Will Anderson: The Green House

Eco-building aficionados are always quick to point out that you should never build a power station on your roof until you've done everything you can to conserve energy in the house below. Why spend a load of money generating energy when cheap-as-chips insulation and draught-proofing will save far more? Indeed this was exactly the response from various green quarters a couple of weeks ago when Currys announced that they were going to start selling photovoltaic (solar-electric) panels through selected stores.

The world is not, however, a very rational place and I for one don't expect people always to do things in the "right" order. We're all driven by different priorities and, if you want a power station on your roof because you like the idea of making energy as well as burning it, that's fine by me. Chances are, once you're producing the stuff and watching your output meter rise and fall across the day, you'll pay a lot more attention to how you consume energy as well.

All insulation ever does is sit there, out of sight, keeping the heat in. How exciting is that? Insulating your loft and lagging your hot water tank are top eco-renovation priorities but they're not much use when it comes to impressing the neighbours. Not that an after-dinner conversation about insulation is unimaginable. For example, can you think of any other product on the market that is made from so many different materials? As well as the familiar DIY-store products made from minerals or glass fibre, you can also buy insulation made from sheep's wool, cork, hemp, straw and chewed up newspapers. There is even a new product made from waste cotton that could make good use of all the clothes that we throw away each year ( www.recovery-insulation.co.uk).

The makers of all these products are keen to draw attention to the particular environmental strengths of their raw materials. Sheep's wool, for example, is manifestly a good material for insulation, given how well it has been doing this job for sheep - and humans - for centuries ( www.secondnatureuk.com). The fact that it is also a renewable resource that helps to sustain upland sheep-farming in Britain clearly adds to its ecological value and installing it is a delight compared to itchy, prickly glass fibre.

Yet the finer points of what insulation is made from are not very critical because the function of insulation - saving energy - is so overwhelmingly important. This is especially true of "embodied energy", the energy that is needed to make the product, which is trivial compared to the energy that insulation saves. You would have to stuff your walls with a ludicrous amount of insulation - around a metre thick - for its embodied energy to become significant over the lifetime of the house.

Focus on specifying the right material for the job and installing it correctly. Thick rolls of sheep's wool might be just the thing for your loft (but put a layer on top of the joists, as well as between them, because wood conducts heat faster than most people imagine). If you are filling a cavity in your wall, you will need something that can be blown in to fill all the hidden nooks and crannies. If you are worried about moisture in your building fabric, use a product that can wick moisture away. Pay special attention to the thermal conductivity of the insulation (the rate at which heat moves through it). If your space for insulation is limited, specify the lowest conductivity that you can afford. And don't be fooled by absurd claims that "space age" reflective materials can replace thick layers of ordinary insulation - they can't.

GREAT BUY

A jacket for your hot-water tank will save you lots of energy. And if you already have one, fit another one on top! They are around £10 from DIY stores.

GREAT WEBSITE

There's lots of insulation information on from the Energy Saving Trust at www.est.org.uk. Check out the Best Practice publications in "housing and buildings".

w.anderson@independent.co.uk