Managing your draughts and ventilation FAQ

How much heat am I losing through draughts?

Your home loses heat partly by conduction through the fabric of your walls, floor, roof, windows etc and partly through draughts and ventilation. You do need some ventilation but if your home is well insulated then draughts can be a large part of your heat loss. Ideally you want the right level of controlled ventilation, not a lot of uncontrolled draughts. This tool will give you a first estimate of how much of your heat loss is due to ventilation/draughts.

However, the only way to be sure is to have your home tested for air tightness. This involves having a big fan installed in your doorway which attempts to blow enough air into the house to maintain a pressure difference between inside and out. It isn’t worth doing this until you think you have at least most of your draughts sorted out.

How much fresh air do I need?

You need fresh air in your house for various reasons – if only to replenish the oxygen you need to breath – but the main one is normally to control moisture. You generate moisture in lots of ways, in particular by breathing, by showering and bathing, cooking and doing the laundry. This means some rooms need more ventilation than others, particularly kitchens, bathrooms and anywhere you dry clothes. However all rooms need some ventilation.

There will be times when you need extra ventilation too. For example new carpets, paint and furniture can give off unhealthy gases called VOCs (volatile organic compounds). Also drying plaster generates lots of moisture, and plaster can take months to dry fully.

Moisture is bad because it can lead to condensation and mould, also it encourages mites which trigger allergies. You get condensation when moist air cools to below its dew point. At 70% humidity (which is the highest you want in your house) the dew point is 14C - below this you will get moisture. If your house is well insulated but there are gaps or ‘thermal bridges’ which are cold on the surface (such as metal window frames) then these areas are particularly prone to condensation.

Under normal circumstances, you need enough ventilation to keep the relative humidity in the air down to 70%, most of the time. Building regulations specifying trickle vents around windows and extractors for bathrooms and kitchens are intended to achieve this comfortably under most circumstances.

How do I find my draughts?

Using the back of your hand
to check for draughts

It can be surprisingly hard to locate the source of your draughts, especially when you consider that whereas it is easy to feel cold air coming in it is much harder to find where the warm air is going out. You may find some draughts are only noticeable when the wind is in a particular direction. Also if there is air leakage through your roof you may never notice the air flow because it is never cold.

A good way to find the draughts then is to pick a cold windy night and watch a stream of smoke. You can borrow a smoke pencil or use some joss sticks or anything else that generates a continuous stream of smoke. Alternatively, use the back of your hand which is more sensitive to cold than the front - run it along the edge of your skirting board for example.

You may find draughts in all sorts of places especially:

  • Between floor boards, or around holes made in the floor boards for plumbing such as radiator heating pies.
  • Behind or under the skirting board
  • Around badly fitting windows and doors (you may be able to see the gaps). Look between the frame and the wall as well as round the opening windows and doors.
  • Around waste pipes, electricity cables or other plumbing that goes through the wall or ceiling (remember to check the floor of the under stairs cupboard).
  • Cat flaps
  • Letter boxes
  • Loft hatch and around pipes into the loft
  • Chimneys
  • Gaps around light fittings or cracked plaster leading to ceiling voids which are themselves vented to outside.
  • Unfinished mortar joints

Also check outside the house and seal around where overflow pipes and aerial wires enter the building.

If you find one or two rooms in the house are particularly draughty, keep those doors shut to help stop the cold spreading.

How do I fix them?

There are various ways to fix draughts depending on where they are, how big and what shape they are.


Here is a review of some products for sealing between your floor boards from the Guardian. The cheapest option is to use old newspapers to make papier-mache as a filler between the boards. We also like Draughtex.

For skirting boards you will almost certainly need a filler as well as a sealant. This video has a good description of the traditional technique. The demonstrator uses a foam core for a filler and then applies a sealant over the top. You can use silicone or a more traditional caulking. Alternatively, you can use caulking strip similar to window draught sealing strip. This video shows how to use stopgap sealant which you fold into a v-shape and stuff down between the boards using a thin card such as a credit card.

E-strip for small gaps

Doors and windows

For gaps around opening doors and windows you need a draught strip. Here is a video showing how to fit it. There are different shapes of strip for different size gaps: E for small gaps and P for bigger ones. The strip contains foam which squashes down to make a good seal with the surface. For sliding surfaces like sash windows or the bottom of doors you can use a brush strip. Alternatively you can fit a curtain to the front of the door with a portiere rod as shown here.

