Diary of an Eco-Builder

'We only need heating because our houses fail to prevent heat from escaping'

So is this it? Is this my heart of darkness? Just when the pace of our ambitious Clapham self-build was finally picking up, the gods have intervened, rendering our site a frozen wasteland where little, if anything, can move forward.

So is this it? Is this my heart of darkness? Just when the pace of our ambitious Clapham self-build was finally picking up, the gods have intervened, rendering our site a frozen wasteland where little, if anything, can move forward.

The brickies have been sent home because it's too cold for their mortar, the timber frame has been put back for yet another week and site stalwarts Steve and George are officially fed up to their back teeth. George spent a day last week 20ft under at the bottom of our neighbour's manhole, drilling through to connect to our drain. Most of the residents of the seven houses which supply the manhole were out at the time, but it was a pretty good image of existential desolation: wading through human effluent in a dank dark hole while the freezing sleet beat down from above. At least he got to wear a fetching yellow outfit for the occasion.

But even snow clouds have silver linings. The sudden winter has provided me with a chilling incentive to return to my long-neglected energy calculations and consider how well Tree House will cope with similar conditions in a year's time.

Most of the energy we burn in our homes goes on space heating, so there's nothing like a cold snap to bring a warm glow to the shareholder value of energy companies. Yet this needn't be the case.

There's a common misunderstanding that when it gets cold outside, our houses inevitably get cold too unless we turn the heating on. But the problem isn't really the cold at all.

We only need heating because our houses fail to prevent the heat that's already in them from escaping, so although a drop in outside temperature will speed up the heat loss, this loss is principally due to the construction of the building.

There's no reason why a house shouldn't remain comfortable throughout the year without any active heating as long as the heat losses never exceed the "passive" heat gains from the sun, the occupants and their cookers and appliances. In practice, this is very difficult to achieve, although at BedZED in South London they have come close by trapping the sun's energy in their huge concrete walls ( www.bedzed.org.uk).

Similarly, in Germany, the Passive House movement has successfully promoted a radical design specification that involves minimal active heating ( www.passivehouse.com). We will do pretty well: on a freezing morning Tree House will lose heat at a rate of 2,200W, a loss that we could replenish using a two-bar electric fire for the whole house, whereas a 1930s house of the same proportions will throw heat away nine times faster, requiring a hard-working boiler to keep the temperature up.

We don't want to ditch heating systems altogether, because we'll always need hot water, so our approach is to combine a very high performance building (exceptional insulation, high quality windows and carefully controlled ventilation) with our own renewable energy generation. If you want to improve the performance of your own home, get in touch with the Energy Saving Trust ( www.est.org.uk or 0845 727 7200).

If my sums are right, in a year's time we will be able watch the snow settle on the boughs of our tree from a warm, fossil fuel-free Tree House. I only hope that in 50 years' time (as opposed to the day after tomorrow) our rapacious oil-driven global economy will have avoided its own heart of darkness and we will still be able to potter down to Brixton market without being chased by daily Arctic gales.