Designing out unintended consequences when applying solid wall insulation - Colin King and Caroline Weeks
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Designing out unintended consequences when applying solid wall insulation Colin King and Caroline Weeks
Designing out unintended consequences when applying solid wall insulation Colin King and Caroline Weeks
ii
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Contents iii Contents 1 Introduction and background 1 2 Types of unintended consequences 3 Specific moisture characteristics with internally applied insulation 3 Specific ventilation characteristics with externally applied insulation 3 Thermal bridging 6 3 Causes of potential problems with solid wall insulation 9 Systemic issues (specification/design) 9 Assessment (surveying) 10 Application (workmanship) 10 4 Tackling the causes of unintended consequences 11 Client guidance 11 Surveying guidance 11 Design guidance 14 Installation guidance 16 5 Conclusions 17 6 References 18
Stone solid wall dwelling where external or internal wall insulation may be considered to reduce heat loss
1 Introduction and background 1
1 Introduction and background
Greenhouse gas reduction strategies, together with recent insulation to either the internal or external façade, which is
financial incentives such as the Carbon Emission Reduction a significantly more costly process than the more common
Target (CERT) fund, Community Energy Saving Programme improvements mentioned earlier. In many cases, it will be
(CESP) and Energy Company Obligation (ECO), have resulted in a necessary to strike a balance between the environmental
significant uptake in improvements to the existing building stock impact and the cost of the proposed measures, while taking
in the UK. Now that the ‘low hanging fruit’ has been tackled into account the aesthetic and cultural issues related to our built
through installing cavity wall insulation, loft insulation and heritage, which may be affected by externally applied solutions,
modern heating systems and boilers, attention is now turning particularly in historic buildings.
to the harder to treat dwellings that offer the greatest scope for
improvements and energy savings. Since it is estimated that around 80% of the existing housing
stock will still be in use in 2050[1] (Figure 2), there is obvious
Generally, these older properties comprise: value in making efforts to improve the energy performance of
this harder to treat stock in order to help reach the UK’s carbon
• solid wall (brick or stone) dwellings
emission targets. However, there have been reports of increased
• non-traditional ‘system-built’ properties (such as steel-
condensation and mould growth and other undesirable effects
frame or panelised concrete)
within some homes following such insulation measures. Recent,
• properties with narrow cavities within the wall where
as yet unpublished studies undertaken by BRE for the Department
installing typical cavity wall insulation is deemed unsuitable
of Energy and Climate Change (DECC), have identified various
due to the risk of causing damp problems.
unintended consequences that could arise following solid wall
insulation. As installing such measures becomes more common,
Examples of each are given in Figure 1. In such cases, improving it is imperative that stakeholders properly appraise the risks that
the thermal properties of the walls is generally done by applying may be associated with these works.
(a) (b)
(c) (d)
Figure 1: Examples of typical wall construction types to which solid wall insulation may be applied: (a) brick solid wall, (b) stone solid wall,
(c) system build (BISF steel frame), (d) narrow cavity wall
2 Designing out unintended consequences when applying solid wall insulation
40
35
30 2046–2050
2041–2045
Number of 25 2036–2040
homes
(millions) 2031–2035
20
2026–2030
15 2021–2025
2016–2020
10 2011–2015
Pre-2010
5
0
2010 2015 2020 2025 2030 2035 2040 2045 2050
Year
Figure 2: Projected age profile for UK domestic building stock. Image courtesy of MacKenzie et al (2006)[1]
It has become apparent that the differences in construction and modelling in circumstances where elevated risk is identified or
building physics between older and more modern buildings suspected. Throughout this guide there is discussion of both
have not been fully understood by practitioners in recent years. internally and externally applied insulation; no preference is
It is vitally important that the principal differences between implied towards either method but different consequences that
old and new construction are taken into consideration when may specifically arise from either are identified where necessary.
choosing suitable improvements. It is necessary to ensure that
the right materials and assessment processes are used in the
right context. It is not possible to provide a standard solution
Box 1: Explanation of terms used
that suits all buildings.
Many older buildings use ‘breathable’ materials which allow Throughout this guide, the distinction between the
some moisture to transfer in and out of a wall in natural following terms should be understood as defined here.
seasonal cycles without causing damage to the wall structure.
Conversely, the strategy in modern buildings is often to use Risk
impermeable materials to prevent moisture from being able The likelihood of something occurring.
to migrate into the structure at all. Risks can be introduced
where these strategies are inappropriately combined — either Consequence
trapping moisture in breathable structures by preventing water The outcome of something occurring.
from being able to evaporate out, or introducing breathable
fabric elements to parts of a vapour-sealed structure that allow Contributing physical factors/variables
moisture to pass through and condense within the structure. Parameters that can exist to varying degrees and which
may or may not be problematic depending on their state or
The purpose of this guide is to raise awareness across the how they are controlled (eg moisture is always likely to be
industry of the potential problems that can arise if inadequate present, but its quantity and the ambient temperature can
consideration is given to the particular circumstances of any make it a problem). Similar to a ‘hazard’ in common risk
installation of solid wall insulation and to provide guidance on assessment terms.
ways to reduce the risks. It focuses particularly on the older
forms of solid wall construction built prior to approximately Cause
1930, but many of the principles will apply to any walls receiving The manmade action that can influence the state of the
external or internal insulation. contributing physical factors/variables. If adequately
addressed or controlled the manmade action can reduce or
This guidance is intended to be informative, but is no substitute eliminate potential risk.
for thorough materials investigations and advanced moisture
2 Types of unintended consequences 3
2 Types of unintended
consequences
Investigation of a number of refurbishment installations has the wall or insulation or both fail (eg the early failure of internal
highlighted a range of unintended consequences that have coverings, such as plaster, or potential frost damage within
occurred following the application of solid wall insulation. the wall over the winter months). Assessing the potential
These have been grouped into the general categories listed in condensation risk is therefore an important step in determining
Box 2, although it should be noted that there may be cross-over the suitability of a building to receive internal wall insulation.
between issues and they often occur together due to the same
underlying causes.
