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Pitched Roof Insulation: Insulating a pitched roof between and below rafters: cold roof insulation

Quinn Therm QL - Kraft PIR Insulation boards

Quinn Therm QR PIR Insulation boards

Quinn Therm QR PIR Insulation boards

Underlay

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Roof covering

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  • Quinn Therm QL - Kraft PIR Insulation boards

  • Quinn Therm QR PIR Insulation boards

  • Quinn Therm QR PIR Insulation boards

  • Underlay

    Read more

  • Roof covering

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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 Quinn Therm QR Roof Insulation boards between the rafters and Quinn Therm QL Kraft Insulation boards below the rafters.

Both products are high performance PIR insulation boards which have been specifically designed for this type of roof construction.

Quinn Therm QF 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.

Quinn Therm QL Kraft 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.

How to install pitched roof insulation beneath and between 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.

Insulation between the rafters

1. Install underlay above the rafters. For an un-ventilated roof construction, a vapour open underlay or breather membrane should be specified. For a ventilated roof structure, a traditional high resistance sarking can be used.

2. For a ventilated roof, fix battens on the inside of the rafters 50mm from the underlay to leave vented air spaces.

3. 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.

4. 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 battens.

5. 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. At eaves, wall insulation should be continued between the rafters until it butts the underside of the Quinn Therm QR boards. Where there is a cavity.

Insulation below the rafters

1. Using suitable mechanical fixings, fix Quinn Therm QL-Kraft boards to the underside of the rafters with the long edge running across the rafters.

2. Install timber noggins where the joints in the insulation are unsupported.

3. To form an AVCL, seal the joists between the boards. Set the boards onto a wide (30mm) bead of vapour resistant sealant applied to the under surface of all the joints and cross noggins.

What thickness of roof insulation is required?

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 Quinn Therm QR and Quinn Therm QL-Kraft PIR insulation boards. To calculate U-Values specific to your roof construction, use our free U-Value calculator.

New build: Quinn Therm QR and Quinn Therm QR below, unvented cavities

Quinn Therm QR Insulation below rafters Quinn Therm QR 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

Refurbishment: Quinn Therm QR and Quinn Therm QR below, vented cavities

Quinn Therm QR Insulation below rafters Quinn Therm QR 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:

Quinn Therm QR 12.5 insulated plasterboard beneath rafters; 150mm deep rafters @400mm centres; Quinn Therm QR 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.

New build: Quinn Therm QR and Quinn Therm QL - Kraft, unvented cavities

Quinn Therm QL-Kraft Insulation below rafters Quinn Therm QR 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

Refurbishment: Quinn Therm QR and Quinn Therm QL-Kraft, vented cavities

Quinn Therm QL-Kraft Insulation below rafters Quinn Therm QR 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:

Quinn Therm QL-Kraft insulated plasterboard beneath rafters; 150mm deep rafters @400mm centres; Quinn Therm QR 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.

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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