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Ventilation and draughts

Introduction to draughts and ventilation

There's quite a bit of confusion about ventilation and draughts. We need fresh air in occupied buildings, but it also makes sense if we can control it. Draughts are uncontrolled ventilation, and are also the cause of considerable heat loss. This can lead to discomfort, or cold, and to higher than necessary heating bills. The good news is that there is something you can do about it. Ventilation is the controlled entry of fresh air into a building.

Draught proofing

Draughts are easy, and cheap, to fix but we tend to learn to live with them.  It’s just a matter of blocking up any holes which allow warm air out, and hot air in. In the typical UK house draughts will account for at least 10% of the total heat loss. If there is an unused open fireplace that figure will rise to over 50%. 

There are all the obvious places to seal – gaps around sash windows and under doors, floor to wall joints, ceiling to wall joints, gaps between floor boards – but the real culprits tend to be where pipes and cables are brought through walls or floors, and there is often a gap around the pipe or cable.

Some draughts will be so bad that they are easy to spot. You can find the others by using a thermal heat detector or by walking round the house with a lit candle, and noticing where the flame flickers.

There are lots of products in your local DIY store for sealing round windows and doors; good old mastic will do the job at the junction of floor and walls, and even gaps between floor boards. Penetrations through walls for pipes and cables can be sealed with mastic, or expanding polyurethane foam for the larger gaps.

Chimney flues can be bricked up or there is a handy balloon-type thing called a Sempaflu that will do the job. It is important to seal the top as well as the bottom to stop rainwater getting in. Sealing the bottom will still allow rainwater into the top, with consequent damp problems now that the flow of air that used to dry it has been blocked.

A note of caution; beware air-bricks. Make sure they can still circulate air where it is needed, typically to the ground floor joists. No air means wet or dry-rot getting established. And make sure you don't block trickle vents in windows or extractor fans.


Ventilation is talked about in terms of air changes per hour. Traditional buildings may experience as many as 4 air changes per hour (this is why draught proofing is worth doing well). A modern building needs a ventilation rate of about one air change an hour. If it's super insulated it will need twice that.

Trickle vents

Modern window frames generally have small vents which you can open and close at will.

Extractor fans

These are generally used in bathrooms and kitchens. They can operate automaticaly when a certain level of humitidy is reached, or come on with a light, or be controlled manually.

Mechanical ventilation

In PassivHaus and other super insulated buildings, mechanical ventilation with heat recovery (MVHR) is used to extract warm, humid air from rooms such as the bathroom and kitchen via a heat exchanger. Fresh air is drawn from outside, warmed in the heat exchanger and delivered to cooler room such as the lounge and bedrooms.

MVHR only works with a high level of air tightness, and must be fitted carefully so that the energy used to drive it is not greater than the heat savings it delivers. The jury is out on whether it is worth retrofitting a system (the ducting is big, bulky and ugly, and you need space in the roof for the air handling unit). However, if you are considering it, it is important to have an air pressure test to find where the leaks are, and then seal them up. The air tightness should be less than 5 cubic meters per hour (that's twice as good as current building regulations).

How to air a room in winter

If you don't have good ventilation in a room, it is better to turn off the heating and open the window for a five minute burst of air, than to leave the heating on and the window half open for a long time. This tip is from the Home Energy Handbook.

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