Wall Insulation: Insulating steel stud frame cavity walls

Steel runners with all ties

Mannok Therm Wall / MW PIR insulation boards

Steel studs

Polyethylene AVCL

Internal finish

When insulating steel stud frame cavity walls, the wall insulation boards can be installed either on the inner or outer face of the metal studs with optional flexible insulation, such as wool insulation, between the studs, dependent on the U-value required and often to improve acoustics.

Installing Mannok Therm Wall / MW insulation boards will provide excellent thermal performance and appropriate fire resistance within the wall cavity. These high performance PIR insulation boards are designed for cavity wall insulation applications and can help achieve excellent U-values with both sides faced with low emissivity composite foil facing.

It is important to ensure a 40-50mm cavity is maintained between the insulation and the outer leaf to minimise the risk of condensation & protection against wind driven rain.

How to install wall insulation in a steel stud frame cavity wall – insulation installed outside steel stud frame

1. Install the steel stud framework according to the manufacturer’s instructions

2. Cut the Mannok Therm Wall / MW insulation boards accurately to ensure they will fit tightly around openings such as window and door openings.

3. Fit the insulation boards to the external face of the steel studs, butted tightly together to ensure a continuous layer of insulation, ensuring vertical board joints coincide with a vertical steelwork member.

4. Fill any gaps between the insulation and the metal framing with expanding polyurethane foam to avoid thermal bridging issues.

5. Fix the wall insulation boards using steel channels which are screwed back to the studs. The screws must be specified carefully to ensure they can withstand the transmitted loading from the cladding, while also minimising heat loss where they penetrate the insulation. U-value calculations must take account of that additional heat loss.

6. Install an AVCL (air and vapour control layer) across the inner face of the steel studs, immediately behind the internal surface finish of the wall. This AVCL is necessary in light steel framed walls to prevent damaging interstitial condensation forming within the wall. Condensation risk analysis should be carried out to ensure the specified AVCL has a sufficiently high vapour resistance for the project. The AVCL will also help reduce air leakage into the stud cavity.

7. Depending on the depth and spacing of the studs and the U-value required of the wall, an additional layer of mineral fibre insulation can be fitted between the steel studs.

Insulated steel stud frame cavity walls: Insulation thickness and typical U-values

The below U-value tables show typical U-values achieved when insulating a steel stud frame cavity wall with Mannok Therm Wall / MW insulation across the metal studs. For tailored U-value use our free U-value calculator

Required thickness of Mannok Therm Wall / MW

No insulation between studs. Mannok Therm Wall / MW across studs.

Insulation thickness (mm)
40 50 60 70 75 80 90 100
U-value (W/M2K) 0.27 0.24 0.22 0.21 0.20 0.19 0.17

100mm wool insulation between studs (0.044). Mannok Therm Wall / MW across studs.

Insulation thickness (mm)
40 50 60 70 75 80 90 100
U-value (W/M2K) 0.24 0.21 0.20 0.19 0.18 0.17 0.16 0.15

150mm wool insulation between studs (0.044). Mannok Therm Wall / MW across studs.

Insulation thickness (mm)
40 50 60 70 75 80 90 100
U-value (W/M2K) 0.20 0.19 0.17 0.16 0.16 0.15 0.14 0.14

U-values based on studs at 400mm centres (3% bridging)

U-value results above based upon wall construction shown:

103mm brick outer leaf; 40 – 50mm low emissivity residual cavity; Mannok Therm Wall / MW; steel studs @ 600mm centres; polyethylene AVCL; 12mm plasterboard. Calculations performed to BS EN ISO 6946 and BRE Digest 465, taking account of repeating thermal bridge.

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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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