Last edited 26 Oct 2020

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Institute of Historic Building Conservation Institute / association Website

Wall insulation and moisture risk



Britain’s housing stock is a key part of our national infrastructure. Efforts to improve its energy efficiency are now being extended to ‘hard-to-treat’, solid wall properties. With more than seven million solid wall dwellings in Great Britain, there are very significant energy savings to be made although, as Colin King explains below, there are significant risks too.

The current focus of attention in the UK building stock is towards the hard-to-treat or ‘HTT’ properties, those with a solid wall construction rather than a cavity between the two wall elements. They were often built using natural, local materials held together with mortar – typically lime-based – and other fine local aggregate to bind it together. The HTT properties have rightly been left alone until recent years. Their inherent moisture movement capacity (breathability) makes the introduction of modern materials risky if the process is not carefully considered, approached with a keen eye on the location of the building and undertaken with a degree of caution.

Until recently, the mainstream construction industry considered the performance of these buildings to be poor in comparison with more modern buildings constructed of cementitious or timber frame materials. However, recent and ongoing research indicates that the U-values of these older structures can be up to 30% better than previously assumed. This further reduces the potential for savings from the introduction of internal or external wall insulation.

When undertaken correctly, external wall insulation is an important measure in helping to reduce heat loss through solid wall buildings. It can also improve the health of occupants, protect buildings from adverse weather and, sometimes, improve appearance and value.

However, it is easy to get it badly wrong, generating a much smaller reduction in heat loss than anticipated. Changes to the physical state of the structure can also cause damage to the building fabric (for example, through mould, dry rot or wet rot), which can subsequently affect the health of the occupants by adversely affecting the indoor air quality. This is made more likely by current industry practice which rarely considers the assessment of adequate ventilation pre- and post insulation.

One would think that the Building Regulations would be sufficient to cover this eventuality, but unfortunately Part F of the Building Regulations offers no real insight into safe or adequate levels of ventilation during the retrofit process, merely defaulting to a ‘make it no worse’ stance. However, without testing air permeability pre- and post-installation, it is almost impossible to decide whether it really is ‘no worse’. In most of the cases I have surveyed and investigated, ventilation was never on the radar of the workforce or procurement process.

Poorly installed external wall insulation.jpg
Poorly detailed external wall insulation resulting from a failure to extend the roof-line of this traditionally constructed building.

The answer is to consider a more risk-conscious approach to the decision-making process and one that is focussed on moisture risk. When considering moisture risk in a built environment context it is vital that the presence of water is considered in in all its different states (ice, liquid water and water vapour).

All buildings and their inhabitants require a level of moisture to be healthy. Efforts should always be made to balance moisture by a combination of fabric protection, heating and ventilation –whether planned or through infiltration such as draughts and gaps in the structure or through physical vents. Solid wall or HTT buildings were constructed to deal with moisture differently than modern buildings and so need to be treated differently. In particular, their construction materials let moisture (as rain and humidity), in and out without the resistance of barriers or cavities.

Again, looking at the regulations it would be reasonable to assume that they are written to protect the fabric of the building and the health of the occupants. However, this is not always the case and other companion guides to the Building Regulations such as BS5250:2011 Code of Practice for Control of Condensation in Buildings are weak in their understanding of breathable structures and only deal adequately with modern buildings or with one aspect of moisture, like condensation.

A more integrated approach to all moisture types which deals specifically with solid wall construction is necessary to reduce moisture risks. It should be noted that there are still many uncertainties about moisture risks in all buildings. At all times, therefore, a precautionary approach should be taken and as much information as possible should be gained in any project through good assessment, monitoring and feedback.

Categories of elevated risk

To understand the risk of undertaking external wall insulation (EWI) on an older building it is important to understand where the risks lie and what can be done at the process and planning stage to minimise them. Research undertaken in this area by the BRE, Sustainable Traditional Buildings Alliance (STBA), Society for the Protection of Ancient Buildings (SPAB) and others demonstrates that there are key stages where risk is most elevated. These can be categorised as set out below. Surveys that do not consider these issues should be considered poor:


What is the building made of? Can EWI be fixed to it (for example, some traditional building materials are very weak)?


Where is the building? Does it get a lot of wind and rain? Is one side more exposed than the others? A building on a cliff in Wales is very different from one on a London street. The more exposed the building is, the more care should be taken over detailing – extended overhangs, wider gutters, proper splash protection and mastic-free seals around windows, for example.

In exposed locations, the exposure of walls to wind driven rain should be assessed using BS 8104 – this is vital for determining material suitability. Other factors such as proximity to sources of deposition (such as industrial outlets, motorways and other sources of airborne pollution) should also be considered at an early stage because they will reduce the robustness and effectiveness of the insulation.


Are there planning constraints or heritage values ascribed to the building as a whole?


Has the building got a complicated shape or details which make it difficult to be sure that there won’t be leakage or cold bridging? Does this form have heritage value? If it is complicated, it may be better not to insulate or to consider internal wall insulation.

Cold bridge through external wall insulation.jpg
Standard detail around a telephone junction box introducing a large cold bridge and the potential for moisture damage


Is the fabric wet internally? Is the building already suffering from damp? If so, has it been assessed correctly? Is it in such a condition that it should be insulated at all?

Fixing insulation on damp or defective buildings

EWI should not be fitted to wet or damp buildings, which need to dry out before being clad, or which need repair (for example, due to high ground levels, leaking roofs, ill-conceived damp-proof courses, and poorly designed and constructed replacement floors). The moisture may be driven into the building by wind or other natural occurrences.


Covering up ventilation by reducing infiltration rates and making the building less draughty is another common cause of risk. Understanding how the insulation will affect the ventilation of the inside is essential to ensure that it does not create unhealthy living conditions.

If already installed, ventilation should always be checked for correct installation and operation at design capacity, and if not installed, assessed and installed where necessary. The main weakness in this area is not knowing how to assess that the building is in moisture balance and how much ventilation is adequate.

Rain during application

Insulation systems sometimes get wet during application or because of poor storage or delivery methods. This can lead to failure of the insulation and render systems from excessive moisture and freeze-thaw cycles.

Insufficient caution

A lack of understanding, skills, budget or time can lead to errors, short-cuts and poor quality work which can cause a variety of moisture risks.

The whole-building approach

In short, the building should be considered as a single entity and a whole-building approach should be used. To fully consider the risk to the building and its inhabitants it is essential that work does not proceed unless it is based on complete and accurate information.

The building should not be insulated until the following factors are known and understood:

If all of these factors are known and the materials and workmanship are of a very good standard, then the introduction of insulation to the external walls might be the right option. However, it is important to remember that nothing is maintenance free and that failure to maintain a building will result in it failing.

This article originally appeared in IHBC's 2017 Yearbook. It was written by Colin King, director of the Building Research Establishment (BRE).

--Institute of Historic Building Conservation

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