Diary Of An Eco-Builder

'As your mother told you, never stick your head - let alone your house - in a plastic bag'

What's the difference between a hat and a scarf? I'm being strictly functional, so ignore the finer points of class and cred, and consider the basic role these garments play in keeping us warm. A woolly hat provides a layer of insulation for your glowing scalp. Although heat is conducted through the hat, this takes time so your head warms up. A scarf does a similar job for your neck but this is secondary to its main function of plugging the air gap at the top of your coat. By making clothing airtight, a scarf helps to reduce your ventilation heat losses.

What's the difference between a hat and a scarf? I'm being strictly functional, so ignore the finer points of class and cred, and consider the basic role these garments play in keeping us warm. A woolly hat provides a layer of insulation for your glowing scalp. Although heat is conducted through the hat, this takes time so your head warms up. A scarf does a similar job for your neck but this is secondary to its main function of plugging the air gap at the top of your coat. By making clothing airtight, a scarf helps to reduce your ventilation heat losses.

Houses conserve heat in exactly the same ways. Think loft insulation for woolly hat, cavity-wall insulation for coat, and for the scarf - well, what? Draught-stripping is part of the answer, but this will not make your home airtight, as there are lots of tiny air pathways through your walls and roof through which heat can escape. It's possible to plug these holes, too, but it's not easy.

In the past week, our little project team has been working out exactly how to install the two plastic bags that will wrap up Tree House, one near the inside of the building envelope that will keep air and moisture out of the walls, and one near the outside that also stops air escaping but lets water vapour through, so that any moisture that does get in the walls can be released (an important consideration for a timber-frame house).

Such airtight ambition has a potential downside. As your mother told you when she dandled you on her knee, never stick your head - let alone your house - in a plastic bag. Sure enough, there is growing concern about the air quality inside some well-insulated modern homes. There are lots of toxic chemicals we bring into our homes (eg formaldehyde in MDF and water-based paints) which will build up if ventilation is poor, and any fire or boiler that is starved of oxygen will produce deadly carbon monoxide. And whenever you make a cup of tea or have a shower, you are emitting a seemingly innocuous but dangerous gas: water vapour. A warm, humid house is a perfect home for dust mites, whose droppings are implicated in the huge rise in childhood asthma.

The answer to these problems is controlled ventilation which supplies enough air to keep you healthy with minimum heat loss. In our airtight house we will achieve this with whole-house mechanical ventilation, a box of tricks that extracts air through hidden ducts from the kitchen and bathroom and brings fresh air into the living room and bedrooms.

Although they don't mix, these two streams of air cross each other through a fine mesh so that the heat from the exhaust air is transferred to the incoming fresh air. The fans in the unit require electricity to run but happily Vent-Axia's Lo Watt system recovers a lot of heat energy for relatively little electrical energy input ( www.vent-axia.com).

Whole-house ventilation with heat recovery is tricky to retro-fit but is worth considering if you are building new, doing a major refurbishment or have problems with asthma. Whatever your circumstances and budget, reducing uncontrolled ventilation with draught-stripping and well-packed insulation will pay you back handsomely in improved comfort and lower bills.

You can then introduce extra fresh air only when it's really needed. If you have little vents built into your window frames, learn to set these to give you only the air you need. Contact the Energy Saving Trust ( www.est.org.uk) for more advice on all these issues.

Unfortunately, my current obsession with the thermal performance of Tree House has not been reflected in sartorial diligence: last weekend I found myself on a windswept Scottish hillside without hat, scarf or even a decent coat. My well-dressed companion, two-year-old Talia, was considerate enough not to scoff from beneath her woollen hat. Give her a year and I have no doubt she'll be sitting on dizzy uncle Will's knee, telling him all about the dangers of plastic bags.