Diary Of An Eco-Builder

It's not easy keeping your building airtight, so we found a man with a fan to turn up the pascals

Paul works for a company called Stroma Technology (www.stroma-ats.co.uk) that pressure tests buildings of all sizes to see how airtight they are. An airtight building can be carefully ventilated to minimise heat loss and optimise indoor air quality. By contrast, a building that is not airtight will leak warm air through cracks and gaps, at worst leading to draughty, cold and uncomfortable interiors.

Builders in Britain have never paid much attention to airtightness. Today, new houses have to be built with much more insulation than in the past but they can still be full of holes. Although there has been a standard of airtightness in the building regulations since 2002, there has been no requirement for new houses to be tested to see if they achieve it. This may finally change in a revision of the building regulations next year.

Tree House, of course, has got to be different. Our ever-patient architect, Peter Smithdale of Constructive Individuals (www.constructiveindividuals.com) tracked down the world's best airtightness detailing for timber frame houses (from Canada) and did his best to design them in. Unfortunately it's not easy to build an airtight house because the downright fiddly business of sealing every junction, crack and service duct is ill-suited to the hurly-burly of a building site. Nonetheless, our guys on site have been attentive, so we were hoping for a good result.

We closed all the windows and external doors and Paul sealed the front door with special panels into which the fan was inserted. Then the fun started. With the fan on and the house under increasing pressure, we were able to go round the building with a little smoke puffer looking for action. Junctions where the smoke made little quiet clouds were good news; places where the smoke whipped away out of sight were trouble.

Happily, we can still take a lot of remedial action to deal with the hot (or rather cold) spots before the build is complete.

Then came the crucial measurement. The fan was ratcheted up until the pressure inside the house was a steady 50 pascals. Paul measured the air flow out of the house and used this to calculate the 'air permeability' of the building: the volume of air that gets flushed through the envelope of the building every hour. The standard in the building regulations is a maximum leakage of 10 cubic metres per hour, for every square metre of the house, at a pressure of 50 pascals. We achieved 3.4, and with remedial action may get this down quite a bit lower. Not bad. In fact, really quite good, especially for a tricksy design with lots of fiddly corners.

As this level of air-tightness would not give us enough fresh air at normal air pressure, in the winter we will be mechanically ventilating the house using a heat recovery unit that changes the air but keeps the heat in (see www.vent-axia.com). In summer, we will open the windows.

Paul reckons that Tree House is likely to be better than the current output of the volume housebuilders by a factor of four or five. If you live in a house built since 2002 that is draughty and expensive to heat, you almost certainly have a case against your builder. Then he shut up the Things in the box with the hook. And Paul went away with a sad kind of look. But he said: "I'll be back." And then he was gone with a tip of his hat.

Will Anderson's 'Diary Of An Eco-Builder' will be published by Green Books next spring. www.treehouseclapham.org.uk