Energy Efficiency and

Historic Buildings
Insulating solid walls

This guidance note is one of a series which explain ways of improving the energy efficiency of roofs, walls and
floors in historic buildings. The full range of guidance is available from the English Heritage website:
www.english-heritage.org.uk/partL
Introduction to the series

This guidance note is one of a series of thirteen documents providing advice on the principles,
risks, materials and methods for improving the energy efficiency of various building elements such
as roofs, walls and floors in older buildings. The complete series includes the following publications:

ROOFS
Insulating pitched roofs at rafter level/warm roofs
Insulating pitched roofs at ceiling level/cold roofs
Insulating flat roofs
Insulating thatched roofs
Open fires, chimneys and flues
Insulating dormer windows

WALLS
Insulating timber framed walls
Insulating solid walls
Early cavity walls

WINDOWS AND DOORS

Draught-proofing windows and doors
Secondary glazing

FLOORS
Insulation of suspended ground floors
Insulating solid ground floors

All these documents can be downloaded free from:

www.english-heritage.org.uk/partL

This series of guidance documents provide more detailed

information to support our principle publication: Energy Efficiency and
Historic Buildings
Energy Efficiency and Historic Buildings: Application of Part L of the
Application of Part L of the Building Regulations to historic and traditionally constructed buildings

Building Regulations to historic and traditionally constructed buildings

This publication has been produced to help prevent conflicts

between the energy efficiency requirements in Part L of the
Building Regulations and the conservation of historic and
traditionally constructed buildings. Much of the advice is also
relevant where thermal upgrading is planned without the
specific need to comply with these regulations.

The advice acts a ‘second tier’ supporting guidance in the This advice acts as supporting guidance in the interpretation of
Approved Documents L1B and L2B that should be taken into account

interpretation of the Building Regulations that should be

when determining appropriate energy performance standards for
works to historic and traditionally constructed buildings

www.english-heritage.org.uk/partL

taken into account when determining appropriate energy

performance standards for works to historic buildings.

COVER IMAGES top right © Philip White bottom left & bottom lower middle © EH/David Pickles 02
CONSERVATION PLANNING
Before contemplating measures to enhance the thermal performance of a historic building
it is important to assess the building and its users to understand:

• the heritage values (significance) of the building

• the construction and condition of the building fabric and building services
• the existing hygrothermal behaviour of the building
• the likely effectiveness and value for money of measures to improve energy performance
• the impact of the measures on heritage values
• the technical risks associated with the measures

This will help to identify the measures best suited to an individual building or household, taking
behaviour into consideration as well as the building envelope and services

TECHNICAL RISKS POSED BY THERMAL UPGRADING

OF OLDER BUILDINGS
Altering the thermal performance of older buildings is not without risks. The most significant risk
is that of creating condensation which can be on the surface of a building component or between
layers of the building fabric, which is referred to as ‘interstitial condensation’. Condensation can
give rise to health problems for occupants as it can lead to mould forming and it can also damage
the building fabric through decay. Avoiding the risk of condensation can be complex as a wide range
of variables come into play.

Where advice is given in this series of guidance notes on adding insulation into existing permeable
construction we generally consider that insulation which has hygroscopic properties is used as
this offers a beneficial ‘buffering’ effect during fluctuations in temperature and vapour pressure,
thus reducing the risk of surface and interstitial condensation occurring. However, high levels of
humidity can still pose problems even when the insulation is hygroscopic. Insulation materials with
low permeability are not entirely incompatible with older construction but careful thought needs
to be given to reducing levels of water vapour moving through such construction either by means
of ventilated cavities or through vapour control layers.

The movement of water vapour through parts of the construction is a key issue when considering
thermal upgrading but many other factors need to be considered to arrive at an optimum solution
such as heating regimes and the orientation and exposure of the particular building.

More research is needed to help us fully understand the passage of moisture through buildings
and how certain forms of construction and materials can mitigate these risks. For older buildings
though there is no ‘one size fits all’ solution, each building needs to be considered and an optimum
solution devised.

