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In response to instances of moisture ingress in homes in north America during the late 1980s, the National HouseBuilding Council (NHBC) stipulated that where timber or steel frames formed the main part of a house's construction, a cavity must be incorporated. Chapter 6.9 of the April 2005 NHBC standards for cladding and curtain walling, that became effective from July 2005, states: 'For timber and steel-framed backing walls, a cavity of at least 15mm should be provided between the wall and the insulation to allow any moisture to drain away.'

On steel-framed structures, the cavity exists solely for drainage purposes, while it also performs a ventilation function in a timber-framed house. The NHBC standard goes on to say that the structure and external wall insulation (EWI) must be 'robustly constructed'.

The challenge for EWI suppliers posed by the cavity is surmountable, but a second problem has been created, one that is currently being overlooked. The cavity is effectively a ue in the building, and complying with the drainage requirements compromises the issue of -re safety. Although the standard dictates that 'materials used for cavity barriers and -re stops shall be capable of providing adequate resistance to -re and smoke', meaning the incorporation of intumescent materials, the absence of a secondary outer sheathing layer means that, in the event of a -re, there would be nothing for the -re barrier to intumesce against. With localised temperatures in the 'ue' reaching up to 1,000¦C, external render will very quickly be left hanging in thin air - not a great prospect on a 12-storey apartment block.

Furthermore, while responsible suppliers of EWI recognise the risks and have developed proprietary systems to negate them, there is a small cost implication attached.

On projects where cost is the key driver, it is feasible for a main contractor to omit the secondary layer from these systems or indeed select an alternative single-skinned system and still comply with the NHBC standard. A simple amendment to the standard to incorporate a second outer layer of 10mm cement particle board to the cavity would make the cavity stable and robust without compromising design, since it would provide a substrate for the intumescent materials to act upon.

BS 8414 Part II is currently at draft stage, and hopefully this new standard will prompt a change to the NHBC standard, but its contents and publication date are as yet unknown.

Unsurprisingly, a system that complies only with the minimum requirement of the NHBC standard has not been -re tested, but it is clear that the cavity ue is a disaster waiting to happen. With 85 per cent of new-build houses in the UK being insured by the NHBC, the scale of the -re risk is considerable. Hopefully we can take preventive measures instead of reacting post event.

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