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Insulation is king

Posted by Tim Pullen on 13 March 2009 at 11:46 am

Let’s be clear, the greatest environmental impact of a house is from the fossil fuels it burns for its energy. No amount of eco-certified, recycled bamboo flooring can compensate for the impact of a gas guzzling house. Conserving energy, minimising the energy needs of the house has to be the first priority.

It is also the cheapest option. The payback on achieving even super-insulation levels will be in the region of four to five years. The sort of figure that solar panel manufacturers can only dream of. In short, insulation is king. But insulation is only part of the story. Draughts are also a big issue.

Air Tightness
All new houses built since April 2006 must meet a strict standard on air tightness. That is, 10 cubic metres of air per hour escaping for every square metre of the envelope surface area. This doesn’t take account of how people like to live in the building. If, like me, you enjoy opening a window and listening to the birds singing while you tap away at a keyboard, your air-tight house will not function as per specification.

Insulation – How much is too much?
The answer is, of course, it depends. Heat loss can never (in practical terms) be reduced to zero, even the best insulation can only slow it down. How close you want to get to zero is the issue.

To give an example: building regs call for a minimum 270mm mineral wool insulation in the loft in new build. That equates to a U-value (measure of heat loss) of 0.16W/m² (watts of energy lost through a square meter of roof). Increasing the thickness to 450mm will reduce the U-value to 0.08W/m². The extra cost might be an extra £5 or £6 per sq metre. But increasing to 600mm will only reduce the U-value to 0.06W/m² - there is a law of diminishing returns.

The Answer
There is a balance between insulation and air-tightness. It is broadly true that the more air passing through the house, the less effective the insulation becomes. The ultimate air tightness is the PassivHaus standard which calls for air tightness of 1m³/h and needs mechanical ventilation. At that level super-insulation is very effective and no active heating system is needed.

However, achieving an air-tight house is difficult, and potentially very expensive. The answer is to put in as much insulation as you reasonably can and accept that life is not perfect.

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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