Diary Of An Eco-Builder

The shape of tree house is certainly striking. despite this, it is basically a big, square bo

January is a puritanical month. If only we scheduled our midwinter festival for February instead of December, we could recover in the knowledge that spring was just around the corner. Instead, we find ourselves in the cold, dark days of January with nothing but haggis-slaying to look forward to.

If nothing else, these are ideal conditions in which to review the thermal performance of our homes.

The domestic experience of warmth and comfort is affected by factors such as insulation, draught-proofing and the efficiency and controls of the heating system. If these are not doing a good job, they can be upgraded (call the Energy Saving Trust for advice on 0800 915 7722, www.est.org.uk). But there is one important building characteristic that you can probably do little about: the shape of your home.

The shape of Tree House was finally revealed in full this week when the last of the scaffolding was removed. It is certainly striking, with its curved timber-clad stair tower, top-floor balcony and big southern-pitched solar roof. Despite these details, however, it is basically a big, square box.

This is a good shape for energy efficiency, as the compactness of the form minimises the number of exterior surfaces through which heat can escape.

It is fascinating to look at the shape of buildings and consider how appropriate they are to their climatic conditions. A sphere has the lowest external surface area for a given internal volume, so it is no surprise to find hemispherical and conical buildings in very cold climates where preservation of heat is a priority, such as igloos, yurts and wigwams.

At the other extreme, the worst shape for a building in a cold climate is a cross, as all the walls are exposed to the elements. Churches have a deserved reputation for being chilly buildings in Britain, but are welcomed as cool sanctuaries in Italy. The first building I ever worked in was Charing Cross Hospital, in west London, a cruciform tower block that gobbled more energy than a sizeable town.

If you have the money and the inclination, you can ignore these issues and simply burn more fuel. Although British country houses were originally fairly compact in form, the increasing wealth of the ruling classes was expressed in far-flung wings reached by long, exposed corridors. A similar profligacy can be seen today further down the social hierarchy in the addition of large heated conservatories to otherwise compact houses. Such extensions add lots of exposed wall and roof to a home and, as they are made from glass, chuck away heat with abandon. The homes with the fewest exposed walls are flats and terraced houses, sandwiched between neighbours who hopefully give as much heat as they take. Although Tree House is detached, it may one day be the first house in a terrace: the northern wall is window-free, designed to enable our neighbours to build right up against us, if and when they choose to.

For simple lessons in the relationship of form to thermal performance, get a cat. I am writing this next to three nearly spherical balls of fur. Cats are very sensitive to temperature and adept at changing their shape to suit. Come the summer, these three balls will unravel and stretch out on the cool, slate ground floor of Tree House, exposing their bellies and maximising their heat loss.

If you are a puritan at heart, follow the example of medieval monks whose exposed cloisters were as cold in the British climate as the churches next to them. If not, I recommend the compact feline approach to winter living, driven by the devil's instinct for total comfort and idleness.

Will Anderson's complete 'Diary of an Eco-Builder' will be published by Green Books this spring ( www.treehouseclapham.org.uk)