Insulating a pitched roof at rafter level can transform a loft into a comfortable, usable space. While it might not be as cost-effective in the short-term as ceiling-level insulation, it offers the significant advantage of creating a habitable space and delivers greater long-term energy savings.
While thermal performance is the priority, there are insulation products on the market that deliver acoustic insulation and improved fire safety as well. Here are the three key steps to insulating a pitched roof at rafter level:
1. Installing the low resistivity underlay and vapour control layer
Effective moisture and vapour control is critical for rafter-level insulation. Before adding the insulation, you will need to install a low resistivity underlay, known as a breather membrane, above the rafters and below the tiling battens. This allows water vapour to escape from inside the structure, while preventing rain driven under the tiles from entering the building.
After the first layer of insulation is installed between the rafters, you will need to add a Vapour Control Layer (VCL) to prevent moisture transfer through the building. The VCL also reduces the risk of condensation forming by preventing warm, moisture-laden air from contacting cold surfaces within the construction and condensing. For the VCL to perform correctly, make sure the layer is continuous and sealed at all laps.
2. Choosing and installing the insulation
Once the low resistivity membrane is installed, you can fit your choice of insulation between the rafters. It’s essential to choose the right product for the application.
Mineral wool is an ideal choice due to its flexibility, ease of correct installation, and ability to adapt to any imperfections and the seasonal change of the structure. Unlike rigid boards, mineral wool can be friction fitted between the rafters, minimising air gaps and ensuring reliable thermal and acoustic performance.
Buildability is another critical factor to consider. Selecting a product that is easy to cut, fit, and handle reduces installation errors and ensures the insulation performs as intended in real-world conditions. If you’re using mineral wool, make sure you choose a product specifically made for rafter level insulation. Knauf Insulation’s Rafter Roll, for example, is provided at a width of 1200mm to account for varying rafter centre widths. Once it is cut to the correct size, it can be friction fitted between rafters.
3. Layering with insulated battens or PIR laminate
To optimise thermal performance and achieve the desired U-values, additional layers of insulation can be added beneath the rafters. Contractors usually face a choice between insulated battens or PIR laminate boards.
PIR laminate boards offer high thermal performance for applications where space is at a premium, but these will require precise installation and taping to ensure the desired U-value is achieved.
On the other hand, using rock mineral wool insulation between battens offers benefits beyond thermal performance and is particularly suitable where space is a lesser concern. Rocksilk® RS45, for example, is non-combustible, with the best possible Euroclass A1 reaction fire rating and provides acoustic absorption too.
Once this step is complete, plasterboard can be added beneath the second insulation layer.
Case study: A timber frame house with a pitched roof
When architect Keith Edwards, was specifying insulation for his own timber frame house, ease of installation was just as important as performance – because he’d literally be building the house himself. Keith said, “I knew it would be much easier and quicker for me to get a really good fit with mineral wool insulation than it is with rigid boards.”
To achieve a U-value of 0.12 W/m²K for the pitched roof, Keith used 100mm of OmniFit® Slab 35 internally between battens, with three layers of 75mm Rafter Roll 32. To prevent the build-up of interstitial condensation, he installed the insulation with an open-jointed sarking board and a vapour control layer.
The right product for the application
Contractors have many options for insulating a pitched roof at rafter level. The right product for the application will depend on the use of the space, its size and the contractors’ own considerations for installation. It requires expertise and experience to make these decisions and ensure the insulation will deliver the required performance effectively when installed and over the building’s lifetime.
For further information on any of the Knauf Insulation products mentioned in this feature use the appropriate link below:
OmniFit® Slab 35 | Glass Mineral Wool