For impermeable, vapour closed systems the strategy is to make
sure the wall is not wet prior to installation and to ensure the
There are common, fundamental physical factors/variables that
surface finish is well sealed from the internal environment using
contribute to many of the problems identified in this chapter,
a vapour control layer (VCL) so moisture cannot migrate behind
particularly moisture, ventilation and thermal bridging. These
the insulation layer to condense on the cold wall surface. If this
issues are often linked, for example the presence of moisture
were to happen, the interstitial condensation would provide
is often exacerbated by inadequate ventilation. It is the control
ideal conditions for mould growth. Adequately sealing the VCL
of these factors that will ultimately determine whether adverse
can be difficult at some junctions, in particular around the joists
consequences are experienced in a building. Unfortunately,
of intermediate floors that penetrate the wall. This can lead to
these variables are usually poorly understood and badly
problems of rot and mould growth on timber joist ends.
managed by practitioners, which leads to the problems detailed
in Box 2.
The second group of vapour open insulation systems do not
contain a VCL moisture seal and continue to allow moisture
As indicated earlier, it is important to understand and
to pass in and out of the wall as necessary. Their composition
complement the way a building is designed to deal with
allows them to buffer moisture, preventing it from accumulating
moisture; some, particularly older walls are intended to be
at detrimental levels at any point in the wall, then allowing it
breathable, ie they can safely manage and buffer moisture
to pass back out of the wall when the relative humidity drops.
to some extent, while other non-breathable walls should be
Although a partially saturated zone will exist within the wall
sealed from moisture. Problems may arise when changes are
at these times, the alkaline nature of the materials present
introduced to the material properties that influence the moisture
will not create a favourable environment for the formation
movement characteristics. This can lead to moisture build-up on
of mould. Despite the accepted presence of moisture within
surfaces or interstitial condensation. Common mechanisms for
the insulation, it would not be expected to notably affect the
such damage are discussed below.
thermal performance of the insulation at the concentrations
anticipated. The manufacturers of these vapour open internal
wall insulation systems acknowledge that there is a limit to their
buffering ability and that they should not be applied to wet
Specific moisture characteristics with walls, where there is rising damp, or in locations with exposed
internally applied insulation weather conditions.
One of the key potential consequences associated with the use
of internal wall insulation is that of condensation forming in parts
of the dwelling where it did not form before the insulation was Specific ventilation characteristics
applied, which could potentially lead to mould growth and an
unhealthy internal environment for occupants. Internally applying
with externally applied insulation
insulation captures heat in the room and prevents most of the
Ventilation is an important and often underestimated
heat from transferring into the wall behind. The temperature
parameter; it is essential in every building to provide a means
of the wall is therefore considerably lower than the surface
by which moisture arising from occupants and activities such as
temperature in the room. If moisture is able to reach these colder
cooking and bathing can be expelled and replaced with fresh air
areas, condensation will form. There are two main risk areas:
from outside so the indoor environment can remain healthy for
• areas of thermal bridging (as discussed later) where the the occupants. Often, older houses are not particularly ‘airtight’
surface temperature at the bridging point will be lower than and a substantial contribution to the building ventilation (albeit
the main parts of the walls (eg at window reveals, party uncontrolled) takes place through natural leakage pathways
walls, intermediate floors, etc.), leading to condensation in the construction (eg around doors and windows). The
and mould growth installation of wall insulation will often improve the airtightness
• areas of interstitial condensation where moisture has of the building by sealing up these uncontrolled ventilation
been able to migrate behind the insulation to the cold wall pathways. While this will help to save more energy, an adequate
surface to cause condensation to form out of sight and ventilation strategy must be maintained in the dwelling to
become trapped; the lack of heat reaching the wall means prevent the build-up of:
that the wall is unable to dry out effectively.
• moisture which can cause condensation in areas that
previously did not suffer from condensation
Interstitial condensation is considered a more significant • pollutants that may inadvertently affect occupant health.
consequence as it may not be apparent to the occupants until
4 Designing out unintended consequences when applying solid wall insulation
Box 2: Consequences commonly experienced following solid wall insulation
Overheating Creation of new thermal bridging/condensation
This has been observed in buildings but it is also possible points
to forecast through simulation modelling. The addition of Application of insulation can create new thermal bridging
extra insulation can keep unwanted heat within buildings, points in the structure. As well as undermining the
particularly in warmer, summer months. Buildings with anticipated thermal performance of the newly insulated
internally applied wall insulation may be more prone to wall, it can also lead to increased risk of condensation
this because heat can no longer be buffered by the wall. formation and mould growth at these points.
