BRE - Designing out unintended consequences when applying solid wall insulation

Organization: BRE
Publication Date: 1 January 2016
Page Count: 28

Introduction and background

Greenhouse gas reduction strategies, together with recent financial incentives such as the Carbon Emission Reduction Target (CERT) fund, Community Energy Saving Programme (CESP) and Energy Company Obligation (ECO), have resulted in a significant uptake in improvements to the existing building stock in the UK. Now that the 'low hanging fruit' has been tackled through installing cavity wall insulation, loft insulation and modern heating systems and boilers, attention is now turning to the harder to treat dwellings that offer the greatest scope for improvements and energy savings.

Generally, these older properties comprise:

• solid wall (brick or stone) dwellings

• non-traditional 'system-built' properties (such as steelframe or panelised concrete)

• properties with narrow cavities within the wall where installing typical cavity wall insulation is deemed unsuitable due to the risk of causing damp problems.

Examples of each are given in Figure 1. In such cases, improving the thermal properties of the walls is generally done by applying insulation to either the internal or external fa├žade, which is a significantly more costly process than the more common improvements mentioned earlier. In many cases, it will be necessary to strike a balance between the environmental impact and the cost of the proposed measures, while taking into account the aesthetic and cultural issues related to our built heritage, which may be affected by externally applied solutions, particularly in historic buildings.

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 value in making efforts to improve the energy performance of this harder to treat stock in order to help reach the UK's carbon emission targets. However, there have been reports of increased condensation and mould growth and other undesirable effects within some homes following such insulation measures. Recent, as yet unpublished studies undertaken by BRE for the Department of Energy and Climate Change (DECC), have identified various unintended consequences that could arise following solid wall insulation. As installing such measures becomes more common, it is imperative that stakeholders properly appraise the risks that may be associated with these works.

It has become apparent that the differences in construction and building physics between older and more modern buildings have not been fully understood by practitioners in recent years. It is vitally important that the principal differences between old and new construction are taken into consideration when 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 that suits all buildings.

Many older buildings use 'breathable' materials which allow some moisture to transfer in and out of a wall in natural seasonal cycles without causing damage to the wall structure. Conversely, the strategy in modern buildings is often to use impermeable materials to prevent moisture from being able to migrate into the structure at all. Risks can be introduced where these strategies are inappropriately combined - either trapping moisture in breathable structures by preventing water from being able to evaporate out, or introducing breathable fabric elements to parts of a vapour-sealed structure that allow moisture to pass through and condense within the structure.

The purpose of this guide is to raise awareness across the industry of the potential problems that can arise if inadequate consideration is given to the particular circumstances of any installation of solid wall insulation and to provide guidance on ways to reduce the risks. It focuses particularly on the older forms of solid wall construction built prior to approximately 1930, but many of the principles will apply to any walls receiving external or internal insulation.

This guidance is intended to be informative, but is no substitute for thorough materials investigations and advanced moisture modelling in circumstances where elevated risk is identified or suspected. Throughout this guide there is discussion of both internally and externally applied insulation; no preference is implied towards either method but different consequences that may specifically arise from either are identified where necessary.