TECHNICAL ILLUSTRATIONS GENERALLY

The technical drawings included in this guidance document are diagrammatic only and are used
to illustrate general principles. They are not intended to be used as drawings for purposes of
construction.

Older buildings need to be evaluated individually to assess the most suitable form of construction
based on a wide variety of possible variables.

English Heritage does not accept liability for loss or damage arising from the use of this information.

03
Content

Introduction 05

01 Issues to consider before adding insulation 06

02 Wall insulation generally- relevant issues 12

03 External insulation – relevant issues 14

04 Internal insulation – relevant issues 16

05 Further information 23

English Heritage Local Offices 24

04
Introduction

This guidance note provides advice on the principles, risks, materials

and methods for insulating solid masonry walls. The insulation of early
forms of cavity construction (early 19th century onwards) is covered
by a separate guidance note.

Traditional solid wall construction is probably the most difficult, and in

some cases the least cost effective building element to insulate. Whether
applied externally or internally, work of this nature can have a significant
impact on the appearance of the building. For listed buildings any form
of wall insulation is likely to require listed building consent and for the
majority of buildings external insulation will usually require planning
permission. External insulation can be particularly difficult to incorporate
into existing buildings as costly ancillary adaptations such as changes to
the eaves and verges of roofs, rainwater goods, and window and door
reveals are often required.

Wall insulation will alter the performance of the solid wall and can
in some cases either exacerbate existing moisture-related problems
or create new ones. It is strongly recommended that insulation is
not applied to damp walls. Adding vapour barriers and materials that
are highly resistant to the passage of water vapour are not normally
appropriate for older buildings as they will tend to trap moisture and
can increase the risk of decay to the fabric.

In some cases the technical risks of adding insulation to solid walls will be
too great and alternative ways of providing a more cost effective long-
term solution to improving energy efficiency may be more appropriate.

05
01 Issues to consider before
adding insulation

Traditional solid walls have very different physical characteristics to modern cavity walls.
The construction and performance of the walls need to be fully understood before adding
insulation or there will be a significant risk of creating long term problems.

CONSTRUCTION
The first step should be to identify the external wall materials and their form of construction.
Many older buildings may have three or four different types of wall construction, reflecting different
stages of their development over many years. Construction can vary from single skin brick and
stone walls of as narrow as 100 mm thick up to rubble-filled walls of a metre thickness or more.
Wall materials can include brick of varying hardness and permeability, rammed earth, dressed stone
blocks of varying types, rubble stone, flint and many more. Mortars can also be earth and/or lime
based, also with wide variations in permeability and durability.

A single wall will often contain more than one material with quite different performance
characteristics. For example, soft porous chalk and hard impervious flint have very different
properties but are commonly found in the same wall.

The presence of voids, irregular bonding patterns and concealed timbers also add to the complexity
of solid wall construction and performance.

Theoretical calculations are frequently used to understand and assess the movement of energy
and moisture through solid walls often using quite sophisticated computer programmes. However
data giving the thermal transmittance and moisture permeability of many traditional materials is
simply not available and calculations at present are based upon idealised, homogenous walls. The
actual variations within the wall and the influence of other variables such as the presence of salts
that occur in reality can make such calculations very misleading when applied to many solid walled
buildings. If ‘theoretical modelling’ is used as a basis for the design of thermal upgrading then
performance should be closely monitored after installation in case any problems occur.

06
01 The first step should be to identify the external wall materials and their form of construction. Construction can vary
from single skin brick and stone to rubble filled walls of a metre thickness or more. A single wall will often also contain
more than one material with quite different performance characteristics.
All images © English Heritage/David Pickles except top left © Oxley Conservation

07
BREATHING PERFORMANCE
Traditional solid walled buildings are colloquially referred to as ‘breathing’ structures, meaning that
they exchange moisture readily with the indoor and outdoor environment. Where insulation is
introduced it is important that this breathing performance is taken fully into consideration.

