Condensation is unpleasant and unattractive and the mould growth as a result of it can have serious issues for our health, especially for people with breathing difficulties from asthma or other respiratory issues. It is an increasing problem as our homes used to be cold and drafty, which whilst not good for our comfort, pretty successfully prevented moisture from building up. However, as we keep our houses warmer and try to prevent drafts, it gives rise to problems with air circulation.
The reasons for it are usually pretty straight forward, and it should not be an insurmountable issue in any house. First, bear in mind that our houses are constantly producing moisture which needs to go somewhere. An average 4 person household will produce a staggering 112 pints of moisture a week from a combination of cooking, showers, boiling the kettle and just breathing!
If we are going to try to avoid it there are just a few things we need to remember.
- Temperature. Warm air holds more moisture than cold air. When warm air cools or comes into contact with a cold surface, the water will condense and settle on the cold surface.
- Ventilation. Moving air carries away moisture. This is why a breeze cools us down and fans help in an environment with high humility.
- Humidity. The dew point of air (the temperature at which water condenses) reduces as humidity decreases, so if you can reduce the water content in the air, it is less likely to condense on a surface.
Bearing these basic principles in mind there are a number of things we can do to reduce the risk of condensation in a house. They all break down into a combination of lifestyle, passive and active fixes. The first two tend to prevent the build up of moisture in the first place and the third deals with the problem once it has occurred. If a lifestyle fix can be used, it is always preferable because it doesn’t cost anything, and a passive fix has a one-time rather than ongoing costs so is preferable to an active one. The problem with the active solutions is that they don’t deal with the root cause but just the symptoms, and so should be less of a priority.
Lifestyle fixes
1 – Heat the house consistently. Keep the whole house at a similar temperature, so that water vapour which is absorbed into the air in the living room doesn’t pass into the bathroom and condense on the surfaces in there.
2 – Heat the house continuously. Try to keep the house at a fairly stable temperature. Keep the thermostat at 15 while you are out so that the building doesn’t cool down dramatically, because this leads to the water in the air condensing and then evaporating when the heating comes on again. Keeping the air at a high enough temperature will physically prevent the moisture in the air from condensing on surfaces. It also means that the building warms up quickly when you get home!
3 – Cover pots when cooking. This will save on the gas or electricity needed to heat your vegetables and cook them quicker because the water gets hotter and they steam a little. Tests show that a pan boils in 4 minutes and 15 seconds with the lid on and 4 minutes and 45 seconds with the lid off. Not a big saving notably, but over a period of time it will save money and time. Families who do a lot of cooking can have significant issues with this.
4 – Dry clothes in a separate room. If possible, dry clothes in a dedicated room and leave the window open while the clothes are drying, or just open them intermittently to let out the moist air without wasting too much heat.
5 – Separate the utility room from the kitchen. Put the washing machine and tumble dryer in a separate room with its own extractor fan and keep the door closed when in use so that the moisture doesn’t get into the rest of the house.
6 – Open windows in good weather and close curtains at night. Get rid of the stale, damp air every now and then by opening the windows when the weather permits. This gets rid of all the moisture which is stuck inside. Also remember to close the curtains at night to keep the heat in and so stop the house from cooling down at night. Thick or lined curtains are the best, although you can also get insulated blinds which are shaped like a fan. All of these help keep the house warmer.
Passive fixes
7 – Seal up gaps around windows and doors. Increase the internal temperature by sealing up gaps prevents drafts. This will also save you money on your heating bills. Seal them from the outside with mastic sealant between the window and the wall, and from the inside with a strip of sticky-backed foam to make the joints in your windows more air-tight.
8 – Install trickle ventilation. Increase natural ventilation in your home by installing trickle ventilators in the windows and doors. These can be retro-fitted into the frame of a UPVC window, and are essentially a few holes drilled in the plastic frame, with cover plates on the inside and out. They are not the most attractive things, but are cheap, at around £4 each and can provide some much-needed ventilation. These are not as good as windows which have them pre-installed, but they are a lot better than nothing.
