How to insulate a pitched roof at rafter level

12 April 2017 Kingspan Insulation UK
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. Joists are the flat horizontal beams at ceiling level, and will be the subject of another blog post in the future.

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 require a breathable membrane, like Kingspan nilvent, to be fitted above the insulation.

The construction build up for this can be seen on our online U-value calculator.

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 50 mm 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.

The construction build up for this can be seen on our online U-value calculator.

Building Regulations

There is a requirement when refurbishing a roof to have a minimum U-value of 0.18 W/m²·K in England and Wales, but for a complete re-roof in Wales the minimum U-Value required is 0.15 W/m²·K. When specifying the insulation for the pitched roof it is important that the thickness of insulation used achieves the required U-value. This can be done using our U-Value calculator.


Related products

Kingspan Insulation Ltd

For queries relating to insulation boards, cavity closers or membranes.





General enquiries: 01544 388 601

Architect Technical Helpline:
01544 387 382