It is important to recognise that moisture in solid walls comes from several possible sources:

• Water from rainfall. This obviously affects solid walls but not all internal damp is a result of
penetrating rain. With the exception of extremely demanding locations such as on exposed
coast or high ground, it is unusual for driving rain to pass through most solid walls in good
condition. Normally it will only saturate the outer part of the wall, which will then dry out
when the rain stops.
• Rising ground moisture can be present in any solid wall which does not have a physical damp
proof course. In such situations the moisture level is generally controlled by the ‘breathability’
of the material, which limits total moisture by allowing the excess to evaporate harmlessly away.
• It is often underestimated how much moisture can be generated by people using a building
internally, simply through breathing but also from cooking and washing. The ‘breathability’ of
external solid walls also significantly helps to control excess moisture and condensation from
these sources.

TRADITIONAL BREATHING PERFORMANCE

Most traditional buildings are made of permeable materials and do not incorporate the
barriers to external moisture such as cavities, rain-screens, damp-proof courses, vapour
barriers and membranes which are standard in modern construction. As a result, the
permeable fabric in historic structures tends to absorb more moisture, which is then
released by internal and external evaporation. When traditional buildings are working
as they were designed to, the evaporation will keep dampness levels in the building
fabric below the levels at which decay can start to develop. This is often referred to as a
‘breathing’ building.

If properly maintained a ‘breathing’ building has definite advantages over a modern

impermeable building. Permeable materials such as lime and/or earth based mortars,
renders, plasters and limewash act as a buffer for environmental moisture, absorbing
it from the air when humidity is high, and releasing it when the air is dry. Modern
construction relies on mechanical extraction to remove water vapour formed by the
activities of occupants.

As traditional buildings need to ‘breathe’ the use of vapour barriers and other
impermeable materials commonly found in modern buildings must be avoided when
making improvements to energy efficiency, as these materials can trap and hold moisture
and create problems for the building. The use of modern materials, if essential, needs to
be based upon an informed analysis of the full implications of their inclusion in order to
minimise the risk of problems arising.

It is also important that buildings are well maintained, otherwise improvements made in
energy efficiency will be cancelled out by the problems associated with water ingress and/
or excessive draughts.

08
Materials used in repair and maintenance must be selected with care to preserve this breathing
performance. Modern impermeable materials – not just vapour control layers but cement renders,
plasters and pointing and many modern paints and coating will significantly impair the breathable
performance and will therefore trap moisture. More often than not this will

increase problems of damp and associated decay of the building fabric, and possibly also create
health risks for the occupants.

02 Hard cement pointing has damaged these soft 03 A cement render has been added to the stone wall of this church
permeable bricks as moisture hasn’t been able to which has caused significant damp problems as the render has
easily evaporate from the mortar joints. altered the breathing performance of the wall. © Robert Gowing
© Philip White

THERMAL MASS
Solid walled buildings, particularly those with thicker walls have comparatively high thermal
capacities, which means they can absorb heat over time and release it relatively slowly as the
surroundings cool down. This is the same principle as a storage heater, although on a larger scale
and can have a significant stabilising effect on the internal environment. External insulation means
little of this heat will be lost to the exterior. This allows a building to maintain a level of warmth
over day-night heating and cooling cycles, improving human comfort and potentially reducing overall
energy use. Internal insulation, whilst reducing short-term heat losses to the exterior will isolate
the internal environment from the benefits of much of this thermal mass.

In summer, when strong sun can cause overheating, the thermal mass of the walls cools the interior
by absorbing excess heat during the day and releasing it slowly during the night. This helps reduce
the need for air conditioning or mechanical cooling.

09
ENVIRONMENTAL INFLUENCES
Location, aspect, and the differing exposure of individual elevations to direct sunlight and wind
driven rain have important influences on a building’s condition and performance which need to be
taken into account when making alterations.

Different parts of a building are affected by very different micro-climates. For example, north facing
elevations can be subject to prolonged damp, as they do not receive the benefit of a drying sun and
are usually sheltered from drying winds. However, they receive little driving rain from the prevailing
south-westerly winds, so conditions are more stable over time. This often means that north-facing
walls deteriorate less than south and south-west facing walls which tend to suffer from accelerated
rates of decay caused by fluctuations in temperature and regular wetting and drying cycles.

