What's the point of a cavity wall?
Posted by Tim Pullen on 5 May 2009 at 2:30 pm
Once again I recently surprised a client by suggesting that single-skin timber-frame wall construction would be a good idea. Shock and horror hardly describe it. It was like I suggested that his children be sold into slavery to fund the new home.
But think about it, what is the point of the cavity? What does it do? More importantly, what is the point of the brick outer skin on a timber-frame wall? Take it away and the house will not fall down, the thermal efficiency will be about the same, the life of the house won’t change. It serves only as a rain barrier and to convince our neighbours that we invested in bricks and mortar – which we didn’t. Not really.
Until the early 20th century solid wall construction was the norm, be it stone, brick or timber. The cavity was introduced as it provided a bit more protection against rain penetration (especially with poor quality construction) and was thought to give a bit of thermal insulation. From 1970s we started putting some insulation in the cavity, and now a cavity filled with insulation is not untypical. So we are in effect back to solid wall construction, but now with some insulation in it.
Take that idea just one small step further and you arrive at single-skin timber frame with the opportunity to install perhaps 250mm of insulation and get a U-value less than half that of a brick & block cavity wall. And it is cheaper and quicker to build.
My confident prediction is that within 10 years single-skin timber frame (with or without structural insulated panels – SIPS) will become the norm. As soon as we accept that energy efficiency is more important than impressing the neighbours, brick & block cavity walls will disappear.
photo © Ryan Klos
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