Perversely, this may result in increased energy use from
mechanical cooling, which would undermine savings made Rot and/or insect attack on structural timbers
during the heating season. If structural timbers are not kept dry they become more
prone to degradation from rot and/or insect attack, which
Increased relative humidity and associated damp can cause subsequent structural problems. Intermediate
and mould growth floor joists and roof timbers may be particularly susceptible
The installation of solid wall insulation often improves to this where the insulation applied creates new thermal
the airtightness of a building as it seals a number of bridging, reducing the temperature at points within the
uncontrolled air leakage pathways. Although this has the timber, making it prone to condensation.
positive effect of reducing heat loss, an increase in internal
humidity can occur if there is inadequate ventilation Frost damage within walls
provision in the building. This can lead to condensation The application of internal wall insulation will prevent
problems, and create the potential for mould growth, heat entering the wall and hence mean that it is no longer
along with associated health complaints, such as asthma. dried by heating the interior of the dwelling. As a result,
moisture entering the wall from outside may not be driven
Reduction in indoor air quality out entirely by external evaporation and when sufficiently
Similarly to the previous consequence, improvements in cold temperatures are experienced the water may freeze,
the airtightness without associated consideration of the expand and damage the wall structure.
ventilation provision in the building can lead to reduced air
quality for occupants and increases in concentration of CO2 Failure of internal surface finishes
and other indoor pollutants. If moisture is not removed from the dwelling and damp
is trapped within the walls, it can cause delamination of
Short-term concentrations of volatile organic internal finishes (including potentially the insulation itself if
compounds (VOCs) internally applied).
Similarly to the above issues, improvement in the
airtightness and reduction of uncontrolled air leakage Interstitial condensation
can lead to increased concentrations of volatile organic If moisture is not adequately controlled within the insulated
compounds from solvents and adhesives likely to be used wall, it is possible that it can migrate to parts of the wall
during the refurbishment. These can have both short- and that are sufficiently cold for it to condense. It may also
longer-term effects on the health of the occupants. be possible for water to penetrate behind the insulation
if seals are inadequate or compromised by degradation
Elevated radon concentrations in dwellings over time. This can cause damage to the wall structure
In areas of the country prone to radon emissions from the and potential mould growth out of sight which could go
ground (especially in areas of South West England, but unnoticed for a considerable time.
also other areas), increased airtightness (without adequate
ventilation) following the installation of solid wall insulation Undesirable visual changes
could increase the radon concentrations in a dwelling, From a cultural or aesthetic point of view, the use of
which can lead to health problems. For maps of affected external wall insulation may have a significant impact on
areas visit www.ukradon.org/information/ukmaps. the appearance and vernacular character of buildings.
Some detailing may also be visually undesirable at difficult
Presence of dust mites and other insects within the junctions if badly designed. This often occurs because it
home is cheaper to create ad-hoc details than to integrate the
Similarly to the above issues, a number of household pests insulation properly (ie extended eaves to cover insulation).
including dust mites, bed bugs and clothes moths are more As it happens, the majority of detailing that may be
active and prevalent in higher humidity environments, deemed visually unappealing is also likely to introduce
which can arise following the installation of solid wall an enhanced risk of thermal bridging and subsequently
insulation without adequate ventilation. more physical problems, as noted above. This is a problem
with many details currently accepted as being ‘standard’
Negative influence on neighbouring property details, where steps and staggers are introduced and there
In some circumstances, the installation of solid wall is significant reliance on sealants to prevent water ingress
insulation on one property can affect uninsulated (rather than better protection of junctions).
neighbouring dwellings, as the relative temperatures of
the walls will be adjusted. New thermal bridging can result, Property value
which may cause increased condensation risk and potential The effect of solid wall insulation on property value is
for mould growth in places where there were previously no uncertain. While some value may be assigned to the
problems. reduced energy consumption that will result, the property
value may be reduced if some aspects of the aesthetics are
poor (see Undesirable visual changes above) or, in the case
of internal insulation, if internal space is reduced.2 Types of unintended consequences 5
Box 2: Consequences commonly experienced following solid wall insulation (continued)
Reduced daylight Disturbance to occupants during works
Solid wall insulation can reduce internal daylight factors The installation of solid wall insulation has the potential for
through increasing the depth of the window reveals and disturbing not only the occupants but also the surrounding
by adding additional insulation within the reveals (which vicinity, with the need for scaffolding, deliveries and other
should be a fundamental part of any installation). This may incidental activities. The extent of the disturbance varies
reduce daylight and solar gains to some extent and may depending on whether insulation is internally or externally
lead to marginal increased energy use from lighting. applied. This could be a disincentive to some occupants to
undertake such improvement works.
Reduced durability and need for ongoing repairs
Solid walls with no insulation applied are generally Potential elevated risk of fire spread in mid/high-rise
robust, sturdy structures. The introduction of lightweight buildings
insulation materials at the exposed surfaces (internal or Externally applied insulation has the potential to offer a
external) that are less resilient to impacts and potential pathway for the spread of fire up the outside of buildings if
damage may lead to the need for occasional repairs to not adequately controlled and designed. This is mainly only
maintain the integrity of the wall. a concern in taller buildings (> 18 m) that may be out of
reach of conventional fire-fighting techniques.