Each building’s exposure to the elements is as much influenced by the proximity and position of
surrounding buildings and its own projections and extensions as by the exposure of the site. For
example, an apparently homogeneous terrace of houses can be affected by quite widely varying
local levels of exposure and shelter. Such complex variations in microclimate would ideally need to
be taken into account in the design of any insulation.

DAMP
If a wall suffers from prolonged damp then a number of problems can occur such as:

• decay in timbers in contact with the masonry

• deterioration of the external fabric of the wall due to freezing and thawing
• movement and crystallisation of salts
• movement of tars and other chemicals through the walls, causing staining at the surface
• growth of mould on the inside surfaces of walls
• corrosion of metallic compounds in contact with, or buried within, the wall

Before making any improvements, it is therefore important to understand how solid walled
buildings ‘manage’ the movement of water, in both vapour and liquid form. This is not only complex
in itself, but may also be affected by the presence of soluble salts (see below).

Most insulation systems are designed and developed solely to limit heat loss and to avoid interstitial
condensation from water vapour generated internally. They do not take account of how they affect
the movement of water and salts already in a traditional wall. So they can easily:

• exacerbate existing problems

• create new problems, such as the displacement of damp and salts and the decay of timbers in
contact with the walls
• create health risks for the occupants, for example from mould growth
• be affected by the moisture, reducing their performance and sometimes failing entirely

Where walls have been damp for a long period of time it can take years for them to dry out.
The selection and design of insulation must take account of the drying-out process, both before
and after installation, and the presence of residual damp and salts.

10
SALTS
Buildings without a damp-proof course can be prone to damp and salt contamination, particularly
at low level, where ground salts are carried in solution. Salts are also commonly found around
fireplaces and chimney breasts where they originated as by-products of combustion. They can also
originate from a previous use of a building such as from animal excrement and storage of fertilisers
in agricultural buildings. Salts may also have been present in the original building materials (stone or
aggregate extracted from marine environments) or from the use of chemicals such as caustic soda to
remove paint. In some old buildings bricks were under-fired leaving a concentration of salts.

04 Damp walls can be prone to salt contamination © Tobit Curteis Associates

Many of these salts are ‘hygroscopic’, that is they have an affinity for water and so exacerbate the
problems of damp by attracting moisture out of the air leading to the phenomenon of surfaces
feeling ‘clammy’ to the touch. They may also re-crystallise at drying faces with changing moisture
levels, and the related expansion within the pores can very effectively turn sound masonry into
powder. The interface between existing walls and added insulation can be susceptible to cycles
of evaporation, condensation and salt crystallisation. As such locations are hidden from view;
major deterioration may have taken place before anybody becomes aware that there is a problem.
Unfortunately salts are notoriously difficult to effectively remove from porous building materials
such as brickwork, masonry and plasters.

11
02 Wall insulation generally-
relevant issues

LOCATION OF INSULATION
Insulation may be added to existing solid walls either externally or internally, but the physical
effects on both the building fabric and the internal environment can be very different. This is
explored in more detail below.

COST-EFFECTIVENESS
The necessity to achieve good building detailing to perimeters and openings can significantly add to
the initial base cost of both external and internal insulation and may significantly reduce its overall
cost-effectiveness.

IMPERMEABLE MATERIALS
Practical experience of the repair and conservation of historic buildings shows that the introduction
of materials and systems that do not maintain the traditional ‘breathing’ performance can seriously
exacerbate existing problems and or create new ones. Examples of incompatible materials and
systems which should be avoided include:

• closed cell and extruded plastic insulation

• plastic vapour barriers
• cement or acrylic based renders
• cement pointing
• plastic based external wall paints
• vinyl wallpaper and emulsion paint

Any of these used on the outside of the wall will trap moisture within the wall and lead to damp
and decay, as well as making the walls feel cold and ‘clammy’. Installed on the inside, they may do
less damage to the building fabric itself, but will negate its ability to buffer moisture levels in the
internal air. Both of these can significantly reduce comfort for people using the building, who tend
to try to compensate by turning the heating up, thus wasting energy.

