Since draft Document L of the Building Regulations appeared, there have been comments about the looming extinction of cavity walls. Blocks used for the inner leaf would allegedly have to be made thicker, which homebuilders would regard as excessive or less dense, which would reduce their loadbearing capacity. If so, one scenario being spoken of is a widespread switch to timber frame.
Is there any validity in this discussion? Not a lot, I think.
As we approach the proposed U values of 0.35 in 2002 and 0.3 in 2004, U value has less to do with inner leaf thickness. In fact, I rate the chances that a leaf of lightweight blocks with dense mortar joints could achieve a U value of 0.35 as zero. If this proposal stands, homebuilders will simply not be allowed to build uninsulated cavity walls.
U value will instead have a great deal to do with what insulation material the wall contains. It will normally be possible to meet the 0.35 target, with no trade-offs, using such combinations as 100 mm mineral fibre with 100 mm dense block or 60 mm polyurethane foam and 100 mm aerated block.
The exact U value depends on how well we regulate the thermal bridging, due to the window reveals, the steel wall ties, the thermal bridges at junctions with floor and roof, and whether the insulation layer is installed properly. These details contribute enormously to heat loss.
As we head towards 0.3, I presume that the UK will finally have to overcome its resistance to full-fill cavity walls. Several thousand UK buildings have used fully-filled cavities of 125-250 mm, which is far in excess of near-term requirements.
Most of these ultra-wide cavities used GRP ties, imported from Denmark, or stainless steel twist-type ties. Nearly all were insulated during construction with mineral fibre batts; one or two used blown-in mineral fibre, which allows homebuilders to double-check cavities for cleanliness before they are filled.
Many UK housing associations reacted to Housing Corporation pressure for higher standards by moving from 50-75 mm cavities to fully-filled 100 mm cavities, with a dense block inner leaf and wire-type stainless steel ties. With care, these meet the immediate U value proposal.
A 1999 Government-sponsored report is believed to study costs of highly-insulated masonry walls in detail. Some findings were revealed at a series of BRECSU workshops in 1999 - that walls with fully-filled cavities were likely to be a more cost-effective and buildable route to the low U values of the future than walls with partly-filled or unfilled cavities.
Pending a thorough debate, which would benefit from this background technical material being published, homebuilders would be wise to familiarise themselves with the reality of masonry building technology.
Lesson one: The UK
In 1977, DETR’s forerunner DoE sponsored the construction and monitoring of two energy-efficient housing estates in Milton Keynes. The estate of 170 terraced and semi-detached houses at Pennyland included 83 homes with fully-filled 100 mm cavities. Eight larger detached houses at Great Linford had similar walls, with 100 mm fully-filled cavities and dense block inner leafs. All houses had plastered walls, masonry partitions, timber-framed double glazing and pitched timber roofs. Final reports on the projects from the Open University/ETSU in 1984 showed the measured energy consumption of 150 kWh/m2yr, gas and electricity combined, was pretty good for a ‘field trial’. Amazingly it still is good.
Build costs of the Linford and Pennyland homes were standard, except the ‘experimental’ energy-efficient features carried a premium of about 1.5%.
Lesson two: Elsewhere in northern Europe
Homes with 125 mm cavity walls upwards have been built elsewhere in Europe since 1975. Nearly all these homes are in Denmark, the Netherlands, and the coast of north Germany - rainy, windy climates where cavity walls took over from solid walls for the same reasons as in the UK - to keep out wind-driven rain. Denmark no longer even debates the alleged risks of fully-filled cavities, they just build them. Research by Prof Van Korsgaard et al at the Technical University showed all cavity walls posed serious risks of water penetration if they had poor design or workmanship. Once these risks were eliminated, fully-filled cavities were as satisfactory as any other construction system.
The Danes also found that the wider the cavity, the lower the risk of water penetration. In fact, there are no recorded cases of water penetrating cavity walls which have been filled with 100 mm or thicker mineral fibre slabs, whereas tests show that rain which penetrates the outer leaf of masonry under wind pressure can cross an unfilled 50 mm cavity and wet the inner leaf. Consequently, since the late 1970s new Danish walls have had fully-filled 100-150 mm cavities.
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Building Homes