Cavity wall insulation is one of the most common methods of insulating walls. It consists of a layer of insulating material placed between two leaves (sides) of the external wall, and is primarily used to reduce heat loss from the building structure. It can significantly reduce energy bills by preventing heat loss, whilst also helping to mitigate the impact of climate change on the environment.

A typical masonry new building regs cavity wall construction typically consists of a central dividing section with a brick skin on each side. A cavity is then formed between these sections and filled with mineral wool or other insulating materials.

As of 15th June 2022, all new builds and extensions must comply with the latest Building Regulations. This is particularly important in Part L, which sets the benchmarks for the thermal performance of buildings. This includes defining minimum U-values for the building envelope.

It’s crucial to understand that achieving the required U-values can be more difficult than expected. This is especially true for those who wish to use a traditional masonry cavity construction.

The latest updates to the Approved Documents set a number of key metrics which must be met in order to achieve compliance. These metrics are set using a notional building specification which is applied to a theoretical construction of the same size as the actual structure. Whilst project teams are free to deviate from these specifications, they must still meet the mandatory metrics.

In order to achieve the notional dwelling values using a traditional mineral wool cavity insulation, a typical construction would require a cavity width of 175 mm and lightweight blockwork. This is a significant increase in thickness and will have knock on impacts for other aspects of the building.

Fortunately, there are other options available to help achieve the required U-values without increasing the overall construction depth. This includes the use of high-performance insulation such as Kooltherm K106, which has a lower thermal conductivity than traditional mineral wool insulants and can often help to reduce construction depths.

The other important factor is the choice of materials and construction details. These must be carefully selected and must be designed to work together. This is particularly important for areas such as weep holes, which are positioned in the external walls to allow water to escape from the building in the event of rain or flooding.

It might seem counterintuitive to focus on these areas when trying to achieve the required U-values, but it’s actually possible to exceed these limits. By selecting the correct insulation and construction materials, it’s much easier than you might think to ensure that your external walls can surpass these minimum standards. You might even be able to do this without extending the cavity width! We’ve put together a simple guide to help you navigate this process. You can download it here. Or get in touch with us if you need more help and advice. We’re here to support you every step of the way!