Solid walls will generally undergo cycles of wetting and drying, Adequate ventilation provision is only part of the solution; its use
both internally and externally, with drying being largely driven by depends on the occupants. This may require some explanation
the heat from inside the dwelling through the winter and by the and education. Occupants may associate ventilation with
sun outside during the summer. Since external wall surfaces will draughts and the loss of valuable heat without understanding
be exposed to the weather, it is necessary for externally applied its importance for controlling moisture build-up, for example,
insulation to be weather-sealed to prevent damage. This creates mechanical fans may be switched off and trickle vents closed or
a barrier to the transfer of any moisture within the wall to the blocked up. Despite all efforts made in its provision, ventilation
outside which means that it can migrate only one way and that is still one of the most difficult aspects to safeguard in use.
is into the dwelling. If the wall in question is dry, this may not When convincing occupants of the importance of maintaining
present much of a problem to the occupants as the dry wall may ventilation, even in cold weather conditions, it is likely to help
be able to provide some level of buffering to moisture arising if any mechanical systems installed incorporate heat recovery.
from household activities. However, if the wall already contains Although mechanical systems with heat recovery are more
moisture (perhaps trapped inside if the insulation was installed commonly considered for ‘whole house’ ventilation systems,
after a long period of wet weather), it is far more likely that the single room ventilation fans with heat recovery are available to
humidity will increase within the dwelling, both from household prevent the majority of the heat from being extracted from the
activities and from the wall itself drying out. It is therefore dwelling while removing moisture.
important that there is adequate ventilation provision to remove
the moisture and prevent this from potentially becoming
problematic, ie occupant health issues, damp, condensation,
increased mould growth, damage to structural elements such as
Relationship between ventilation
premature decay of timbers. and overheating
In any case, as an absolute minimum, the requirements of the Ventilation can be an important aspect of controlling potential
relevant applicable national building regulations for ventilation overheating risk. Despite provision of adequate ventilation for
should be met[2–5]. However, the requirements for existing the purposes of controlling moisture, additional ventilation via
buildings generally follow the principle of making the building openable windows is likely to be required in the summer to
no worse than it was prior to any improvements, which may purge excess heat. Again, education may need to be given to
not actually represent a truly adequate level of ventilation. A occupants to effectively control overheating. If temperatures
clause for ‘traditional buildings’ means that adequate ventilation outside come within a few degrees of those indoors, keeping
should be provided as far as reasonably and practically possible windows closed will likely be the best option, while opening
and that the work should not prejudice the character of the windows overnight when outdoor temperatures are cooler will
host building. In those cases without restrictions relating to help re-balance indoor temperatures and avoid compounding
historic buildings, it may be more prudent to follow the ‘new gains day after day.
build’ requirements of the regulations, which provide minimum
extraction rates for wet rooms (utilising mechanical ventilation) Social landlords will need to give consideration to how this may
and/or whole dwelling ventilation rates that should be met be facilitated, particularly in ground-floor properties where
(depending on dwelling size). This should only be considered security associated with opening windows would need to be
the starting point when assessing ventilation provision since considered. Proprietary window/vent covers may need to be
the introduction of thermal upgrades may influence moisture installed.
movement within a building, and so ventilation requirements
should be specifically assessed on a case-by-case basis.6 Designing out unintended consequences when applying solid wall insulation
Window reveals, when uninsulated, offer direct thermal
Thermal bridging bridging pathways for heat to transfer around windows.
Reduced temperatures will often occur, resulting in an increased
Thermal bridge is a term given to any part of the building
risk of condensation and mould growth. The enhanced heat loss
envelope where there is an increased heat flow compared with
through reveals is visualised by the bright yellow colours in the
the adjacent parts. This creates areas of reduced temperature
thermographic image (Figure 3(a)).
at internal surfaces since heat moves away from these areas
more readily (hence the phenomenon is also often called ‘cold
Ground-floor junctions, where it is often difficult to ensure
bridging’). If temperatures become too low, there is a risk of
continuity of insulation into the floor slab. With external
surface condensation and mould growth.
insulation, it is possible to extend the insulation into the ground
to offer some improved resistance to additional heat loss.
Current industry practice with its emphasis on cost control
Typically, however, the insulation stops short of the ground to
generally does little or nothing to minimise the effects of
retain a damp-proof course, although there are other ways
thermal bridging or prevent inconsistency in thermal envelope
to do this without leaving large segments of the external wall
performance when insulation is installed in solid wall dwellings.
uninsulated. The effect of the heat loss at ground-floor level
Making the effort to minimise thermal bridges is considered
where insulation is not continuous is demonstrated by the bright
likely to add more time and expense to what is already regarded
yellow regions in the thermographic image (Figure 3(b)).
as a costly improvement measure. However, by ignoring such
bridging effects, it is likely that the predicted improvements
Intermediate floors (with internal wall insulation), where the
from installing solid wall insulation will be undermined and
insulation continuity is disrupted by the intermediate floor (solid
the creation of concentrated heat loss points can make areas
floor) or timber joists (suspended floor). Thermal bridging at this
of a building more vulnerable to condensation, leading to
point can reduce the temperature experienced at the timber joist
subsequent damage and degradation. The cost of dealing
ends, which can become vulnerable to moisture damage, rot
with these consequences is likely to be more expensive than
and degradation.
addressing the thermal bridging in the first place.
