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.
Mannok Therm Roof / MR 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.
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.
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; Mannok Therm Roof / MR between rafters; Mannok Therm Roof / MR 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.
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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