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

Quinn Therm QR PIR Insulation boards between rafters and supported by battens

Quinn Therm QR PIR Insulation boards over rafters

Stop batten

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Counter battens

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Vapour open underlay

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Roof covering

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  • Quinn Therm QR PIR Insulation boards between rafters and supported by battens

  • Quinn Therm QR PIR Insulation boards over rafters

  • Stop batten

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  • Counter battens

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  • Vapour open underlay

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  • Roof covering

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Warm roof construction is where the roof insulation is fitted above the rafters, therefore bringing the rafters into the insulated envelope of the building. It is the most thermally efficient way of fitting roof insulation.

In some warm roof designs, the insulation is fitted both above and between the rafters. This method can be chosen for a number of reasons. It is typically a space saving design as the depth of the continuous layer of roof insulation laid above the rafters can be reduced when adding more insulation between the rafters. This reduced depth of insulation also reduces the load on fixings and makes installation easier.

Whilst this method is not quite as thermally efficient as installing a thicker layer of insulation above the rafters, it is a good compromise to maximise the living space in the room whilst still achieving a comfortable living area.

Quinn Therm QR roof insulation boards are recommended for installing insulation above and between the rafters given the excellent thermal properties combined with the lightweight and easy to cut and install features of these rigid PIR insulation boards.

These roof insulation boards are faced on both sides with a low emissivity composite foil facing, which increases the insulation’s thermal resistance to adjacent unventilated air space, improving the roof’s overall thermal performance.

How to install pitched roof insulation above and between rafters

Insulation above the rafters

1.  Install a timber stop batten across the rafters as shown in position C above, with the batten equalling the thickness of the Quinn Therm insulation.

2. Lay Quinn Therm QR 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.

Insulation between the rafters

1. Measure the space between the rafters where the insulation will be placed and cut the insulation boards marginally wider than the space to ensure a friction fit upon installation.

2. Fit the cut Quinn Therm QR insulation boards tightly between the rafters, with the Quinn Therm branded side facing into the building. The boards should be pushed right into the space so they are tight up against the layer of insulation fitted above the rafters.

3. Install battens along the side of the rafters to support the roof insulation boards.

4. To limit heat loss and prevent problems such as condensation, mould growth and staining occurring at cold spots in the construction, insulation should be continuous at junctions between the roof and other elements, particularly the eaves and gable. Tightly fit the insulation to the ridge plate and carry over and tightly butt the wall plate at eaves.

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

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

What depth of pitched roof insulation should I use?

The thickness of roof insulation is determined by the U-Value requirements. It is common to use the same thickness of insulation between the rafters as you have used above the rafters. The table below is based on a typical warm roof construction with insulation above and between the rafters. To calculate U-Values specific to a different warm roof design, use our free online U-Value calculator.

Required insulation thickness of Quinn Therm QR:

Insulation above rafters (mm)
Insulation between rafters 60 70 80 90 100 110 120 125
Thickness (mm) U-value (W/m2K)
50 0.20 0.18 0.17 0.16 0.15 0.14 0.13 0.13
60 0.19 0.17 0.16 0.15 0.14 0.13 0.13 0.12
70 - 0.16 0.15 0.14 0.13 0.12 0.12 0.12
80 - - 0.14 0.13 0.13 0.12 0.11 0.11
90 - - - 0.13 0.12 0.11 0.11 0.11
100 - - - - 0.12 0.11 0.11 0.10
110 - - - - - 0.11 0.10 0.10
120 - - - - - - 0.10 0.10

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

150mm deep rafters @400mm centres; Quinn Therm QR between rafters; Quinn Therm QR above rafters; vapour open underlay; 50mm 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 Quinn team.

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  • Pitched Roof Insulation
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  • Wall Insulation