When can I use full fill cavity wall insulation?
Cavities were first introduced into walls as a way of managing moisture which enters a construction, preventing it from getting inside the property and causing damp. However, improvements in building materials and construction approaches mean it can now be possible to create a ‘moisture closed’ construction, which allows far lower levels of moisture ingress. This can allow the use of full fill cavity solutions, such as Kooltherm K106 Cavity Board, which require little or no residual cavity meaning a greater thickness of insulation can be used without increasing the overall wall depth.
Before specifying a full fill insulation, however, it is important to understand how likely the property is to be exposed to wind-driven rain. The greater the exposure risk, the more likely moisture is to enter a construction. A map of exposure risk is provided in section 6.1.6 of the NHBC Standards. This separates properties into four risk categories: Very Severe, Severe, Moderate and Sheltered. Depending on the risk for a particular property, you may need to use a partial fill insulation solution, such as Kooltherm K108 Cavity Board, with a minimum 50 mm residual cavity.
What thickness of cavity wall insulation should I install?
This will depend on several factors— most importantly the thermal conductivity of the cavity wall insulation and the other construction materials. The lower the thermal conductivity of the materials, the more effective they are at stopping heat loss. This can reduce the overall thickness of insulation needed to achieve a desired U-value.
Find out more about thermal conductivities.
Kooltherm K106 and K108 have a thermal conductivity of 0.019 W/mK. This is lower (better) than some other commonly used options, such as mineral wool insulation, and can have a significant impact on the overall construction depth.
In the table below, we’ve looked at a few different options for insulating a typical cavity wall construction with a cavity width of between 100 and 150 mm, in line with standard industry practice.
Construction | Insulation and residual cavity | Blockwork type and wall U-value (W/m2K) |
Aerated (0.11) | Lightweight (0.15) | Medium (0.51) | Dense (1.13) |
102.5 mm brick outer leaf
100 mm blockwork
12.5 mm skimmed plasterboard on dabs | 1. 90 mm Kooltherm K106 with 10 mm cavity | 0.16 | 0.17 | 0.18 | 0.18 |
2. 115 mm Kooltherm K106 with 10 mm cavity |
