Wall Insulation: Insulating masonry cavity walls with full fill insulation

External brickwork

Mannok IsoShield Insulation

Cavity Tray

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Internal block-work

Internal finish

Full fill cavity wall insulation allows more of the cavity between the inner and outer leaf to be filled with insulation, providing a much better insulated structure without increasing the wall width. Partial fill wall insulation typically requires a 50mm cavity to be maintained, however with advanced manufacturing techniques, full fill PIR insulation boards, such as Mannok IsoShield insulation, means this cavity can be reduced to only 3mm.

This provides a more thermally efficient structure, reducing heat loss and allowing specifiers to achieve even lower U-values.

Mannok IsoShield PIR insulation boards provide a superior cavity wall insulation solution which combines ease of construction with all the benefits of Full Fill PIR. The boards have a thermal conductivity of 0.022W/mK, precision cut rebate edges and premium stucco embossed facings, combining thermal performance with ease of construction.

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 Full Fill Cavity Wall Insulation

1. For cavities up to 100mm, install wall ties in the inner leaf at maximum 900mm horizontal centres and 450mm vertical centres, with additional ties around openings and at corners.

2. For cavities greater than 100mm, wall ties should be installed at reduced centres.

3. Wall ties must slope down from the inner leaf in order to shed water into the cavity.

4. Continue building the inner leaf as you install wall ties, with ties secured by the next row of blockwork.

5. Ensure the blockwork structure is kept clean as you build. The faces of masonry must be kept clean and clear of mortar snots and droppings to enable the

Mannok IsoShield insulation boards to be installed properly. A cavity board should also be used to protect the cavity from droppings.

6. Once the wall ties are secured, the full fill insulation boards can be installed. Wall insulation boards must extend 150mm below the upper edge of floor insulation to prevent thermal bridging at the floor perimeter.

7. Mannok IsoShield insulation boards should be fitted tightly against the leading leaf of masonry supported on the first row of wall ties. The higher side of the rebated edge should always be against the inner leaf.

8. Ensure each insulation board is fitted tight against the inner leaf and joints are tightly butted. Each board should be secured by wall ties at a minimum of three points.

9. At corner junctions, the rebated edge should be carefully removed with a handsaw or knife to allow a straight butt joint with no gaps. The corner joint should be staggered and the exposed edge of the board should be protected by a vertical DPC of an appropriate width.

10. Build the outer masonry leaf to the top of the insulation boards, leaving a cavity of 3mm from the insulation boards.

Important considerations when using full fill PIR wall insulation boards

  • Installation is carried out to the highest level on each wall or the top edge of the insulation is protected by a cavity tray
  • Cavity trays are used with appropriate stop ends and weep holes at lintel level
  • Cavity battens and/or boards are used during construction to prevent bridging by mortar droppings.
  • Wall ties are installed correctly and are thoroughly clean
  • Excess mortar is cleaned from the cavity face of the leading leaf and any debris removed from the cavity
  • Mortar droppings are cleaned from the exposed edges of installed boards
  • DPCs at ground level do not project into the cavity as they can form a trap for mortar bridging
  • Mannok IsoShield insulation boards are properly installed and butt-jointed
  • Raked or recessed mortar joints are avoided in very severe exposure areas.

Masonry cavity wall insulation thickness and U-values

Mannok IsoShield insulation with plasterboard on 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)
72 0.26 0.25 0.24 0.24 0.23 0.22
97 0.20 0.20 0.19 0.19 0.18 0.18
122 0.16 0.16 0.16 0.15 0.15 0.15
147 0.14 0.14 0.13 0.13 0.13 0.13

Mannok IsoShield insulation with 12mm plasterfinish

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)
72 0.27 0.26 0.25 0.24 0.24 0.23
97 0.27 0.26 0.25 0.24 0.24 0.23
122 0.17 0.16 0.16 0.16 0.16 0.15
147 0.14 0.14 0.14 0.13 0.13 0.13

U-value results above based upon wall construction shown:

103mm brick outer leaf; Mannok IsoShield insulation, 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.

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