Brush excluders are also good for letter boxes. If you have a large key hole you get a cover that you can swing out of the way when you want to put a key in.

I have not found anything that really works for cat flaps. It is very hard to draught strip a swinging flap. Why can’t cats learn to press a button which could activate a sliding door?


Chimneys must have some ventilation through because otherwise if rain gets into the chimney it will stay wet. However if they are left completely open, even with a gas fire in front, can be a major source of heat loss

If you have an open fire but you don't use it much, then you can block off most of the chimney with plastic bags filled with newspaper or get a chimney balloon. Here is one supplier and here is a video showing how to put one in. Another possibility is a chimney sheep which is a sort of fleecy draught excluder designed for chimneys. If you do use the fire frequently then you could consider getting a flue damper fitted. This allows you to open and close the chimney more easily. Your sweep will be able to help. Alternatively, you can consider getting a wood stove instead. They are much more efficient in any case.

If you have a gas fire then regulations require that you keep the chimney open permanently. This also applies to the type of gas fire that is placed in front of an existing chimney opening and heats ceramic bricks – they hide a hole roughly 20cm x 30cm. Usually the only solution is to replace it with an enclosed fire or stove.

Other holes

For other gaps such as around light fittings in ceilings or pipes going through the floor you will need some kind of filler.

Will I get condensation if I fix the draughts?

If you fix all your draughts and don’t have enough ventilation then you will get condensation and mould and all sorts of problems. You are especially likely to get moisture in areas where air does not flow freely, such as behind the sofa if it is close to the wall or behind plastic blinds, or where there is a cold surface.

Why am I getting mould on window sills?

This is caused by a combination of the sill being a cold surface and poor air flow. It is particularly common if you have heavy blinds which prevent air flow at night. The cold can be due to draughty windows, there may even be a draught under the window sill if this is not well fitted. Anything that makes your sill cold will encourage condensation. Also if you have windows with metal frames they should have a thermal break in them otherwise the frames will get cold. Condensation on the windows or frames will drip onto the sill.

To fix this the first thing is to make sure the surfaces are warm by fixing draughts and fitting good quality double glazed windows. Also if you have roller blinds consider replacing them with curtains or with a different material which will ‘breathe’ allowing the moisture in the air behind to escape into the room.

Why is there mould behind the sofa?

This is because of a combination of poor air flow and a cold surface i.e. the wall. You can improve the air flow by pulling the sofa away from the wall and/or by insulating the wall.

Why is there mould on the bathroom ceiling?

You get a lot of warm moist air when you shower, bath, or even just run the hot tap. The bathroom often gets steamy and the ceiling is often the coldest surface so it gets the most mould. To fix this you need to make sure your ceiling is well insulated so it does not get so cold, and that you ventilate the steam away quickly with some kind of extractor fan.

What are my ventilation options?

You have a wide range of options with a wide range of costs.

  • Opening the windows (see shock ventilation below)
  • Fixed vents, such as trickle vents around windows
  • Extractors in kitchens and bathrooms, on timers or with a humidity sensor
  • Whole house ducted ventilation
    • using natural convection or with a mechanical fan to force air flow,
    • with or without a heat exchanger
    • fixed rate or with a humidity sensor

Opening the windows (and shock ventilation)

If you wait until the room is stuffy before opening the window the air may be unhealthily moist before you notice a problem. Even if you do, in winter you may not want to leave a window open all the time and you might prefer to be stuffy and warm than well ventilated and cold. This is not ideal.

The other problem with opening windows is that you can’t control the direction of air flow. When the air is moist, for example after you have been cooking or showering, you want the steam to go out of the house rather than into the other rooms. Merely opening the window allows flow either way.

None the less, there is a good technique for ventilating your home just by opening the windows, if you take the trouble. This has long been recommended in Germany where it is known as 'Stoß-Lüften' which means ‘shock ventilation. The idea is that you open the windows and internal doors just for a few minutes (5 minutes at the most), several times a day. Opening everything gives you a good through draught which means it takes very little time to change the air thoroughly. However, you only lose the small amount of heat that is in the air, not the heat in the walls, floors, furniture etc because they don't have time to cool down.

Fixed vents such as trickle vents around windows

Trickle vents above a window

Fixed ventilation systems such as trickle vents are designed to give you enough ventilation most of the time which means inevitably you get more than enough – and you are losing more heat than strictly necessary. Also in steamy areas like kitchens and bathrooms they may be inadequate and they don’t control the direction of flow – you definitely want the moist air to go out of the window rather than into the other rooms.

