Masonry cavity wall

Cavity wall is still probably the most common form of construction in the UK, so should not concern to most builders, but the approach needs to be adopted to achieve the more rigorous Passivhaus requirements. For masonry cavity wall to meet Passivhaus standards we need to be able to cope with the natural rough surfaces of masonry walling without leaving air passageways for thermal bypass. One way to do this is to use soft fibrous insulation, such as mineral wool, where mineral wool cavity batts are woven in vertical layers and are designed to be able to get wet in the first 10 – 15mm from any wind driven rain penetration through the naturally porous brick or stone. The use of well-installed cavity trays over window and door openings, and vigilance in keeping insulation batts clean from mortar droppings ensures thermal performance is as designed.

The repeating thermal bridging of stainless steel wall ties within the super insulation would diminish the overall U-value of the wall, and create a danger of cold spots on the internal plasterwork, potentially leading to condensation. Thermal bridging from wall ties should therefore be minimised by the use of specialist wall ties made of lower conductivity materials such as basalt and resin or nylon.

Airtightness can be adequately dealt with by traditional two-coat plasters. Where the plaster meets another material, such as timber, OSB, plywood, etc, cracking will occur through differential movement, seriously affecting the airtightness strategy. Specialist airtightness tapes are needed to address and cope with this issue.

Components such as intermediate timber floors, staircases, SVPs, etc that are installed before plastering require a preliminary simple weak sand, cement/lime parge coat applied to the blockwork. The parge coat should give adequate airtightness and blend with the plastering later.

Advantages and disadvantages of masonry cavity wall

Advantages Materials are readily available, and the construction method is familiar to local labour, though not necessarily to the stringent Passivhaus standards. Being a heavyweight construction, traditional blockwork gives excellent thermal storage and soundproofing properties. There is no danger of interstitial condensation and holes in the plaster airtightness layer are always repaired as part of redecoration work. Masonry cavity wall can be the cheapest construction method and is still the most common house building construction method in the UK so its use will avoid taking builders outside of their experience and ‘comfort zone’, and hence avoid over-pricing due to perceptions of risk.

Disadvantages Masonry cavity wall is a slower construction method than timber frame and it is more difficult to check the quality of the continuity of insulation, (eg, the cleanliness from mortar dropping, etc.). Airtightness testing occurs later in the building programme which can make it more difficult to rectify hidden faults. The absence of a service void means that more care is needed in services design and execution. Although electric cabling can be buried in the plaster effectively, badly installed back boxes can create weak points. A longer structural drying out period will affect the first season’s heating demand.

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