Diary Of An Eco-Builder

There are three ways of dealing with water vapour - and we are pursuing all of them

Actually, the colour I'm picking is for the render board that will provide much of the "rain screen" for the building, complementing the timber-clad trunk that wraps around our staircase. Both types of cladding will soon be nailed in place on timber battens, leaving a ventilation gap between the rain screen and the main wall.

Our walls must obviously be robust enough to stop water getting in, but this is a simple task compared with dealing with moisture going the other way. The principal movement of moisture within walls is from the warm, humid interior to the cooler, drier exterior.

We generate huge amounts of water vapour in our homes: cooking, bathing, washing and breathing all increase humidity. This can cause all sorts of problems, not least asthma: 80 per cent of asthmatics are allergic to the droppings of dust mites, which proliferate in warm, humid conditions. For every wisp of steam that you see rising from your coffee cup, potato pan or bath tub, there's a whole lot more that you can't see. In fact, what you see over your coffee is not vapour at all but the condensation where the rising gas meets cooler air.

Condensation occurs whenever air cannot hold any more water. This happens in hot conditions if there is lots of moisture in the air, such as in a steam room or a tropical city in the wet season. But condensation is much more common in Britain in cool conditions because cold air can hold very little moisture. Cold air is very dry and may not even absorb our rapidly condensing winter breath.

In houses, condensation is common in poorly insulated cold spots such as window frames where warm air meets the cold surface. However, it is a greater problem if it occurs in areas you can't see, such as inside your walls or roof, where it can do everything from undermining your insulation to rotting your building fabric.

There are three ways of dealing with water vapour: (a) get rid of it with good ventilation; (b) put a barrier in your wall to stop it getting in; or (c) design your wall to cope with it. The most advanced version of (c) is a "breathing wall" which absorbs the water vapour produced in the building and carries it outside without the use of any active ventilation systems. This is the method used by the two billion people who live in houses made of earth and clay.

Our approach is to pursue all these strategies at once with an eye for energy conservation. For ventilation, we will open the windows in summer but turn on the mechanical ventilation in winter: our Vent-Axia LoWatt whole-house ventilation system combines low power consumption with a very high rate of heat transfer from the outgoing to the incoming air (www.vent-axia.com).

To stop the vapour getting into the walls, the entire shell of the building has an internal lining of recycled black plastic that also helps to stop valuable warm air escaping. Finally, if moisture does get into the walls, the Warmcel recycled newspaper insulation (www.excelfibre.com) will carry it out to the ventilation gap behind the rain screen, preventing any moisture build-up within the timber frame. The result will be a house with high air quality, low humidity and low energy loss.

As for that colour choice, I didn't even look at the lovely charts in the catalogue. The Bauhaus school recommended white to draw attention to the form of a building rather than its surface. As Tree House is designed to be subtly tree-like in its form, I think this recommendation provides an excellent post-hoc rationalisation, on this occasion, for following the contemporary crowd.