Cavity Wall Insulation U-Value Explained: How Does It Work?

U-value, sometimes also called thermal transmittance, tells us about how well a material is in resisting heat transfer. The value itself shows us the rate at which heat transfer takes place, so a lower u-value means a better-insulated structure.

With the dwindling efficiency of the UK housing stock becoming a focus point on the path to Net Zero emissions by 2050, homes nationwide are getting insulated to improve their thermal efficiency. U-value is an important indicator of how effectively a home is insulated.

This ultimate guide by GreenMatch UK will break down everything there is to know about u-value in regards to wall cavity insulation, including what u-value should be achieved, any regulations governing this, and how to improve u-value.

Ready to get cavity wall insulation? Look no further than GreenMatch UK for a one-stop solution to all your needs. Instead of spending countless hours researching and vetting installers, simply spend 30 seconds filling out our online form and receive up to 3 free home-tailored quotes delivered right to you. Click the button below to begin! 

If you’re wondering what is u-value insulation, we’ll break it down for you. The u-value for a cavity wall, or any material, tells you how well it is at resisting heat transfer. In other words, the u-value measures how well of an insulator a material is. 

An average cavity wall has an u-value of about 1.6 W/m2k before being insulated and can be improved up to 0.6 - 0.18 W/m2K after proper cavity wall insulation. 

If you notice, an improved u-value is lower in number. This is because the u-value shows us the rate of heat transfer per metre and hour. A lower u-value indicates less heat transfer, which indicates a better insulating capacity.

Simply put, a cavity wall insulation u-value is calculated by the formula U = 1/(Sum of all R-value). In this case, the R-value is a sort of opposite of the U-value. So R = 1/(Sum of all U-value). They are inverses of each other. To eliminate the confusion, we've created the r-value insulation chart in the UK so you can compare and spot the differences.

While the R-value also tells us about how good a material is at resisting heat transfer, it’s considered to be the flipped version, which means a higher R-value means the material is better at resisting heat transfer. 

Makes sense? Probably not, but don’t worry if it doesn’t! Understanding how these values are calculated is nowhere near as important as understanding what they represent.  

Now that we know a lower u-value means a material is better at insulating, cavity wall insulation aims to lower the u-value of your cavity walls. 

In this case, common building regulations throughout the UK recommend aiming for a u-value of 0.6 - 0.18 W/m2K after installing cavity wall insulation. This is compared to an average u-value of around 1.6 W/m2k for uninsulated cavity walls.

This is a drastic drop from the u-value of an uninsulated wall, showing just how much better these walls become at resisting heat transfer with proper insulation measures.

The UK Building Regulations 2010 document outlines u-value expectations for the conservation of fuel and power in domestic housing and commercial properties. 

Most of these regulations tend to focus on new builds instead of existing structures, but may still be relevant if you plan on constructing a new home. 

According to the updated Part L section of the document, the expected u-value for external walls and cavity walls are:

Builders, contractors, and insulation experts need to be aware of these regulations to make sure they are complied with. For that reason, it’s very important to work with a professional installer. That way, you can make sure to receive a quality job that is in line with government expectations. 

However, finding the perfect installer can be a daunting job that consumes many hours of research and vetting. That’s where GreenMatch UK can help you out. 

Instead of spending endless hours on websites and phone calls, just spend 30 seconds filling out our simple online form, and receive up to 3 free home-tailored quotes from our nationwide network of accredited installers. No extra charges and non-binding. Click the button below to begin!