Insulating a pitched roof between and below the rafters is a very common roof insulation choice, particularly for retrofit and refurbishment projects.
This method of installing roof insulation allows you to maximise the headroom by using a thinner layer of insulation beneath the rafters, whilst achieving U-Values which will ensure you comply with Building Regulations and have a roof space or loft which is comfortable for its occupants.
The rafters themselves will allow more heat loss than the insulation material, so the layer of insulation installed beneath the rafters is essential to combatting this thermal bridging effect.
For this method of pitched roof insulation, we recommend using Mannok Therm Roof / MR Insulation boards between the rafters and Mannok Therm Laminate-Kraft / MLK Insulation boards below the rafters.
Both products are high performance PIR insulation boards which have been specifically designed for this type of roof construction.
Mannok Therm Roof / MR 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.
Mannok Therm Laminate-Kraft / MLK PIR Insulation has a kraft paper facing on one side and plasterboard bonded to the other side, making it ideal for use below the rafters to reduce heat loss through the roof and eliminate thermal bridging through the rafters.
Insulation can be installed between the rafters using a ventilated roof construction or an un-ventilated roof construction. The decision on which method to use is usually dependent on the sarking, or underlay. If the underlay is a low vapour resistance, ‘breathable’ sarking membrane, then a ventilated space is not required. This un-ventilated structure is more likely to be used in new builds or retrofit projects where the roof covering is being removed.
The more traditional method of roof construction is a ventilated roof, where the underlay is a traditional high resistance sarking membrane.
The depth of insulation is dependent on the U-values required and the depth of the rafters. The below U-Value tables show what thickness of insulation to use to achieve a range of U-Values using Mannok Therm Roof / MR and Mannok Therm Laminate-Kraft / MLK PIR insulation boards. To calculate U-Values specific to your roof construction, use our free U-Value calculator.
Mannok Therm Roof / MR Insulation below rafters | Mannok Therm Roof / MR Insulation between rafters (mm) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
75 | 100 | 125 | 150 | 175 | ||||||
Thickness (mm) | U-value (W/m2K) | |||||||||
25 | - | 0.20 | 0.17 | 0.15 | - | |||||
30 | - | 0.19 | 0.17 | 0.15 | 0.24 | |||||
40 | 0.20 | 0.18 | 0.15 | 0.14 | 0.22 | |||||
50 | 0.18 | 0.16 | 0.14 | 0.13 | 0.21 | |||||
60 | 0.17 | 0.15 | 0.13 | 0.12 | 0.20 | |||||
70 | 0.16 | 0.14 | 0.12 | 0.11 | 0.18 | |||||
80 | - | 0.13 | 0.12 | 0.11 | 0.17 |
Mannok Therm Roof / MR Insulation below rafters | Mannok Therm Roof / MR Insulation between rafters (mm) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
75 | 100 | 125 | 150 | 175 | ||||||
Thickness (mm) | U-value (W/m2K) | |||||||||
25 | - | - | 0.19 | 0.16 | 0.15 | |||||
30 | - | - | 0.18 | 0.16 | 0.14 | |||||
40 | - | 0.19 | 0.17 | 0.15 | 0.13 | |||||
50 | 0.20 | 0.17 | 0.15 | 0.14 | 0.12 | |||||
60 | 0.18 | 0.16 | 0.14 | 0.13 | 0.12 | |||||
70 | 0.17 | 0.15 | 0.13 | 0.12 | 0.11 | |||||
80 | - | 0.14 | 0.12 | 0.11 | 0.10 |
U-value results above based upon pitched roof construction shown:
Mannok Therm Roof / MR 12.5 insulated plasterboard beneath rafters; 150mm deep rafters @400mm centres; Mannok Therm Roof / MR between rafters (new build); 50mm unvented cavity, vapour open underlay (refurbishment); 50mm vented cavity; bituminous underlay; large format concrete tiles. Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.
Mannok Therm Laminate-Kraft / MLK Insulation below rafters | Mannok Therm Roof / MR Insulation between rafters (mm) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
75 | 100 | 125 | 150 | 175 | ||||||
Thickness (mm) | U-value (W/m2K) | |||||||||
37.5 | - | 0.20 | 0.17 | 0.15 | - | |||||
42.5 | - | 0.19 | 0.17 | 0.15 | 0.24 | |||||
50.5 | 0.20 | 0.18 | 0.15 | 0.14 | 0.22 | |||||
62.5 | 0.18 | 0.16 | 0.14 | 0.13 | 0.21 | |||||
72.5 | 0.17 | 0.15 | 0.13 | 0.12 | 0.20 | |||||
82.5 | 0.16 | 0.14 | 0.12 | 0.11 | 0.18 | |||||
92.5 | - | 0.13 | 0.12 | 0.11 | 0.17 |
Mannok Therm Laminate-Kraft / MLK Insulation below rafters | Mannok Therm Roof / MR Insulation between rafters (mm) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
75 | 100 | 125 | 150 | 175 | ||||||
Thickness (mm) | U-value (W/m2K) | |||||||||
37.5 | - | - | 0.19 | 0.16 | 0.15 | |||||
42.5 | - | - | 0.18 | 0.16 | 0.14 | |||||
50.5 | - | 0.19 | 0.17 | 0.15 | 0.13 | |||||
62.5 | 0.20 | 0.17 | 0.15 | 0.14 | 0.12 | |||||
72.5 | 0.18 | 0.16 | 0.14 | 0.13 | 0.12 | |||||
82.5 | 0.17 | 0.15 | 0.13 | 0.12 | 0.11 | |||||
92.5 | - | 0.14 | 0.12 | 0.11 | 0.10 |
U-value results above based upon pitched roof construction shown:
Mannok Therm Laminate-Kraft / MLK insulated plasterboard beneath rafters; 150mm deep rafters @400mm centres; Mannok Therm Roof / MR between rafters (new build); 50mm unvented cavity, vapour open underlay (refurbishment); 50mm vented cavity; bituminous underlay; 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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