Clearly, if the walls are already damp before installing insulation these effects will be exacerbated.
Under these circumstances it is particularly important to allow walls to ‘breathe’ in order to dry to
the outside as effectively as possible. Drying to the inside is significantly less effective, and may be
extremely unpleasant for users of the building.

12
THERMAL BRIDGES
Whenever insulation is added to an existing building there is a danger of creating thermal bridges at
critical details where full coverage may be interrupted. When insulation is added externally these
weak points are typically at window and door reveals, but with internal insulation they may also be
formed at the points where floors meet external walls.

Areas left with reduced or no insulation coverage will not only be colder because of the lack
of protection from the outside environment, but will also attract relatively more condensation
because the majority of other surfaces are warmer and can no longer share the load. The result
can be severe local decay, particularly to timber and finishes. For example, the ends of floor joists
embedded in the external walls are at increased risk of decay from condensation.

Great care needs to be taken to ensure adequate detailing around window and door openings to
avoid potential thermal bridges, and this can significantly increase the overall cost of both design
and installation. The necessary level of detailing can even be impossible to incorporate in certain
circumstances, in which case, depending on the potential severity of the consequences, it may even
be better not to install insulation at all.

FINANCIAL COST AND PAYBACK

As noted, the addition of external or internal insulation to solid walled buildings tends to be
expensive, and financial payback times are potentially correspondingly long. It is important not
to underestimate the costs associated with the necessary levels of care in detailing to avoid cold
bridges. Full payback periods are typically 30 years or more, but they will inevitably vary greatly
between individual instances.

This suggests that in the majority of cases it would not be worth considering the insulation of
external walls until the full range of easier and more immediately rewarding upgrades to traditionally-
constructed buildings have been carried out. These would include actions such as repairing and
draught-stripping windows and doors; insulating roofs and suspended ground floors, and possibly even
installing condensing boilers. Significantly, most of these upgrades will also have considerably fewer
detrimental effects on the character and cultural significance of historic buildings.

13
03 External insulation –
relevant issues

Most external insulation systems comprise an insulation layer fixed to the existing wall and a
protective render or cladding installed on top to protect the insulation from the weather and
mechanical damage (impact or abrasion).

PHYSICAL ADAPTATION OF THE BUILDING

The increased depth of an external render or insulation system will require adaptation to the roof
and wall junctions, around window and door openings and the repositioning of rainwater down-
pipes. These alterations will require scaffolding access and possibly a temporary roof to reduce the
risk of water penetration during the work.

CHANGES IN THE APPEARANCE AND CHARACTER OF A BUILDING

External insulation will radically alter a building’s appearance, even if it is already rendered. Even
then, decorative architectural features such as cornicing, string courses and window surrounds will
be affected. Even where the elevations are quite plain, simple alterations such as the deepening of
window and door reveals and the alteration of eaves lines can markedly alter a building’s appearance.

In many cases it will be necessary to actually relocate windows and doors further forward in the
overall wall thickness in order to minimise the danger of creating cold bridges at the reveals. This
can reduce some of the visual impact, but will inevitably impact on the building’s character.

Planning permission will be required for external insulation in the majority of instances, whether or
not the building is listed; the local planning authority should be consulted before work commences.
For listed buildings, consent will be needed, and will normally only be likely to be granted in very
special circumstances.

SOLID WALL: EXTERNAL INSULATION

Existing plaster

Solid wall

Permeable insulation

Wire mesh

Insulating lime render

05 This shows a permeable solution with an insulation such as hemp-lime or wood-fibre batts fixed to the masonry
and finished with a ‘breathable’ lime render. Alternatively a non-breathable external insulation could be used such as
expanded polystyrene (EPS) depending on the type of construction.

14
CHANGES IN MOISTURE MOVEMENT WITHIN THE WALL
It is important that the insulation and protective finish installed externally should have low vapour
resistance in order to retain the necessary ‘breathability’, and allow moisture to evaporate away
harmlessly. A useful rule of thumb is that all layers of an insulated solid wall should become
progressively more permeable from the interior to the exterior. Whilst it is important to protect
external insulation from rain, this should not be done in any way that will trap moisture from within
the fabric or from the ground within the solid wall material.

