How to insulate a pitched roof at rafter level
When insulating a pitched roof at rafter level there are a number of key design considerations which will affect the build-up of the roof and the choice of insulation. In this blog post we explore some of the options available.
Firstly, just to make it clear, when we are talking about rafter level we mean the timbers that support the pitched roof. We aren’t talking about joists, which are the flat horizontal beams at ceiling level.
The main decision to make when insulating a pitched roof is if you are going to have a warm roof or a cold roof. This will depend on the construction on your roof, and if it is a refurbishment, whether the roof tiles are going to be removed.
The main difference between the two is the location of the insulation.
- A warm roof is where it is above and between the rafters (so the rafters are kept warm).
- A cold roof is where the insulation is between and below the rafters (and the rafters remain cold).
For a new build, or if a detached dwelling is being re-roofed, then there is scope to choose the build-up of roof that suits your purposes best, however if the roof insulation is being added to an existing roof, for example as part of a loft conversion, then a cold roof is the most likely solution as you can’t insulate above the rafters.
Warm Pitched Roof
To create a warm pitched roof, insulation such as Kingspan Kooltherm K107 Pitched Roof Board should be fitted between and over the rafters. As this requires fitting insulation above the rafters the roof covering will have to be removed in order to fit the insulation. This roof type will also require a breathable membrane, like Kingspan nilvent, to be fitted above the insulation.
Cold Pitched Roof
In this case we would suggest fitting Kooltherm K107 Pitched Roof Board between the rafters and Kooltherm K118 Insulated Plasterboard to the inside face of the rafters. Usually this construction is a ventilated roof which will have sarking felt. Ventilated roofs require a 50mm air gap above the insulation, timber stop battens can be nailed into the side of the rafters to give the correct air gap when installing the insulation. The insulated plasterboard should then be placed with the long edges running across the rafters.
When refurbishing a pitched roof in England or Wales, there is a requirement to reach a U-value of at least 0.18 W/m2.K. For a complete re-roof in Wales the U-Value required is 0.15 W/m2.K. Scotland is similar, but you can be required to have a U-value of as low as just 0.13 W/m².K. Detailed guidance on Building Regulations is available here.
When choosing the insulation for you pitched roof, it is important to ensure that the thickness of insulation used achieves the required U-value for your area. This can be done using our U-value Calculator or by contacting our Technical Services department, who can provide advice about thicknesses and also provide a Condensation Risk Analysis which ensures that, with the chosen insulation and construction build up, there is no risk of condensation occurring within the roof build up.
Further information on how to install the pitched roof insulation products mentioned above can be found in the respective product literature:
- Kingspan Kooltherm K107 Pitched Roof Board
- Kingspan Kooltherm K118 Insulated Plasterboard
- Kingspan nilvent
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