Choosing materials for cavity wall construction

Learn the basics of cavity wall insulation and how you should adapt your choice to meet the requirements of your home's location.

By Hugh Metcalf | 28 August 2020

Take a fabric first approach and ensure your self build’s cavity wall system is the right choice for your project.

SureCav cavity wall insulation - grand designs

Image: Surecav

Getting the fundamentals right is an essential consideration for any self builder and well-built cavity walls will guarantee a robust envelope for the home, while also preventing overheating, drafts and moisture penetration.

A number of factors contribute to a high-spec cavty wall system, aside from the ability of the builder you employ. Imminent changes to ‘Part L’ of Building Regulations has encouraged a ‘fabric first’ approach, highlighting the importance of using products which promote energy efficiency. Further, other material considerations such as safety, standardisation and climate are also important.

Charlie Ayers, managing director of SureCav, shares his top tips for anyone specifying materials for a cavity wall system.

Understanding the language

‘U-values’, ‘thermal performance’, ‘Part L’, ‘fabric first’, ‘thermal bridging’… the list goes on. These are all terms familiar to the built environment professional but can be confusing to the layman. However, in order to achieve a cavity wall which keeps you warm in the winter and cool in the summer, it’s important to understand them as it will influence material choices.

Good ‘thermal performance’ is one of the most critical requirements in building construction. Essentially, it relates to the efficiency with which a material retains, or prevents, the passage of heat. This is measured in ‘U-values’, which calculates the rates at which heat passes through a structure, particularly walls, floors and roofing. Ideally, you are looking to attain as low a value as possible, indicating low transmittance and optimal heat retention and regulation.

This dovetails into ‘Part L’, referring to a new regulation to improve a building’s energy efficiency, with the goal to create a carbon neutral society through a better choice of construction materials. It’s a ‘fabric first’ approach, maximising the thermal performance of building products, before mechanical or electrical heating, cooling and ventilation solutions are sought.

 Grand designs tv house - haringey

Image: While parts of this project included the renovation of this dilapidated coach house, the original cavity walls lacked ties, meaning the exterior brickwork had to be taken down and rebuilt. Photo: Fraser Marr 

Finally, ‘thermal bridging’, the greatest barrier to achieving optimal thermal performance. It describes an occurrence where one or more elements in a building’s envelope make a direct connection between the interior and exterior. Where these materials possess higher thermal conductivity than the rest of the fabric it creates a thermal bridge, allowing heat to rapidly escape. To put this in context, a small bridge (e.g. steel tie poking through the insulation), can lead to around 30% heat loss. This means more waste and higher energy bills. As this most commonly occurs through poor construction and cheap components, the choice of quality services and materials is essential.

How to choose a cavity wall product

There are thousands of building products available on the market so it can be difficult to know what to choose.

Look for official accreditation. Much as you would with the selection of a construction professional, certification from recognised bodies such as BBA, NHBC or BRE will provide assurance that what you purchase has undergone rigorous testing, backing up the manufacturer’s claims.

For further verification, check to see if the product is listed on NBS Source, the official platform for building product manufacturers to share information and up-to-date specification documents. Although the concept of cavity wall construction may seem simple, what you specify for it will have lasting implications for how the home performs, not just thermally, but structurally too.