Eaves junctions (with external wall insulation), where the eaves
Recent research carried out for DECC[6] on the impact of thermal
overhang is shallow and there is insufficient space for the
bridging on internal wall insulation heavily emphasises the
loft insulation to overhang the externally applied insulation
importance of ensuring that the reveals of window openings
effectively and thus provide continuity. This results in an
are insulated. Thermal modelling showed that if this was not
exaggerated step out from the eaves at the wall insulation. Not
done, there would be minimal benefit from installing insulation
only is this sort of detail likely to be unappealing aesthetically,
any thicker than approximately 40–80 mm on the walls; the
but it also creates a significant thermal bridging point. This
additional energy savings from thicker wall insulation would be
detail is often not addressed correctly due to the cost and
counteracted by additional, exaggerated heat loss from thermal
inconvenience of extending the eaves.
bridging at the reveals. Thermal bridging studies by BRE[7] have
quantified the effects of poor detailing at key junctions when
Other obstructions to the continuity of insulation, such as (but
installing solid wall insulation and provided recommendations
by no means limited to) meter boxes, gas and mains water
that should be followed to reduce or eliminate these effects.
connections (external insulation) or internal walls adjoining the
main wall (internal insulation).
Common areas for concern include those discussed in the
sections below.
(a) (b)
Figure 3: Thermal images showing: (a) increased heat loss at window and door reveals when left uninsulated (image courtesy of Joanne Hopper),
(b) increased heat loss at ground-floor level where insulation has not been continued to the ground2 Types of unintended consequences 7 For internally applied wall insulation, internal partition walls are often a difficult or impossible junction point to address. Generally, other internal obstacles, such as distribution pipework or incoming service points are suitably dealt with during internal insulation projects by removing, extending and replacing the pipes/services. However, with externally applied insulation, such obstacles are often not removed, extended and replaced to allow the insulation to be continuous. Each interruption of this kind will create a thermal bridge and a potential point for condensation to form on the internal wall surface, not to mention being visually unappealing. Figure 4 shows the development of mould on the inner wall surface corresponding to the area where insulation was not continued behind a meter box on the external wall. In most cases such carelessness is inexcusable and is simply a result of lazy workmanship and/or a lack of contingency budgeting to Figure 4: Distinct square of mould growth on the inside of the area deal with site-specific issues. Box 3 (overleaf) shows a range of where a utility meter box breaks the continuity of externally applied examples of poor detailing that could lead to similar effects. insulation Needless to say, these features should not be emulated.
8 Designing out unintended consequences when applying solid wall insulation
Box 3: Examples of poor detailing creating thermal bridges
Insulation has been cut around a security alarm instead of removing Insulation has been cut around pipes and there is an exaggerated
the alarm and re-fitting it gap around the air brick
Insulation has been cut around a pipe inlet into the dwelling instead Insulation has been stepped at the eaves and gable instead of
of removing the pipe and re-fitting it extending the roof line. Seals at the stepped junctions will be more
vulnerable to degradation and subsequent water ingress than if they
were protected by an overhang
Cut outs have been made to allow external window openings. Insulation has been damaged and poorly fixed. Insulation boards
Changes should instead have been made to the window opening should be closely butted with no gaps, facilitated by clean cut lines
arrangement (eg to open internally or by installing a smaller high- instead of being badly cut (see image on left). Care should be taken
level casement, although this may still have prevented insulation of not to damage insulation boards when applying fixings (see image
the reveals) on right)3 Causes of potential problems with solid wall insulation 9
3 Causes of potential problems
with solid wall insulation
As implied in chapter 2, various actions taken in the design
and installation of solid wall insulation (or ignorance of the Energy reduction targets
implications of certain actions) can impel factors such as
moisture, ventilation provision and thermal bridging to become
a problem. These are the manmade causes of potential Insulation suitability/
unintended consequences and they can be classified into three material choice
overall categories, as shown in Figure 5. However, as will be Systemic issues
seen, there are areas of cross-over depending on how works are (specification/ Detailing
procured, designed and installed. design)
Example causes in each of the contributing categories are
discussed in the sections below. Fire risk/design
Condition of structure
Systemic issues (specification/design)
Energy reduction targets
U-value targets from national building regulations may be Exposure conditions
too demanding in some cases. For example, to achieve the
target values in breathable, historic structures, there may be an Assessment
unnecessarily high risk of introducing problems. If elevated risk (surveying) Sources of moisture
can be demonstrated, national building regulations do make
allowance for relaxing the target U values.
Ventilation
Insulation suitability/materials choice
Failure can be caused by, for example:
• inappropriate selection of materials/systems for the natural
Heritage
mechanisms of the substrate wall (eg selection of vapour
closed products for breathable walls)
• products that are unsuitable for the local climate conditions.
