Draughtproofing and Insulation (eBook)

Chapter Three

Ventilation, Draughts and Condensation

There are conflicts between ventilation, draughts and preventing condensation. We see that good ventilation is crucially important. The adage ‘build tight, ventilate right’ is a very good one and we could extend it to say, ‘live airtight, ventilate right’. It is especially important to ventilate underfloor voids and most lofts, roofs and chimneys (see Chapter 7).

We often come across people who say they have no condensation in their homes, and we suspect they may be living in very draughty buildings. Others think that draughts are good because they get rid of condensation.

A more constructive way to look at it, however, is that draughts are bad because they take away a lot of heat from the home, and that it is far better not to have any draughts and to open windows or switch on fans when ventilation is needed. The reason for this is very simple: you don’t need to ventilate all the time. We certainly don’t need to run a bathroom fan or have a window open all day; in fact, we like them to be closed most of the time, especially in winter. They should be opened for a short period every morning in bedrooms and the rest of the home should be ventilated as and when required.

So how should we get rid of condensation? We advocate opening bathroom or shower room windows or running a fan for a short period of time after the room has been used for bathing or showering. Additionally, we suggest squeegeeing or wiping tiled surfaces, shower doors, screens, and so on after use. If carried out it immediately, this simple action removes most of the water that has condensed on those surfaces, meaning that less ventilation time is required to dry the surfaces and a lot less heat is lost.

Draughts from Fans

Extractor Fans

Extractor fans are now a statutory requirement and must be fitted in all new bathrooms, utility rooms and kitchens. Herein lies a huge problem. Extractor fans do exactly what they say they do: they extract air from the home, dumping it outside. The drawback is that they take out heated air and throw it away. In a completely airtight building, an extractor fan would extract a small amount of air from the building, depressurising it slightly, and then do nothing except make a noise. There is something illogical going on when we combine the idea of airtightness with an extractor fan. Sadly, in the real world this is never a problem because our buildings are so leaky that air is sucked in through all the little gaps and cracks that we at DraughtBusters strive to seal up.

My own house was designed to be very airtight and is probably one of the most airtight masonry homes of its age in the country. I installed a mechanical heat recovery ventilation system (MVHR). I have a big box with two fans in it in my plant room. Inside it there is a heat exchanger that strips the heat out of the outgoing air stream and transfers it to the incoming fresh air stream. New fresh air flows into all my living rooms and bedrooms through outlets in the ceilings of each of those rooms and stale, used air is extracted from bathrooms, cloakrooms, utility room, hallway, plant room and kitchen. I don’t have a heating system and can’t afford to throw away any heat; by using an MVHR, the air in my house is always lovely and fresh and we don’t need to open windows – although my wife still loves opening windows in the morning!

Extractor fans in the homes of those in energy poverty never get switched on and sometimes they are blocked up deliberately by tenants or homeowners. This is bad for both the building and the occupants. As soon as there is mould around, there are health hazards, which can be very serious, and the fabric of the building starts to decay too.

One of the main reasons that people block up extractor fans is because they are draughty. We often advise our clients to keep the bathroom or shower room door shut after use until everything has dried out. We even advocate opening a window very slightly in bathrooms and shower rooms and keeping the door shut to try to prevent damp, mould and condensation. This strategy does work, but it needs help: a diligent homeowner who is prepared to squeegee and wipe surfaces after showering or bathing and then hang the cloth or towel outside and not on the radiator will have fewer problems and less often than one who doesn’t.

Extractor fans are available that have shutters on them that open and close depending on whether the fan is on or off; these are considerably more expensive than the fans that are usually fitted, which are the cheapest ones available from the supplier.

Cooker Hoods

I was recently helping a friend to seal some stains on their kitchen ceiling. We used gloss paint, which was quite smelly, so I switched on their cooker hood to try to help evacuate the fumes from the paint more quickly. The cooker hood didn’t like it, however, and soon started flashing an error code. I later discovered that they had put a big ball of polythene in carrier bags inside the cooker hood between the filters and the fan, as it made the kitchen cold whether it was switched on or not, particularly if it was windy outside or in winter. Not surprisingly, when I then tried to use it, the fan motor got a bit too warm and switched itself off! Thankfully it worked again once the fan motor had cooled down, and I returned later to do the sealing.

So, what can be done about cooker hoods? They can have positive shutters so that they are closed when switched off; it would be sensible if the regulations required this. When it’s very cold, you can push a big ball of polythene into the outlet pipe to stop draughts – just don’t forget it’s there when you need to use the hood! Alternatively, find an automated choke, throttle or electrically operated shutter for the outlet duct or terminal.

Bathroom and Utility Room Fans

These fans sit in a 100mm- or 150mm-diameter hole in the wall or ceiling of the room in question. That hole, even with a fan in it, causes draughts. This should not be the case, so ideally the requirements would be changed so that all extractor fans have positive close shutters on them, not the ones that flap and blow around in the wind and still let air out any time the wind gusts strongly enough. They need to be able to shut off the air flow, either by closing the pipe with an iris shutter or with an electrically operated choke. Sometimes we can see daylight through extractor fans, which means that they will be draughty.

Airtightness

Airtightness is a measure of how much air leaks into or out of a building. So that results can be easily compared, this is done at a pressure difference of 50 pascals (Pa). To measure airtightness, we install a fan into the building that can either pressurise or depressurise the building, and then measure the volume of air that is needed to maintain that pressure. The results are expressed as either air changes per hour at 50 Pa, or as cubic metres per hour per square metre of the envelope area. In the UK, the results usually fall in the range between ten and one. It is very difficult to find houses with results at the lower end of this range. I worked very hard on my own house, which is of masonry construction but wet plastered, and the result was just less than one. In Canada results commonly range from 0.3 to below 0.1. For new houses in the UK tested using this process, we usually now get results in the range of three to five or above.

Airtightness sealing is the process of finding and sealing up all the little gaps and cracks in the envelope, and is normally done during the construction process. It is very difficult to do it once the building is finished because some of the gaps are impossible to get at, for instance between the floor and the ceiling, in a pipe box or behind the plasterboard linings.

Trying to heat a leaky building is difficult and wasteful of energy.

Figs 3.13 and 3.14 show the blower door test being carried out on my house. The engineer told me he frequently can’t pressurise new houses to 50 Pa and was surprised when I guaranteed that he would be able to get mine there. He then had problems turning the fan speed down very low and had to fit all his flow restrictor discs to be able to measure the volume of air my house used to keep it at 50 Pa. The process of testing, as mentioned above, is carried out by both pressurising and depressurising the building. Fitting restrictor discs reduces the volume of air that the fan can shift and is a very good indication of airtightness.

Condensation

Condensation has many causes: lack of ventilation, missing insulation, very cold indoor surfaces, high humidity, drying washing indoors, cooking (particularly boiling), showering and bathing, and even breathing, as well as any other activity that adds moisture to the air inside a home.

The area of damp mould and condensation shown in Fig 3.15 was caused by a lack of ventilation. A chest of drawers had been pushed into the corner of the room. It was one of those that have a plinth and chipboard sides that come right down to the floor, so there was no air flow underneath it,...