8 JUNE 05

Our windows arrived this week: a consignment of high performance glass and timber shipped all the way from Scandinavia by Swedish Timber Products. As they will be integral to both the environmental performance and the human experience of Tree House, we cut no corners here. Windows are magical: they breach the boundary between inside and outside without sacrificing the security and shelter that define our experience of home. And if you’re investing in magic, there’s really no room for compromise.

Windows play a complex eco-role: they bring in the warmth and light of the sun and so can reduce demand for heat and power, but they are also remarkably good at chucking heat away. Lots of glazing can also lead to over-heating and reliance on energy-guzzling air-conditioning, an increasing risk in our warming climate. The optimal eco-window is therefore a matter of good building design as well as good window technology.

Although both of the main living spaces in Tree House have substantial glazing, each room is protected from over-heating by careful shading: external venetian blinds on the ground floor and the tree itself on the top floor. The tree contributes its own magic to the design, shading the house in summer and letting light through in winter when extra warmth is most needed.

We can only incorporate these outside-in spaces into our ultra-efficient house because the windows themselves let very little heat escape - less than a fifth of the heat lost by an ordinary sash window. This is achieved by tackling all three modes of heat flow: conduction, convection and radiation.

Conduction is the direct flow of heat through a material, such as through the handle of a poorly designed pan on a stove. This is how heat moves through a pane of glass, so if you add more panes - our windows have three - the heat loss takes longer.

Convection takes place when heat is carried along by the movement of a liquid or gas. The swirling of the water around the potatoes in my pan is evidence of such convection currents. The heat transfer from pane to pane in double glazing is largely due to convection currents in the air gap, so with a heavier gas – our windows are filled with argon - the currents slow down.

Radiation is a universal phenomenon: everything radiates heat. Even when I have turned the stove off and served my potatoes, I can tell the ring is still hot because of the heat it radiates. Every window pane radiates heat out to the sky but different materials do this at different rates: our windows have two invisible 'low-emissivity' coatings that reduce this heat loss.

Finally, because our windows are extremely air-tight, they will lose a minimum of heat from draughty joinery, a major and uncomfortable problem in older houses.

Window frames are also an important eco-issue. It's something of a disaster that u-PVC windows have triumphed in the market, given how ugly they are and how much toxic waste is created in their manufacture. Why use such a poisonous material when you can use wood instead? Happily, Swedish Timber Products has FSC accreditation, so we can even be sure of the sustainable management of the forests from where our windows began their journey to Clapham. The only eco-downside is the boat trip from Sweden, but this is a small price to pay for the efficiency savings that the windows will bring over the life-time of the house.

By the way, my cooking analogy is not entirely accurate. For a start, our ultra-efficient AEG-Electrolux induction hob generates heat in the pan but doesn’t get hot itself. Furthermore, I'm on a low carbohydrate diet and have long since given up cooking potatoes. But that is definitely another story.

Contact Swedish Timber Products on 01347 825 610, www.swedishtimberproducts.co.uk.