Wall Insulation: Insulating masonry cavity walls with partial fill insulation

External brickwork

Mannok Therm Wall / MW PIR insulation boards

Cavity Tray

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Internal block-work

Internal finish

Masonry cavity walls consist of an inner leaf of blockwork and an outer leaf of either brick or blockwork with an air cavity between the two, which should be insulated using rigid PIR cavity wall insulation boards.

The Mannok Therm Wall / MW boards should be fixed to the inner leaf using appropriate wall ties, leaving a narrow, unventilated air cavity between the insulation boards and the outer leaf. Wall ties can have a significant impact on the overall U-value of the wall. It is therefore important to select ties which will minimise heat loss, such as stainless steel double triangle ties. Ties should comply with EN 845–1 and PD 6697.

Partial fill cavity wall insulation is suitable for new builds and extensions and can achieve excellent thermal performance using Mannok Therm Cavity / MC insulation, which is foil faced on both sides to further improve the structure’s thermal resistance, therefore achieving improved U-values.

Using partial fill cavity wall insulation leaves a 50mm gap between the insulation layer and the outer leaf of the wall, which acts as a barrier to prevent external moisture reaching the internal leaf.

Insulating a solid ground floor below the ground bearing concrete slab can be done using either PIR insulation boards or EPS insulation boards. EPS can be a cost-effective alternative to PIR for floor insulation, and the same thermal performance can be achieved with increased thickness.

Using insulation below the slab helps maintain steady internal temperatures in the building by bringing the ground bearing concrete slab into the insulated envelope of the building making use of the concrete thermal mass.

How to install partial fill cavity wall insulation in masonry walls

1. The cavity wall insulation will be held in place by the wall ties which tie the inner and outer masonry skins of the wall together.

As the inner leaf is being constructed, wall ties should be installed and the wall built up in sections.

Fix wall ties to the inner leaf at the appropriate distance, which will depend on the width of the cavity.

Cavities up to 100mm wide require wall ties at maximum 900mm horizontal centres and 450mm vertical centres, with additional ties around openings and at corners. Cavities wider than 100mm may require ties at reduced centres.

When fitting the wall ties, ensure that they slope down from the inner leaf in order to shed water into the cavity.

Wall ties should comply with EN 845–1 and PD 6697.

For ground floors, the first row of supporting wall ties should be fixed at least one course below the damp proof course (DPC).

2. Once each row of wall ties are in position, continue to build up of the inner leaf to secure the ties and continue to build up in sections

3. Ensure the face of the masonry is kept clean as you build, removing any excess mortar or drops to ensure the Mannok Therm Wall / MW insulation can be installed correctly.

4. Wall insulation must extend 150mm below the upper edge of floor insulation to prevent thermal bridging at the floor perimeter. If the cavity includes a radon barrier the first run of boards should be supported on the cavity tray.

5. Mannok Therm Wall / MW insulation boards should be fitted tight against the inner leaf of masonry and held in place by wall-tie clips or collars. Ensure the insulation boards are butted tightly together at all horizontal and vertical joints to give a continuous layer of insulation, and cut neatly to fit tight to the backs of frames, sills, cavity closers and lintels. Each insulation board should be secured at a minimum of three points to ensure adequate retention.

6. When raising the outer leaf, ensure you maintain a 50mm cavity between the wall insulation layer and the outer masonry leaf. Use a timber cavity board to protect the cavity from mortar droppings. The outer leaf should be built up to the top of the insulation.

Masonry cavity wall insulation thickness and U-values

Use the below U-value tables as a guide to required insulation thickness in this common wall construction, or for project specific U-values refer to our online U-value calculator.

Required thickness of Mannok Therm Wall / MW for different blocks

Partial fill Mannok Therm Wall / MW insulation with internal plasterboard on plaster dabs:

Inner leaf Conductivity (W/m2K) Dense Block Others Others Aircrete Thermal Blocks
1.13 0.45 0.3 0.19 0.17 0.12
Thickness (mm) U-value (W/m2K)
50 0.29 0.28 0.27 0.26 0.26 0.24
60 0.25 0.25 0.24 0.23 0.23 0.22
65 0.24 0.23 0.23 0.22 0.22 0.21
70 0.23 0.22 0.22 0.21 0.21 0.20
75 0.22 0.21 0.21 0.20 0.20 0.19
80 0.21 0.20 0.20 0.19 0.19 0.18
100 0.17 0.17 0.17 0.16 0.16 0.16
110 0.16 0.16 0.15 0.15 0.15 0.15

Partial fill Mannok Therm Wall / MW insulation with internal wet plaster:

Inner leaf Conductivity (W/m2K) Dense Block Others Others Aircrete Thermal Blocks
1.13 0.45 0.3 0.19 0.17 0.12
Thickness (mm) U-value (W/m2K)
50 0.30 0.29 0.28 0.27 0.26 0.25
60 0.26 0.25 0.25 0.24 0.24 0.23
65 0.25 0.24 0.23 0.23 0.22 0.22
70 0.23 0.23 0.22 0.22 0.21 0.21
75 0.22 0.22 0.21 0.20 0.20 0.20
80 0.21 0.21 0.20 0.20 0.20 0.19
100 0.18 0.17 0.17 0.17 0.17 0.16
110 0.16 0.16 0.16 0.15 0.15 0.15

U-value results above based upon wall construction shown:

103mm brick outer leaf; 40 – 50mm low emissivity residual cavity; Mannok Therm Wall / MW, 100mm concrete block (conductivity as shown); 12mm plaster. Brick and block leaves with 10mm nominal mortar joints.

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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