Wall Insulation: Insulating timber frame cavity walls

Vapour open membrane

Timber sheeting

Mannok Therm Wall / MW PIR insulation boards

Mannok Therm Wall/MW or Mannok Therm Laminate-Kraft/MLK PIR insulation boards over studs

Polyethylene AVCL

Internal finish

Insulating cavity walls in a timber frame construction should be completed using wall insulation which retains an air gap between the insulation layer and the outer leaf of the wall. This prevents the risk of condensation forming in the cavity which can happen when alternative wall insulation methods are used. Condensation in the cavity space can lead to the timber rotting and so should be avoided.

High performance PIR insulation boards, such as Mannok Therm Wall / MW insulation boards, are the ideal solution to ensure the cavity remains free from condensation and the wall construction achieves the required thermal performance and excellent U-values.

The boards are faced on both sides with a low emissivity composite foil facing which increases the thermal resistance of an unventilated cavity adjacent to the insulation, reducing the overall U-value of the wall.

Mannok Therm Laminate-Kraft / MLK PIR insulation boards, with a kraft paper facing on one side and plasterboard bonded to the other side, can alternatively be used over the studs to provide a plasterboard finish in one product.

How to install wall insulation in timber frame cavity walls

1. Measure the spacings between the timber studs and cut the Mannok Therm Wall / MW insulation boards if required to ensure a tight fit between the studs

2. Install the wall insulation boards between the studs and to door and window framing, ensuring they are lightly butted to give continuity of insulation

3. Fill any gaps between the insulation and studs with expanding polyurethane foam

4. Install softwood timber battens flush with the timber studs to act as a stop and prevent the insulation boards from moving within the stud cavity.

5. Depending on the depth and spacing of the studs and the U-value required of the wall, an additional layer of Mannok Therm Wall / MW or Mannok Therm Laminate-Kraft / MLK insulation boards may be required, fixed across the inner face of the studs.

6. Timber framed walls require a vapour control layer to prevent damaging interstitial condensation forming on the back of the sheathing. Where a continuous layer of Mannok Therm Wall / MW insulation is fitted across the inner face of the studs, this vapour control layer may be formed by taping the joints between boards with metalised tape.

Where Mannok Therm Wall / MW insulation boards are fitted only between the studs a vapour resistant membrane, such as 500 gauge polyethylene, should be installed across the outer face of the studs.

Condensation risk analysis should be carried out to ensure the specified AVCL has a sufficiently high vapour resistance for the project.

7. A service cavity can be formed by fitting plasterboard on battens, rather than nailing directly to the studs. Electrical and other services can then be run in the cavity without compromising the vapour control layer.

8. The outer masonry leaf should be constructed in the conventional manner and secured using appropriate wall ties.

What thickness of wall isulation is required?

The thickness of insulation to use in the timber frame cavity wall will be dependent on the desired wall U-value. Use the below U-value table as a guide, or use our free online U-value calculator for project specific calculations.

Therm Wall between studs and Mannok Therm Wall / MW or Mannok Therm Laminate-Kraft / MLK across studs:

Between + Across Studs
89mm stud U-Value (W/m2K) 140mm stud U-Value (W/m2K)
50 + 25 0.26 0.25
60 + 25 0.24 0.24
75 + 25 0.22
90 + 25 0.20
100 + 25 0.19
110 + 25 0.18
110 + 40 0.16
110 + 50 0.15
110 + 65 0.13

U-value results above based upon wall construction shown:

103mm brick outer leaf; 40 – 50mm cavity; vapour open membrane*; 12mm plywood sheathing; 89 or 140mm timber studs @ 600mm centres (timber bridging fraction 15%); Mannok Therm Wall / MW between studs; Mannok Therm Wall / MW or Mannok Therm Laminate-Kraft / MLK over studs; polyethylene AVCL; 12mm plasterboard. Calculations performed to BS EN ISO 6946, taking account of repeating thermal bridges.
* Note: improved U-values may be achieved using reflective vapour open membranes.

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.

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