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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

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

5 comments - read them below or add one

fuelexplorer

fuelexplorerComment left on: 20 May 2010 at 12:55 am

Bingo!  Pre-fabricated engineered domestic dwellings are clearly the future.  At least 300mm insulation, draft tight, will almost zero the need for heat. Just hot water ( solar ) and electricity ( PV ) to minimise your fuel costs.  Get rid of low energy flourescent bulbs, switch to the new 3000° LEDs.  Navy's on a building site need only fabricate ihe structural base raft, the rest should all be engineered off-site, and erected in a few days by specialists.   

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 18 August 2009 at 1:05 pm

As to whether this idea will "catch on", it is a bit late to worry. All of the Code for Sustainable Homes Level 6 house in the UK build in this way. All Passivhaus' build in this way, virtually all off-site manufactured houses build in this way. The majority of house in mainland Europe are built this way - and have been for many years. It is really only the UK house-building industry that does want to and that is because they have pre-determined design and construction methods based on masonry cavity walls. They then convince the rest of us that this is the proper, the solid, the British way of doing things. Any right thinking person can work out that the cavity is pointless and the reasoning flows from that.

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Advanced Insulation Manufacturing

Advanced Insulation ManufacturingComment left on: 17 August 2009 at 4:10 pm

Its a very interesting idea, but I'm not sure it would catch on unless it meant the cost of the house to the end user were significantly reduced. If you were doing it as a self-build project it might well make sense, especially if you were planning to stay there for an extended period. I just wonder if it would catch on at large when faith in the construction industry is generally low - my main worry would be that developers see it as a way of just reducing their own costs and not passing the savings on to people buying the homes.

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Tim Pullen

Tim Pullen from Weather WorksComment left on: 8 May 2009 at 11:16 am

Short answer is that it depends on the timber - and the brick, come to that. Quality will last and poor quality will fail irrespective of the material. This country has many examples of timber houses 400+ years old. The old building in the world is a timber temple in Japan - some 4,000 years old. The Building Research Establishment set out a design life for houses of 80 years (only recently increased from 60 years). To achieve that you don't even need durable timber, just good construction. 

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Morgan Property Services

Morgan Property ServicesComment left on: 8 May 2009 at 10:16 am

What is the difference in life expectancy of a timber only building against a timber building with brick exterior?

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