Energy Efficiency in Historic Buildings
Early Cavity walls
This guidance note provides advice on the principles, risks, materials and methods for improving the thermal performance of buildings built with early forms of masonry cavity walls dating from before the Second World War
Modern cavity walls (i.e. post-Second World War) often provide a good opportunity to improve thermal performance through cavity wall insulation. Such insulation is cheap and quick to install and invisible. As a result cavity wall insulation is widely promoted, and even subsidised. But cavity wall insulation is not generally suitable for early cavity walls.
At the start of the 19th century ‘hollow walls’ with two separate leaves of brickwork were developed to provide as much protection as possible from the elements, especially driving rain. Other benefits included enhanced stability and economy of materials and methods. These walls first appeared in exposed areas of the UK such as seaside towns. Early cavity walls may not be easy to recognise by the brick bond. Measurement of the wall thickness together with careful assessment, perhaps using a boroscope (an instrument that can see into the cavity) is necessary to determine the nature of the cavity and the method of tying the two leaves of brickwork together.
Cavity wall insulation must always be carried out by a certified installer. They are required by law to carry out a thorough survey of the cavity with a boroscope (in accordance with British Standard 8208) prior to the work being carried out. The purpose of this survey is to check that the cavity meets the stringent requirements of these standards and means that in practice few pre-1930s houses are suitable for insulation. It would be wise to monitor how cavity filling affects the house following the work because little is known about long-term performance.
If the wall is not suitable for cavity wall insulation, it will most probably exhibit the same sort of performance characteristics as traditional solid walls. Many early cavity walled houses built in exposed locations might benefit from external wall insulation protecting the wall from the weather, but the problems of a changed appearance, additional works to other parts of the building and the overall cost may still outweigh the benefits.
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