gain Heat Loss

Fabric heat Ventilation heat

Conduction Convection Radiation

“ Wasteful use of energy is affecting

Reuse, Reduce, Recycle

◉ In heat transfer, the Thermal

◉ Heat moves from a warm place

◉ Dense materials, where molecules are more closely packed

◉ Heat transfer through a fluid (liquid, gases, etc)

◉ Air surrounding a surface that is hotter than

◉ Cold air dense, therefore, it settles down but

◉ The warm air may come in contact with a

◉ Sunlight is part of thermal radiation

◉Temperature of the surfaces and

◉ Examples of thermal conductivity, k

Material k (in W/mK)

◉ Heat energy flows from a hot object to a

◉ Whenever there is a temperature

◉ It can never be stopped, and it can only

◉ The characteristic is dependent on the thermal conductivity & the

t: thickness of material (m)

◉ U- Value: Overall heat transfer

◉ It measures the rate of heat transfer

◉ U is the inverse of R (Thermal

◉ Thermal transmittance is a measure of the speed with which

◉ The lower the U-value, the better insulation a material can

◉ The albedo of an object or a surface is the extent to

◉ Generally, a light coloured surface has a higher albedo.

Eg. Material SRI

• A dark colored roof absorbs most sunlight

• Increasing solar reflectance – reduces solar heat gain,

• High thermal emittance – facilitates radiative cooling,

• Lowering roof temperature – can reduce electricity

◉The OTTV should not exceed 50 W/m²

OTTV = (A01 x OTTV1) + (A02 x OTTV2)… + (A0n x OTTVn)

A1 : Gross exterior wall area for orientation1;

OTTVi = 15 α (1 – WWR) Uw + 6 (WWR) Ug + (194 x CF x WWR x SC)

◉ Heat conduction through walls

OTTVi = 15 α (1 – WWR) Uw + 6 (WWR) Ug + (194 x CF x WWR x SC)

◉ Heat conduction through walls

α = Solar Absorption = Colour of Walls

15 x α x Wall area ratio x U-Value of Wall

◉ Heat Conduction through Windows

OTTVi = 15 α (1 – WWR) Uw + 6 (WWR) Ug + (194 x CF x WWR x SC)

◉ Heat conduction through windows

6 x Window to wall ratio x U-value of Window

Glazing Type U-Values

Single glazed clear 5.7 to 6.2

Laminated clear/PVB/Clear 4.5 to 5.5

Double glazed clear/air/clear 2.6 to 2.9

OTTVi = 15 α (1 – WWR) Uw + 6 (WWR) Uf + (194 x CF x WWR x SC)