Pitched Roof Insulation: Insulating a pitched roof above the rafters: warm roof insulation

Mannok Therm Roof / MR PIR insulation boards

Mannok Therm Roof / MR PIR insulation boards

Stop batten

Counter battens

Vapour open underlay

Roof covering

Insulating a pitched roof above the rafters is the common method chosen for new builds and re-roofing existing detached buildings as it provides superior thermal performance for the building. Insulating the roof above the rafters is known as warm roof insulation as the rafters are brought into the insulated envelope of the building.

This over-rafter insulation method also reduces thermal bridging, giving an airtight continuous layer of high-performance insulation and results in a well-insulated attic space.

The recommended roof insulation for this method of warm roof construction is Mannok Therm Roof / MR insulation, which are lightweight and thermally efficient rigid PIR roof insulation boards. The boards are easy to handle and install and can help achieve excellent U-Values with just a single layer of insulation in the roof construction.

How to install roof insulation above the rafters

When installing Mannok Therm Roof / MR above the rafters on warm pitched roofs consult BS 5534 Code of practice for slating and tiling (including shingles) – Annex B for guidance on calculation methods and fixings.

1. Install a timber stop batten across the rafters as shown in position C, with the batten equalling the thickness of the Mannok Therm Roof / MR insulation.

2. Lay Mannok Therm Roof / MR insulation boards above the rafters, butted tightly against the stop batten and supported on both sides by the rafters. If required, cut the insulation boards to ensure they fit tightly and secure temporarily with appropriate nails.

3. Install the vapour open underlay over the layer of insulation. The underlay should be vapour open (vapour resistance <0.25MNs/g) but water resistant, to protect against wind-driven rain and snow.

4. Install counter battens over the vapour open underlay along the layer insulation, with each counter batten in line with the position of the rafters beneath. The counter battens should be nailed to the stop batten previously installed.

5. Using the calculated fixing requirements specific to your design, install the fixings along the counter batten at maximum 400mm intervals.

6. Fix tile battens and roof covering over the counter battens.

7. On the internal side of the roof, fit a vapour control layer the insulation and the internal finish.

8. Finally, install the internal finish, such as plasterboard.

What thickness of roof insulation do I need?

The roof insulation thickness will vary dependent on the required U-Value. The below table shows the thickness of insulation required for a typical warm roof construction with insulation fitted above the rafters. Our U-value calculator is free and simple to use for calculating U-Values specific to your roof construction design.

Required insulation thickness of Mannok Therm Roof / MR:

Mannok Therm Roof / MR Insulation (mm)
100 110 120 125 130 140 150
U-Value (W / m2K) 0.19 0.17 0.16 0.16 0.15 0.14 0.13

U-value results above based upon pitched roof construction shown:

150mm deep rafters @400mm centres; Mannok Therm Roof / MR; vapour open underlay; 38mm cavity formed by counterbattens; large format concrete tiles. Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.

Need technical support?

For help and advice on any of our products and how they might fit your needs please contact our Technical by Mannok team.

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