In most places, however trickle vents are quite good enough. You only need to worry if your house is well insulated and the ventilation is a large part of your heat loss. In that case it makes sense to try not to ventilate more than you need and trickle vents are not controllable.

Extractors in bathrooms and kitchens

You need extra ventilation in areas which get steamy such as bathrooms and kitchens or places you dry clothes. It is really important to make sure that the moist air leaves the building rather than going into the other rooms: an extractor will do this. Some extractors have a heat exchanger to capture heat from the outgoing air - this is called heat recovery.

Extractor fans are rated according to the air flow they will generate. A bathroom fan should be at least 15 l/s which is 54 m3/hour. If your bathroom is 2.5m x 2.0m x 2.2m, its volume is 11 m3 so this rate would give you a complete change of air in 12 minutes i.e. 5 ACH (air changes per hour). In practice some of the steam will hide in corners so you won’t get a complete change but running the fan for 15 minutes should clear the air adequately. Fans are also rated for noise. Larger fans are usually quieter.

Fitting a fan involves electrical wiring and you may need to add a fused spur to a lighting circuit. This is a notifiable work if done in a kitchen or bathroom so you will need to be a qualified electrician to do it yourself. However you may be able to get around this by using a 12V fan and wiring the switch and transformer outside the bathroom. The rules are here.

It is important to allow air into the room to replace the air being extracted. Opening a window next to the extractor won’t work well because the air flow will bypass most of the room. You need to bring air in from opposite the extractor which usually means the bathroom door leading to the rest of the house. Sometimes it can be useful to put a vent in the door but usually there will be sufficient gaps around and under it: 10mm is recommended However, if the door is too airtight the extractor will struggle, and that applies to the rest of the house too: the more air tight your house is the less effective the extractors. When it gets to that point you need to consider having a whole house ducted system.

You can have your extractor switched on by a cord and turn off automatically after a given time, or you can get one which turns on automatically. This can be an advantage if you (or your fellow house mates) are prone to forgetting to turn the fan on. The trigger can be a humidity sensor or a presence/motion sensor. The latter has the advantage it is easy to tell if it is working since you know it should come on when you enter. Also humidity is not the only reason why you might want ventilation in a bathroom. (I have not yet heard of an odour activated fan.) You can also have the fan activate when the light is switched on but lights are not always needed so this isn't ideal either.

Whole house ducted systems

Most new homes are built with MVHR – mechanical ventilation with heat recovery. This is a whole house ducted ventilation system with a mechanical pump driving the air flow between rooms and a heat exchanger to reduce heat loss. Each kitchen or bathroom has an extract and the other rooms have vents for fresh air to flow in . The heat exchanger takes heat from the extracts and uses it to warm the fresh air coming in – it should be able to reduce heat loss by 90% or more.

MVHR unit in a loft

An MVHR unit should be sized to give you at least 0.5 ACH (air changes per hour) in most rooms and at least 2 ACH in kitchens and bathrooms. Usually you have 2 or 3 levels: medium for most of the time, boost for if you’ve just been plastering or got new carpet and low for if it is annoying you at night (you can hear the fan, a bit, just like an air conditioning system).

MVHR can be retrofitted into existing houses, though this is not always easy. It helps if the joists between floors are deep enough for the ducts. However, MVHR is expensive to fit and has significant running costs. It takes power to drive the pump and it has filters to keep the heat exchanger clear that need to be replaced regularly. A pair of filters can cost £30 or more and should be replaced at least every year. An MVHR system fan should use no more than 1 W per l/s air flow, or 1W per 3.6 m3/hour. With a 5C difference between inside and outside you would be saving 90% of 3.6 x 5 x 0.33 W = 5.3 W of heat, per 1W of MVHR power.

You can also have mechanical ventilation without the heat recovery. The need for filters is less and the power demand is also less. The heat loss is greater but this can be reduced by controlling the ventilation according to need. For example the AERECO system has vents which respond to humidity (without needing any electrical power).

It is also possible to run the ventilation system using natural convection, though this is usually very difficult to fit into existing houses. You need bigger ducts, with as few bends as possible so that the air flows easily under minimum pressure. The driving force is simply that warm air rises, so some heat loss is inevitable. However, in combination with humidity sensing controls this does give you whole house ventilation and good air quality with no extra electricity bills and reduced heat loss.