MATERIALS
The need to prevent impermeable layers within the external insulation precludes the use of
modern closed-cell foam and other plastic-based insulations, as well as the use of protective
finishes which bar moisture vapour movement. As most suitable external insulations will also
need to be protected from external rain and from mechanical damage, external insulation should
normally be considered as a two-component system where all layers need to work together.

Useful materials for the external insulation itself include:

• Hemp-lime composites
• Mineral wool
• Wood fibre panels

All these insulation materials need to be protected from both the weather and mechanical damage,
although to differing degrees. Suitable moisture-permeable finishes include:

• Lime renders
• Rain-screen cladding (tile hanging etc.) with lapped joints

Materials which can be used as a single coat are available, such as insulating lime renders containing
expanded vermiculite, but these tend to give significantly lower insulating values. They can,
however, sometimes be applied in circumstances where other types of external insulation would be
detrimental to the character of a historic building.

15
04 Internal insulation –
relevant issues

Internal insulation is usually applied directly to the inner face of the relevant external wall, and then
a finish is applied to the room side.

Rigid board insulations can often be fixed directly to the wall face itself, and then the finish applied
to conceal them without any additional structure. In its most convenient form, plasterboard can be
obtained with a factory-applied foam insulation backing which can be fixed to the inner face of the
wall very easily, although such systems alone do not offer very great insulating performance overall.

For significant insulation thicknesses a non-rigid insulating material will often be installed between
timber studs or battens erected internally to the wall, with the new internal finish applied to the
timber structure. Occasionally, the structure and insulation may be erected as a separate inner leaf,
with a cavity between the insulation and the original wall.

In all cases it is necessary to very carefully consider the control of vapour from the warm internal air
entering and condensing within the insulation, or within vulnerable parts of the original solid wall.

SOLID WALL: INTERNAL INSULATION

VAPOUR EXCLUDED

Solid wall

Existing plaster retained

Impermeable rigid insulation

fixed mechanically or with adhesive dabs

Air and vapour control layer

New plaster

06 A rigid non-permeable insulation is shown here fixed either with adhesive dabs or mechanically fixed. A vapour
control layer is added to the room side face before plastering. Care needs to be taken to make sure this layer is not
punctured by fittings or fixtures otherwise water vapour could find its way into the construction and condense on
the cold side of the insulation.

16
PHYSICAL ADAPTATION OF THE BUILDING
As with external insulation, care needs to be taken with the design and installation of internal
insulation at critical details in order to avoid cold bridging, particularly at the reveals of windows
and doors and wall/floor junctions. It is also often necessary to relocate services (radiators and
associated pipe runs, electric power points and light switches) as well as making adjustments to
skirting boards and door architraves, fitted furniture etc.

(Thermal break at floor junction)

300mm of insulation
between floor joists

POTENTIAL
COLD BRIDGE
IF NO INSULATION

Existing ceiling

Sealing tape to ensure air-tightness

Wood fibre insulation and lime plaster

External wall

07 To avoid a thermal break at a floor junction insulation should be added within the perimeter of the floor zone. It is also
important to seal insulation at junctions with the ceiling to maintain air-tightness.

17
The construction of a separate insulated inner leaf could include ventilation to the cavity. However,
there is a risk that there will be insufficient air movement within the cavity and any vents could alter
the character or appearance of the building. There is no point in ventilating such a cavity to the inside
of the building, as the air movement will simply by-pass the insulation, rendering it ineffective.

SOLID WALL: INTERNAL INSULATION WITH CAVITY

VAPOUR EXCLUDED

Solid wall

Existing plaster retained

Cavity

Rigid insulation

Air and vapour control layer needed

if impermeable insulation is used

New plaster or plasterboard

08 The insulation here is kept entirely separate from the external wall by means of a cavity. If impermeable insulation
is used then a vapour control layer would still be recommended as the air movement within the cavity might be quite
minimal. With this arrangement the benefits of thermal mass of the wall are lost.

Thick, high-performing internal insulation installations will often significantly reduce the floor area
of rooms and corridors, sometimes to the extent that they cannot be used as before.

