1st April 2015


Early Cavity Walls

Builders and architects started to experiment with cavity or ‘hollow walls’ from early in the Victorian period. By the first decade of the 20th century, most pattern books for houses included examples of outer walls with two separate leaves of brickwork. Initially the development of the ‘hollow’ or cavity wall was to provide as much protection as possible from the elements, especially driving rain together with enhanced stability and economy of materials. These types of wall first appeared in exposed areas, particularly in coastal locations.

The use of cavity wall construction became increasingly common particularly for the house building boom of the inter-war years where developers saw its economic advantages over solid wall construction. Around 80% of the existing housing in the UK is now estimated to be of cavity wall construction.

Since the energy crisis of the early 1970s it has become common to fill or partially fill the cavity with a variety of insulation materials to reduce the heat loss through the wall. Modern cavity walls (post-Second World War) often provide a good opportunity to improve thermal performance with cavity insulation. Such insulation is invisible, cheap and quick to install, with no reduction to room sizes. As a result cavity insulation is widely promoted, and even subsidised. But cavity insulation is not generally suitable for all cavity walls particularly those classed as ‘early cavity walls’.

The performance of early cavity walls will differ from that of later cavity walls and these differences need to be taken into account when considering the addition of insulation. However, some early cavity walls can be insulated using cavity insulation and this guidance discusses how to determine whether any particular wall is suitable. For some early cavity walls, cavity fill insulation will always be unsuitable and the construction needs to be treated as a solid wall and insulated either internally or externally.


 Solid Wall Heat Losses and the Potential for Energy Savings – Literature Review

Insulating the solid wall housing stock is one of the greatest challenges for UK Government energy efficiency policy.  It also potentially offers some of the most significant savings, as there are more than 7 million solid wall dwellings in Great Britain.  Policies to deliver this work, such as the Green Deal and ECO, rely on modelled estimates of savings from solid wall insulation. It is therefore essential that the models used to provide these estimates are based on robust, up to date and accurate data. Questions exist around the assumptions used when predicting savings from solid wall insulation.  In particular there is a gap between calculated predictions and actual realised energy savings following the application of this measure.

The UK Governments Department for Energy and Climate Change (DECC) have developed a research project to answer these questions through a collection of work packages, each designed to address a particular aspect of the performance gap.

Here we provide Work Package 1 – Literature review, which is a search of existing literature on the subjects relevant to the study. The objective is to inform the design of the research conducted in this project and to avoid unnecessary duplication. The literature review has been completed and can be downloaded from the ‘downloads’ section of this page.


The other packages are:

Work Package 2 – Understanding heat losses from solid walls: A field survey to investigate in detail the thermal performance of walls of 80 dwellings and a laboratory study of the thermal performance of specially constructed wall sections in a hot-box.

Work package 3 – Pre- and post-insulation survey: The performance gap: The collection of energy consumption data, together with other on-site physical measurements, from a sample of 30 dwellings pre- and post-installation of solid wall insulation.

Work Package 4 – Methodologies for measuring and calculating U-values: The validation of the current best practice methodologies for measuring U-values in-situ and an investigation into the method and assumptions used for calculating U-values from the physical properties of the wall.

Work package 5 – Monitoring and investigating unintended consequences: The production of a guidance document for assessing and monitoring unintended consequences.

Work Package 6 – A rapid and reliable in-situ U-value measurement tool: A competition to design a tool that can be used by Green Deal assessors to measure wall U-values in-situ and the construction of a database that can eventually be built into RDSAP to allow assessors to vary default U-value assumptions.


 External Wall Insulation in Traditional Buildings

Insulating solid walls has become a considerable focus in recent years, particularly with the introduction of the Government’s Green Deal and Energy Company Obligation finance schemes which place an emphasis on the thermal retrofit of traditional solid-walled housing. Given the technical risks inherent in insulating walls internally, external insulation is likely to be a serious consideration for many traditionally-constructed properties, where aesthetics allow. However, there are numerous considerations – technical and otherwise – to be addressed when considering this measure, and further research is required in order to have a fuller understanding of these.

The background and technical details of three external wall insulation projects in the North of England have been evaluated that includes householder feedback and analysis to identify common and diverging findings. The projects were then analysed to highlight common conclusions and areas requiring further research.


Insulating Timber-Framed Walls

Wall insulation alters the hygrothermal behaviour of traditional buildings, and can exacerbate existing moisture-related problems or create new ones. Any installation requires great caution and it is strongly recommended that insulation is not added to damp walls. Modern methods of upgrading thermal performance which include 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 increase the risk of condensation and trap moisture and can increase the risk of decay to the fabric – including the structural timber frame. Here we look at the methods, materials and risks involved with insulating the walls of timber framed buildings. The benefits of making improvements will include improved comfort for occupants as well as lower fuel bills and carbon emissions.


 Insulating Solid Walls

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 some 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. Insulation should not be 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.