Restrictions/obstacles
Detailing Application
Failures in detailing are often linked to cost, for example, (workmanship)
choosing not to insulate within window reveals, not ensuring On-site checks/
continuity of insulation when detailing around obstacles, quality assurance
omitting the temporary removal and/or extension of incoming
services, etc., all of which can create thermal bridging and
associated consequences (see Figure 4 and Box 2). Handover information
Fire risk/design
Although spread of fire, both internally and externally, is covered
by national building regulations[2–5], the rapidly developing solid Systemic (specification/design)
wall insulation market makes it difficult to offer prescriptive Failures introduced essentially through the insulation solution
guidance for any given solution. In mid- and high-rise structures, chosen, decisions made relating to the design and specification of
fire spread through external insulating cladding systems could the system that are fundamentally likely to introduce elevated risks.
be exacerbated if appropriate measures are not taken at the Assessment (surveying)
design stage[8]. The knowledge and process used to assess the condition of
the building and determine whether any corrective actions will
Condition of structure be required prior to installation. The identification of physical
If there are inherent defects within the structure, any installation constraints or conditions that may give rise to future problems.
may be more prone to failure.
Application (workmanship)
The quality of the installation and the skill required to identify
restrictions or problems that could be introduced during application.
The attention to detail and implementation of the design intentions.
Figure 5: Classification of the key causes of unintended consequences10 Designing out unintended consequences when applying solid wall insulation
Assessment (surveying) Application (workmanship)
Condition of structure Restrictions/obstacles/heritage
Poor assessment of the baseline condition of the structure can Installers should recognise features that may influence
lead to the introduction or aggravation of problems. A survey the nature of the installation and, if these have not been
should include as an absolute minimum: appropriately addressed by the pre-installation survey and
the system design, the installation should not proceed until
• assessing the state of repair of the wall and other related
appropriate solutions are identified.
elements (including drainage)
• identification of any defects that should be addressed
On-site checks/quality assurance
before insulation works commence.
It is inevitable that installers will have different levels of
experience and skills. Installations should always be checked
Exposure conditions/local climate
and signed off at key stages by a competent person prior to
If inadequate assessment is made of the prevailing climate
completion and acceptance. Ideally, this would be done by an
conditions (shelter, solar gain, wind-driven rain), inappropriate
independent third party to ensure that issues that may result in
choices of materials and solutions may be made at the design
additional costs for rectification are not simply ignored.
stage.
Handover information
Sources of moisture
The installation crew are likely to be the last point of contact
This is an important but difficult aspect to assess and, unless
with the occupants following the works so it is logical for them
better or more convenient test methodologies become available,
to be responsible for transferring any useful information to the
a conservative approach should be taken in the assessment.
occupants, such as warranty information, care and maintenance
While rising or penetrating damp may be more readily identified
considerations to prevent future damage and degradation to the
(and will usually preclude the application of solid wall insulation),
system, the importance of ventilation, etc.
the moisture movement patterns within solid walls are usually
not well understood.
Ventilation
Distinguishing the causes in this way implies a defined process
It is necessary to assess the current ventilation provision and
towards solid wall implementation from initial design and
determine whether it will continue to be adequate following
specification of the works, to surveying and on-site installation.
building improvements.
However, some design aspects may not be considered at all if
installers are directly selling their services as approved suppliers
Heritage
or installers for a manufactured system. Limited attention may
Surveyors should understand features likely to be considered
be paid to the surveying aspect, depending on how much the
of heritage value to planning authorities that may influence the
installers’ surveyors know about construction and building
choice of solution (eg external versus internal insulation). It will
physics.
also be important to understand historic building construction
techniques and materials to assess correctly their likely
It therefore follows that many of these causes exist as a
interaction with newly applied solutions.
consequence of current prevalent procurement practices. This is
also fundamentally linked to the pricing of the works; solid wall
Restrictions/obstacles to the installation
insulation is inherently a relatively costly improvement measure
Unless potential obstacles or difficult detailing aspects are
so it is easy to see how facilitating steps that may be viewed
identified during the initial surveys, these cannot be dealt
as an extra burden could be overlooked, while not necessarily
with appropriately during the design and specification of the
realising the potential long-term knock-on effects.
solution. This forces contractors to improvise on site, often
resulting in less than ideal solutions and thus elevated risks.
As will be seen in chapter 4, it is likely that the current approach
to planning and procuring solid wall insulation works will need
to be reconsidered in order to eliminate the underlying causes of
potential problems.4 Tackling the causes of unintended consequences 11
4 Tackling the causes of
unintended consequences
The only way to avoid some of the consequences detailed in systems (eg extract fans, trickle vents, etc.). It should be
chapter 2 (eg visual and aesthetic aspects, practical loss of space) emphasised that any limited additional heat retention caused by
may be to decide not to undertake the insulation measures. blocking vents or switching off fans could come at a significant
However, choosing between externally and internally applied price to the integrity of the structure and the quality of the
insulation may overcome some of the issues. A combination occupants’ internal environment.
of both may be adopted where there is a desire to retain the
appearance of front elevations to maintain character and place. Post-construction assessment and monitoring by occupants
For other issues, the obvious action is to tackle the causes and housing managers should be actively encouraged in
identified in chapter 3 by: order to look for signs of moisture problems with the aim of
catching and rectifying any identified issues at the earliest
• thorough surveys possible opportunity before they cause more significant
• good detailed design damage. Maintenance schedules should list key points for future
• good implementation (ie workmanship) inspection; this should prevent the development of problems by
• appointing an appropriate team of professionals and encouraging, for example, good upkeep of external finishes.
managing the expectations of the occupants.