SOLID WALL: INTERNAL INSULATION WITH TIMBER BATTENS

VAPOUR EXCLUDED

Solid wall

Existing plaster retained

Quilt or rigid insulation

Air and vapour control layer

New plaster

09 The use of timber battens can allow other types of insulation to be used other than rigid insulation. In some proprietary
systems the battens have insulation bonded to them to minimise cold bridging through the timber. Quilt insulation can be
held in place between the battens or materials such as cellulose can fill the cavity. A vapour control layer is shown in this
detail as the insulation is non- breathable.

18
CHANGES IN THE APPEARANCE AND CHARACTER OF A BUILDING
Significant internal features such as plaster cornices and joinery components such as picture
rails, skirting boards and door architraves may all be affected by internal wall insulation. They will
inevitably be either concealed or disturbed to accommodate the insulation. Although it is normally
possible to replicate such details on the inner face of the new insulation, the effect of revised room
proportions on the design of adjacent wall finishes needs to be carefully considered at the design
stage, as the side walls of an insulated room will become shorter.

The disturbance to the internal appearance can be compounded by the need to extend insulation
back from the external wall onto party walls, other internal walls, floors and ceilings to reduce the
risk of thermal bridging.

In listed buildings, consent will be required for any internal alterations that affect the appearance
and character, including any materials, details and finishes of historic or architectural interest. In
many cases this may simply make the installation of insulation unacceptable.

CHANGES IN MOISTURE MOVEMENT WITHIN THE WALL

As noted above, it is a useful rule of thumb that all layers of an insulated solid wall should become
progressively more permeable from the interior to the exterior. In order to protect internal
insulation from condensation occurring within its thickness it is generally necessary to separate it
effectively from the warm, moisture-bearing air of the building’s interior. This will require either the
use of impermeable closed-cell foam insulation or an effective vapour control layer. Alternatively a
vapour permeable system such as wood fibre can be used.

SOLID WALL: INTERNAL INSULATION WITH SERVICES ZONE

VAPOUR EXCLUDED

Solid wall

Existing plaster retained

25mm services zone minimises

risk of damage to air and vapour
control layer

Impermeable foil faced

rigid insulation

Timber battens

Plasterboard

13 An impermeable foil back rigid insulation board is shown here with battens fixed over. These provide a fixing for the
plasterboard without puncturing the foil-face as well as providing a services zone.

19
14 Wood-fibre board internal wall insulation being fixed in place.
© EH/David Pickles

Effective vapour control is, in practical terms, very difficult to achieve. Air and vapour control
layers are positioned on the warm side of the behind the new finish. These membranes are easily
damaged by building users nailing through walls or modifying electrical fitting etc. They can also
be punctured during the construction process itself. All penetrations will allow moisture vapour
through in very large quantities, which will condense either within or adjacent to the insulation,
causing rot and decay in a hidden location. Closed-cell foams are inherently vapour-impermeable,
but are vulnerable to vapour penetration at the joints.

Both forms of vapour control are vulnerable at the perimeter, particularly in a traditional
permeable structure, where moisture can by-pass the physical vapour barrier through adjoining
walls and floors.

However, many of these problems can be overcome by using insulation systems that are
hygroscopic and vapour permeable (eg wood-fibre).

20
MATERIALS
Almost any insulation material available can be used internally, subject to proper control of vapour
and careful isolation from sources of dampness. The full range of possible internal finishes can also
be applied, either to copy the original or to introduce a new design.

New insulation products are continually being developed, particularly those that have a very
minimal thickness (around 10mm). The benefits of such products to reduce overall energy
consumption will be small and they can be relatively expensive. What they will do however is to
make a room feel more comfortable by raising the surface temperature of the walls and possibly
reducing the risk of condensation occurring on the decorated surface

In all cases, however, it is vital to understand the likely effects of proposals at the design stage in
order to avoid damage to both new and valuable historic building fabric.

INTERNAL SOLID WALL INSULATION

(With no vapour control layer)

VAPOUR PERMEABLE

Lime plaster

Wood fibreboard insulation

Existing lime plaster replaced

depending upon evenness

Solid wall

15 This shows a fully permeable insulation system using wood-fibre board and lime plaster. A new lime plaster may
need to be added to the existing wall to provide an even surface if the existing plaster surface is particularly uneven.

