Pitched Roof Insulation: Insulating a loft or attic wall in pitched roof space

Mannok Therm Wall / MW PIR insulation boards

Mannok Therm Wall / MW PIR insulation boards

Timber studs

AVCL

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Plasterboard

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Insulating the dwarf wall in an attic or loft space is essential to any loft conversion or new build attic room to ensure a fully insulated envelope. It is important to ensure the appropriate ventilation and airtightness is considered in the design to avoid condensation.

The recommended PIR insulation product to insulate the dormer wall is Mannok Therm Wall / MW insulation, which is a foil faced rigid insulation board with excellent thermal properties to suit the pitched roof space. The low emissivity foil facings increase the thermal resistance, reducing the overall U-value of the wall. In addition, it is easy to form an effective vapour control layer using the foil facings by simply taping the joints with an aluminium foil tape.

To avoid thermal bridging, exposed gable walls should also be insulated with Mannok Therm Wall / MW insulation boards fixed by battens or with Mannok Therm Laminate-Kraft / MLK  insulation boards. Find out more information on how to insulate the exposed gable wall on pages 20-23 of our product and installation guide.

How to install dwarf wall insulation in a pitched roof space

1. Accurately measure the space between timber studs and cut the Mannok Therm Wall / MW insulation boards marginally larger.

2. Install the insulation boards tightly between the timber studs to give a friction fit. Ensure the insulation in the stud walls is continuous with that in the ceiling and meets the insulation laid across the joists in the remaining roof cavity.

3. Partially drive stainless steel nails into the studs at 400mm centres to support the insulation boards

4. Where the required U-value cannot be obtained by insulating between the studs, an additional continuous layer of Mannok Therm Wall / MW insulation should be installed across the face of the studs.

5. Fit a vapour control layer on the warm side of the insulation to avoid condensation. The vapour control layer may consist of minimum 1,000 gauge (250µm) polythene sheet, with all joints lapped and sealed, and should be sealed to the vapour control layer in the ceiling.

6. Alternatively, a vapour control layer can be formed by taping of the joints on the Mannok Therm Wall / MW insulation boards placed across the studs with an aluminium foil tape.

Calculating insulation thickness and U-Values

In order to determine the correct thickness of insulation to use, you should know the U-Value you want to achieve. The U-Value table below shows the correct thickness of Mannok Therm Wall / MW insulation boards to use in a typical dormer wall construction, or you can use our free U-Value calculator to calculate U-Values and insulation thickness based on your specifications.

Required insulation thickness of Mannok Therm Wall / MW:

Mannok Therm Wall / MW between & across joists U-value (W / m2K)
100 + 0 0.26
100 + 25 0.20
100 + 35 0.18
100 + 40 0.17
100 + 50 0.16
100 + 60 0.15
100 + 70 0.14
100 + 80 0.13

U-value results above based upon wall construction shown:

Plasterboard; timber battens; AVCL; Mannok Therm Wall / MW over timber studs; 50mm x 100mm timber studs @ 400mm centres; Mannok Therm Wall / MW between studs; timber battens fixed to studs to secure boards. 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.

  • Floor Insulation
  • Pitched Roof Insulation
  • Flat Roof Insulation
  • Wall Insulation