Sufficient budget and timescale should be allowed for all
actions (surveying, design and detailing) to be fully considered
and implemented. In wetter seasons, it may be necessary
Client guidance to incorporate significant contingency time so that external
insulation works do not need to take place in wet weather as
This section provides general guidance for clients on wider this would inevitably introduce moisture into the walls that may
aspects and emphasises the need for a joined-up approach subsequently be difficult to eradicate.
towards surveying, designing and installing insulation systems.
Communication between surveyors and designers and,
subsequently, designers and installers is important throughout Surveying guidance
the whole process to ensure risks are fully appraised and
solutions are passed through the whole delivery chain. This section specifically focuses on the surveying requirements
Consideration should be given to appointing a ‘moisture when considering solid wall insulation as these can make a
champion’ with a good understanding of the actions and significant contribution to the success of an installation (or
implications of moisture movement in buildings. The moisture failings, if not appropriately carried out). Surveyors need to
champion would oversee the entire project to ensure that have the appropriate skills to recognise specific constructions
individual elements are considered in an integrated way and that and materials, and to understand moisture issues, particularly
the whole building works as intended. within historic buildings. An understanding of the process of
installing solid wall insulation is also fundamental to identifying
Occupants need to be informed of: potential obstacles and detailing aspects that will need to be
addressed on a case-by-case basis by designers. If the necessary
• the full visual impact on a property,
skills are not present, then issues may be missed resulting in
• compromises on internal or external space
potential problems. Surveyors must be confident and capable of
• considerations for future use (eg new hanging points to fix
addressing the key steps set out in Table 1.
things to walls will need to be provided to prevent future
damage to insulation).
Step 1: Building condition
While it has been noted in the previous sections that surveyors
should be able to identify such issues, they may not always be in Before any fabric improvements to enhance the thermal
a position to pass this information on to the occupants. Clearly, performance of a building are considered, it is essential that
it is important that the occupants understand the visual impacts the actual building condition is assessed to ensure that it is in a
and any loss of space prior to the start of any works. However, good state of repair. Buildings in good condition are generally
information on maintenance and the installation of fixing points much more capable of dealing with upgrades than buildings
may be more appropriately provided by the installers at handover. already experiencing issues. It is essential that there are no
A strategy therefore should be agreed between the client, signs of excessive weathering, poor drying, trapped moisture,
surveyor, designer and installers, clarifying who will be best penetrating or rising damp, other sources of moisture or
placed to inform the occupants of any necessary considerations, leaks. If persistent sources of moisture exist, or the external
the form this should take and when it should be done. surfaces cannot adequately protect from wind-driven rain in
winter or vapour diffusion in summer, moisture may become
It is necessary to understand how occupants use the building trapped within the solid wall once insulated. This can lead to
and to inform them of the importance of maintaining adequate the consequences of interstitial condensation, elevated indoor
ventilation so they do not override or undermine any installed humidity levels (external insulation) or frost damage within walls12 Designing out unintended consequences when applying solid wall insulation
Table 1: Key steps to be addressed when surveying for
the installation of solid wall insulation
Step 2: Assess construction materials
Step Assessment Description As discussed in chapter 2, walls essentially use one of two
methods to deal with moisture arising from the internal or
1 Building Check that the building is external environment:
condition in a good state of repair, or
identify repair needs. Identify • vapour closed, whereby the strategy is to make sure the
the presence and sources of wall is not wet prior to installation and to ensure the surface
moisture. finish is well sealed from the internal environment using a
vapour control layer (VCL)
2 Construction Assess the wall composition • vapour open systems that do not contain a VCL , whereby
materials and whether it is fundamentally the strategy should be to ensure that they continue to allow
vapour open or vapour closed. relatively small amounts of moisture to pass in and out of
3 Exposure Determine the suitability of the wall as necessary and have some buffering potential.
conditions insulation/finishes to be chosen
during design and for feeding
It is necessary to identify accurately the construction materials
into moisture risk modelling
present and whether the existing wall currently relies on
calculations.
a vapour open or vapour closed approach to dealing with
4 Ventilation Assess whether ventilation moisture. An appropriate insulation solution can then be
assessment provision will be adequate recommended to complement this.
following works or if additional
measures will be required.
5 Areas of specific Highlight areas for specific
Step 3: Exposure conditions
detailing attention during design and
The performance of any building will be influenced by the
installation to avoid thermal
environmental conditions to which it is exposed, such as
bridging.
wind-driven rain that may cause elevated wetting. There may
6 Complementary Identify whether any other also be localised sheltering that can protect walls in otherwise
building building elements should be relatively high exposure locations. Such factors may influence
elements considered for upgrade at the the insulation and/or finishes that may be suitable for any given
same time as the installation of location.
solid wall insulation.
7 Occupant Consider issues likely to cause Generalised exposure maps and guidance are given in
consequences short-term disruption or BRE Report, Thermal insulation – avoiding risks (BR 262)[9],
longer term considerations for although this does not take account of the localised context.
occupants. For areas suspected to be at elevated risk, buildings should
be assessed according to BS 8104:1992 Code of practice for
assessing exposure of walls to wind-driven rain[10].