21
 16 Permeable insulation such as this
wood-fibre board are compatible with
the ‘breathable’ nature of traditional
construction. © EH/David Pickles

17 Closed cell insulation bonded to plasterboard. © EH/David Pickles

22
05 Further information

English Heritage, 2012, Practical Building Historic Scotland; 2008, Technical Paper 1,
Conservation: Glass and Glazing, Ashgate The Thermal Performance of Traditional Windows;
Historic Scotland
English Heritage, 2009, Improving the
Performance of Traditional Windows, English Newton, R, 2002, The Conservation and
Heritage, London Restoration of Glass, Butterworth & Heinemann,
London,
Essex County Council Planning Department,
2000; The Conservation and Renewal of Timber Tutton,M and Hirst, E (ed); 2007, Windows
Windows, Essex County Council History Repair and Conservation; Donhead;

Georgian Group, n.d. Georgian Windows: A brief Victorian Society, 1999, Timber Windows:
guide to the History and Replacement of Windows No. 9 in the Care for Victorian Houses Series;
in Georgian Buildings, The Georgian Group. The Victorian Society.

Historic Scotland; 2002, Guide for Practitioners

3: Conservation of Timber Sash and Case Windows;
Historic Scotland

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English Heritage Local Offices

NORTH EAST EAST OF ENGLAND

English Heritage English Heritage
Bessie Surtees House Brooklands
41 - 44 Sandhill 24 Brooklands Avenue
Newcastle upon Tyne Cambridge
NE1 3JF CB2 8BU
Tel: 0191 269 1200 Tel: 01223 582700
E-mail: northeast@english-heritage.org.uk E-mail: eastofengland@english-heritage.org.uk

NORTH WEST LONDON

English Heritage English Heritage
3rd floor Canada House 1 Waterhouse Square
3 Chepstow Street 138 - 142 Holborn
Manchester London
M1 5FW EC1N 2ST
Tel: 0161 242 1400 Tel: 020 7973 3000
E-mail: northwest@english-heritage.org.uk E-mail: london@english-heritage.org.uk

YORKSHIRE AND THE HUMBER SOUTH WEST

English Heritage English Heritage
37 Tanner Row 29 Queen Square
York Bristol
YO1 6WP BS1 4ND
Tel: 01904 601901 Tel: 0117 975 0700
E-mail: yorkshire@english-heritage.org.uk E-mail: southwest@english-heritage.org.uk

WEST MIDLANDS SOUTH EAST

English Heritage English Heritage
The Axis Eastgate Court
10 Holliday Street 195-205 High Street
Birmingham Guildford
B1 1TG GU1 3EH
Tel: 0121 625 6820 Tel: 01483 252000
E-mail: westmidlands@english-heritage.org.uk E-mail: southeast@english-heritage.org.uk

EAST MIDLANDS CONSERVATION DEPARTMENT

English Heritage English Heritage
44 Derngate The Engine House
Northampton Fire Fly Avenue
NN1 1UH Swindon
Tel: 01604 735400 SN2 2EH
E-mail: eastmidlands@english-heritage.org.uk Tel: 01793 414963
E-mail: conservation@english-heritage.org.uk

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English Heritage is the Government’s statutory adviser on the historic environment. English
Heritage provides expert advice to the Government about all matters relating to the historic
environment and its conservation.

The Conservation Department promotes standards, provides specialist technical services and
strategic leadership on all aspects of the repair, maintenance and management of the historic
environment and its landscape.

This guidance has been prepared on behalf of English Heritage by Oxley Conservation under the
direction of Phil Ogley and has been edited by David Pickles, Ian Brocklebank and Chris Wood

Illustrations by Simon Revill

First published by English Heritage, February 2010.

This edition published March 2012

Product code: 51585

www.english-heritage.org.uk

If you would like this document in a different format,

please contact our Customer Services Department:

Telephone: 0870 333 1181


Fax: 01793 414926
Textphone: 01793 414878

E-mail: customers@english-heritage.org.uk

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