Extra Wide Masonry Cavity Walls- Technical Detail

External Masonry Cavity Walls

Masonry Cavity Walls

Masonry Cavity Wall Design

Wider cavities

With the move towards carbon zero buildings, the use of wider cavities in masonry cavity walls is likely to become much more common.

DriTherm® Cavity Slab (glass) can be installed in multiple layers to fully fill cavities up to 400mm wide. Proprietary cavity closers and folded steel lintels are widely available for cavities up to 100mm wide. For cavities over 100mm wide, the choice of proprietary products is more limited. For cavities over 150mm wide, the detailing of openings may need to be altered radically.


Cavities over 150mm wide present problems at the jambs. One solution to this is to use a plywood liner to the opening that can also double up as a former for the window frame. The diagrams below indicate this option for a 200mm wide cavity.

The plywood box solves several problems:

  • It closes off the cavity

  • It can be sealed against the inner and outer leaf to limit air leakage

  • It provides an accurate template for the window frame

  • The temporary polythene front cover provides weather protection until the window frame is installed

  • The window frames can be installed after the brickwork is complete – reducing the risk of damage from mortar, etc.

Typical use of a plywood box as former for window opening

Eave’s detail showing installation of window frame in ply box


The use of separate lintels for the inner and outer leaf becomes more common and practical as the cavity width increases. Separate lintels not only avoid the thermal bridging problems of one-piece steel lintels, but they also provide the designer with a greater level of design freedom. A concrete or aircrete lintel for the inner leaf is a simple, economic and firesafe option.

The lintel in the outer leaf can be anything from reconstituted stone to plain brickwork supported on a steel angle.


Wall ties

 DriTherm®Cavity Slab is supplied in 1200 x 455mm slabs for use between wall ties at 450mm vertical centres. For cavities up to 150mm wide, stainless steel wire ties are recommended by Knauf Insulation because:

  • Stainless steel has a thermal conductivity of 17 W/mK, compared with 50 W/mK for mild steel

  • Wire ties have a much smaller cross-sectional area than flat metal ties

For these two reasons, stainless steel wire ties present a smaller thermal bridge through the cavity insulation than other types of metal wall tie.

For cavities up to 150mm wide, double triangle stainless steel wire ties are available up to 275mm long.

These ties conform to BS EN 845 -1: 2003 as a Type 2 tie. It is recommended that they are embedded 62.5mm into the inner leaf to help keep the cantilevered section of the tie horizontal during the build.

With the use of wider cavities, it is anticipated that other designs of stainless-steel wire ties will be developed for cavities up to 150mm wide.

For cavities over 150mm wide, Knauf Insulation recommend the use of two-part stainless-steel ties. These ties overcome the problem of keeping a long tie horizontal when built into the inner leaf. However, they have a much greater cross-sectional area than wire ties and their thermal bridging effect must be considered when calculating the wall U-value. They are suitable for cavities up to 400mm wide.

Typical detail using separate lintels

Typical detail using separate lintels and rebated window frame

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