The exposure rating may also be used in moisture modelling
(described in the next section). In more extreme exposure
conditions, it may be pertinent to consider extra protection
within the solid wall insulation detailing, such as extended eaves
overhangs, wider gutters, splash protection, particular attention
(internal insulation) as highlighted earlier. Particular attention
to the seals around windows, etc. in order to reduce the risks
should be given to identifying:
of moisture ingress. It should be noted that while BRE Report
• cracked and damaged render BR 262 suggests that external wall insulation systems with
• mortar continuity and pointing quality external cladding are generally suitable in all exposure categories
• rising or penetrating damp (1–4), it qualifies this by saying that reference should be made
• rainwater goods and drainage issues (to ensure no leaks or to manufacturer and third-party certification guidance on
water accumulation at walls) exposure suitability. This statement on suitability also assumes
• ground levels relative to internal floor levels (to ensure that the installation will be of high quality, and, in practice,
moisture cannot penetrate walls above floor level). either any defects introduced through poor workmanship or the
degradation of sealants over time may dramatically increase the
exposure risk.
The moisture assessment is fundamental to any potential
insulation measures, yet unfortunately there are no simple
physical measurements that can be taken to give absolute values Step 4: Ventilation assessment
for moisture content, which may of course vary from season
to season. (Surface measurement devices are not suitable for Ventilation should be checked for correct operation if already
assessing moisture within walls.) Clearly, more work is needed installed (Figure 6) or should be specified where not already
to determine a method for assessing moisture content to assist present. This may involve the provision of mechanical ventilation
surveyors. Any issues will need to be rectified before insulation units in kitchens and bathrooms to eliminate moisture at its
may be considered and, if they cannot be remedied, the source. However, in some older properties the walls are so thick
recommendation should be that solid wall insulation should not that it may be difficult to source fans with long enough ducts to
be installed. traverse the width of the wall. Potential issues like this that may4 Tackling the causes of unintended consequences 13
not be given consideration during the design stage and it will
be left to the installers on site to create an ad-hoc solution with
no additional budget for any special fixings or extra facilitating
works.
Step 6: Upgrade of complementary
building elements
Although the fundamental purpose of the survey is to evaluate
the viability of installing solid wall insulation, it is worthwhile
taking a whole building approach and assessing the condition
of other building elements. If there are items that may require
upgrade, maintenance or repair in the short term, there may
be merit in combining them with the wall insulation works if it
provides opportunities to better address thermal bridging or
detailing issues.
Figure 6: Ventilation units may be physically tested to ensure ventilation For example, windows are a common source of difficulty with
rates are as intended solid wall insulation as there may not be adequate space to
insulate within the reveals while still allowing the windows to
open. However, if the windows are reaching the end of their
life and are likely to be replaced in the short term, this could be
brought forward ahead of the insulation so that the window
frame profiles and openings can be designed to accommodate
the necessary insulation. It would also allow the insulation and
affect the provision of good ventilation in dwellings should be
finishes to be properly sealed to the windows rather than risk
identified during initial surveys for solid wall insulation and an
them being disturbed if they are replaced in the short term
adequate ventilation strategy incorporated into the proposed
following solid wall insulation, potentially creating areas of
solution. The assessment also needs to consider whether any
weakness for moisture penetration.
existing provision will be adequate after the proposed works
(ie if the airtightness of the building is subsequently improved
Some other examples of elements which may need to be
following insulation). Surveys should also consider whether
upgraded in the short term and which might be done in
adequate ventilation provision is maintained under suspended
conjunction with solid wall insulation include:
floors, in vented roof spaces and, if necessary, for combustion.
• replacement of roof coverings which may facilitate the
Where ‘air bricks’ are present in a wall to aid the combustion extension of eaves overhangs to accommodate externally
of a heating system (eg open fire, non room-sealed stove), applied insulation
assessments should consider whether these are still required. If • replacement of kitchen units or bathroom suites which
the heating system has been changed and no longer requires would need to be removed and re-installed on the walls
such ventilation, the survey should note a need for them to during installation of internal wall insulation.
be appropriately blocked up. If they are still required, the
ventilation gap will need to be maintained through the applied
insulation; appropriate detailing and finishes should be specified
by the designer. It is important that the intended outcome is
Step 7: Other consequences including
communicated through the various stakeholders to the installers disruption for occupants
so that they are confident about how to deal with air bricks; they
will not risk blocking a vent unless they are assured that it is no Surveyors should consider any other issues that may be
longer required. All other ventilation grilles should be similarly important to the building occupants, even if these issues do not
maintained if deemed to be necessary. fundamentally influence the design of the proposed system. A
key aspect of the success of any home improvement measures
will be managing the expectations of the occupants. Anything
Step 5: Areas of specific detailing that may cause disruption to the occupants should be noted
and communicated to them, such as the need for space to
Surveyors need to identify areas of the building that will require provide scaffolding access for external works or to move/
detailing at the design stage. Specific attention should be store furniture for internal works. Any loss of space should also
given to potential obstacles to the continuity of insulation so be communicated, as well as information about how some
that appropriate detailing can be considered and priced. For key visual aspects may be detailed and the need to maintain
example, mains/services pipework, meter boxes, satellite dishes, good ventilation. If any aspects are deemed unacceptable
external alarms, window openings (particularly within close to the occupants, there is still time and scope for alternative
proximity to other walls receiving insulation), adjoining walls/ suggestions to be considered (eg a fundamental switch from
garages, etc., as shown in Figure 7 should all be considered. If external to internal insulation if the occupants are unhappy with
such items are not flagged during the survey, they will inevitably a change in appearance or the loss of external space).You can also read
