Diary Of An Eco-Builder

Winter will pass, spring will come, and we'll be here to watch every bud unfurling on our tree

One of the joys of living in the country (allegedly) is a heightened awareness of the passing seasons. Which is not to say that we urbanites fail to notice the frost; only that we easily miss the subtleties of the changing flora and fauna around us. Central London may not be devoid of nature's gifts, but they do get pushed into the background by a surfeit of bricks, mortar and tarmac.

Tree House has been designed to redress the balance. In our open-plan study at the top of the building we have a private view of the seasons through the floor-to-ceiling windows that open on to a balcony right in the crown of our tree. Sure enough, in my regular morning visits to the site I have been struck not only by the longevity of the leaves this year, but also by the subtly changing light as they fade from deep summer green to a translucent yellow, variegated by the cracked browns of the wind-burnt leaf edges.

The study faces east, so as the leaves fall more sunlight penetrates the room, warming the house at the beginning of the day. The tree's timing is impeccable: lifting its shade when the temperature drops, and then restoring it in spring to prevent the room from overheating in the summer.

External shading has never been a high priority in the design of British homes, but with rising summer temperatures this ought to change. Forty thousand domestic air-conditioning units were sold in 2004, a 27 per cent increase on sales in 1996. Recent work by the University of Manchester has highlighted the alarming possibility that hotter summers could lead, by 2050, to carbon emissions from cooling outstripping all other domestic emissions put together. Do we want to end up like New York, which ground to a halt when summer electricity demand for air-conditioning tripped out the entire power supply to the city?

This scenario can be avoided if we design buildings to stay cool passively. Good natural ventilation across or up a building can remove warm air quickly, and exposed heavyweight materials, such as tiled floors, will smooth temperature swings across the day by absorbing and slowly releasing heat.

But the most robust strategy is to stop the heat getting inside in the first place. Better insulation, especially in the roof, will help to keep heat out in the summer, but you still need to control the impact of direct sunlight pouring through windows. External shading performs this function well but is tricky to get right because daylight and winter solar warmth both make a valuable contribution to a low-energy house.

In the top room of Tree House, the tree's excellent season-specific shading is complemented by deep eaves that will shade the high summer sun but let through the low winter sun. All our windows have deep architraves, for the same reason. On the ground floor, the glazing along the width of the living space will be protected by exterior venetian blinds (www.lynnwestward.com) and, where the view is less critical, a fixed framework of horizontal wooden slats (a "brise soleil"). There'll be a pergola beyond the windows over which we will train a deciduous climber. External shutters are widely used in southern Europe to keep the heat out during siesta, but as we don't plan to have an afternoon snooze every day we have stuck to design strategies that will help keep the house cool but bright.

Alas, our big bright study is not yet graced with books and furniture but with dusty piles of timber and insulation off-cuts. As another completion date slips by with plenty still to do, I feel remarkably calm. Winter will pass; spring will come. The tree will flourish again and we will be here to watch every bud unfurl.

Will Anderson's complete 'Diary of an Eco-Builder' will be published by Green Books in spring 2006 (for details, see www.treehouseclapham.org.uk )