Wall Insulation: Insulating masonry cavity walls with internal dry lining

Internal block-work

Original plaster finish

Dry well adhesive

Plaster dabs

Mannok Therm Laminate-Kraft / MLK PIR insulation boards

Internal finish

Internal wall insulation is commonly used in existing buildings and in retrofit projects. The thermal performance of insulated cavity walls can be significantly improved by adding a layer of PIR insulation boards on the internal face of the walls. For masonry walls, the fixing method used will be dependent on the internal finish of the existing wall. In order for the wall insulation to be fitted correctly and achieve maximum performance, preparation of the internal surface is essential.

For walls with a very uneven surface, it is unlikely that dot and dab application of the insulation boards will be suitable, so using mechanical fixings will give the best result. This could include the fixing of timber battens on the wall surface to provide an even surface for the fixing of insulation.

For walls that are in good condition with an even surface, adhesive fixing of the wall insulation boards can be used.

For masonry surfaces with cracked or damaged plaster, it is important to ensure the damage is repaired prior to the installation of the internal insulation boards. For brick or block walls, pointing with mortar should be completed in order to prevent any air gaps and minimise thermal bridging issues.

All walls should be assessed to determine if they are suitable for dry lining prior to works commencing.

Mannok Therm Laminate-Kraft / MLK insulation boards are recommended for internal dry lining of masonry cavity walls. These high performance PIR insulation boards are faced on one side with a kraft paper facing which is suitable for adhesive or mechanical fixing methods, and plasterboard bonded on the other side to give a suitable internal wall finish.

How to install insulation in masonry cavity walls with internal dry lining

Installing the cavity wall insulation 

For new builds, the cavity wall insulation should be installed correctly according to the wall construction. Installation guides are available for cavity wall insulation:

Masonry cavity walls with partial fill insulation

Masonry cavity walls with full fill insulation

Insulating timber frame cavity walls

Insulating steel stud frame cavity walls


Installing internal dry lining insulation

1. Prepare the wall surface to ensure it is free from all obstructions. Remove skirting boards, picture rails and window boards, as well as any wall coverings with high vapour resistance, such as vinyl wallpaper or gloss paint.

2. Fit Mannok Therm Laminate-Kraft / MLK insulation boards against the inner leaf of masonry and hold in place using wall-tie clips or collars. Boards must be butted tightly together at corners to give a continuous layer of insulation and avoid thermal bridging issues. The insulation boards should be cut neatly to fit tight to the backs of frames, sills, cavity closers and lintels.

3. To minimise air infiltration, a continuous strip of dry-lining adhesive should be used to seal the perimeter of the Mannok Therm Laminate-Kraft / MLK boards, including around window and door openings.

4. For heavy fixtures such as wall cupboards or radiators, install timber fixing battens within the layer of insulation to carry them. Otherwise, the fixtures should be fixed directly to the masonry structure using appropriate mechanical fixings.

5. Fix Mannok Therm Laminate-Kraft / MLK insulated plasterboard to the wall with adhesive dabs and secure with 2 secondary mechanical fixings at mid height and at a minimum of 15mm from the board edge. The screws which fix Mannok Therm Laminate-Kraft / MLK to the battens should be selected to minimise heat loss: U-value calculations for the wall should account for the additional heat loss resulting from mechanical fasteners.

What thickness of insulation is required?

The thickness of wall insulation required will be dependent on the U-values you wish to achieve. Installing internal insulation impacts on the floor space of the internal rooms, so careful detailing can help minimise thickness of insulation to minimise the impact on floor space whilst achieving excellent thermal performance. See the below table for U-value results based on the wall construction shown. For project specific U-values use our free U-value calculator.

Required thickness of Mannok Therm Laminate-Kraft / MLK:

Cavity wall with internal drylining

Mannok Therm Laminate-Kraft / MLK Partial fill cavity insulation (mm)
50 60 75 80 100
Thickness (mm) U-value (W/m2K)
37.5 0.22 0.20 0.17 0.17 0.15
42.5 0.21 0.19 0.17 0.16 0.14
50.5 0.19 0.18 0.16 0.15 0.13
62.5 0.17 0.16 0.14 0.14 0.12
72.5 0.16 0.15 0.14 0.13 0.12
82.5 0.15 0.14 0.13 0.12 0.11
92.5 0.14 0.13 0.12 0.12 0.11

103mm brick outer leaf; 40 – 50mm low emissivity residual cavity; Mannok Therm Wall / MW; 100mm concrete block (conductivity as shown); 12mm plaster with dry wall adhesive; Mannok Therm Laminate-Kraft / MLK insulated plasterboard on plaster dabs. 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.

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