9 – Undercut your doors. Improve natural ventilation throughout your house by cutting a little off the bottom of the doors to allow air to move around the house more freely, and prevent moisture from building up in any particular area. A 10-15mm gap between the underside of the door and the floor finish is ideal.
10 – Use absorbent materials. Salt or water-retaining crystals can be placed in bowls to absorb the moisture from the air. They can be placed on window sills where the condensation is most likely to occur, and bear in mind that they need to be periodically emptied so that they can carry on being effective.
11 – Improve insulation. Increase the internal temperature by adding insulation to your home. This can be added to the walls, roofs and floors and will help keep your home warm. Refer to the article on Insulating Your Home which gives more detail on the options for doing this.
12 – Install double glazing. This also helps increase internal temperature, and the two layers of glazing reduce the temperature differential between the two sides of the glass which means that moisture is less likely to condense on the surface. Make sure that the windows have trickle vents installed!
13 – Vent a tumble dryer. Make sure that a tumble dryer is vented to the outside air if it is designed to be used with one. Ideally these should be located against an external wall and duct straight to the outside, but they can be ducted through the house in a similar way to an extractor fan, or you can buy a condenser which connects to the end of a duct which collects the condensate (available from most DIY stores) but they are not recommended by the manufacturers because they are not always effective and can still lead to significant moisture build-up, but they are used widely and are a cheap and convenient solution to a problem (all be it not a long term solution) and the strictly correct method can be complex and expensive.
Whilst long ducts for extractor fans are common, they are more of an issue with dryers as the air from a tumble dryer tends to be carrying lint with the expelled air which can settle in the pipe over time. You should firstly consult the manufacturer’s literature to see the maximum duct length for the design of your vent, i.e. with the number of bends needed for your installation. It should also be possible to clean the duct occasionally (probably once a year for a domestic installation). Then decide whether you are going to use flexible or rigid metal ducting, as rigid ducting allows longer ducts to be used because it has no ribs for the lint to collect on. Whilst plastic ducting is smooth and so should suit dryer ducting, it tends to have a static charge which leads to lint collecting on the inside of the pipe. If you have serious issues, you can also install a pump into the route of the duct to force the air through at a higher pressure.
Active fixes
14 – Buy a hygrometer. These are a cheap way to monitor the level of humidity in the home, so that you can see when you need to take action. You should try to keep the humidity level 50%, and run extractor fans to remove the humidity levels when they rise above this. At around £20 they are good value, and let you know the extent of the problem.
15 – Use extractor fans and hoods. These remove air from the areas where it is most likely to be damp, i.e. the kitchen and bathroom, and prevent it from moving into the rest of the house. Set the overrun to be 15-20 minutes after the light is switched off rather than the standard 5. If you have a kitchen or bathroom without a working extractor fan, then it does not meet building regulations at the moment, and installing or replacing one can be a good idea. At £20 for a basic model, these are a great way to get rid of unwanted humidity and smells.
16 – Upgrade your extractor fan to one with a humidistat. These monitor the humidity in the air and only turn off when it has been reduced to a suitable level. These are more precise than a simple timer overrun, although they can be quite a lot more expensive. There are also issues reported with the reliability of the humidistats, meaning that a lot comes down to the setup, as they can be set to run too long or not enough. These cost around £40, so are a good value solution.
17 – Upgrade your extractor fan to a dMEV unit. This stands for a decentralised Mechanical Extract Ventilation unit. As modern houses are increasingly airtight, there is an increasing concern over air quality, and if unventilated, carbon dioxide levels rise significantly, especially over winter. These provide a constant extraction of air at a much slower rate than a standard extractor fan, with a boost for when they are needed (i.e. activated by the light). They run very efficiently and so have a minimal cost, (around £4 per year) and are silent in low power mode so that the noise does not become a nuisance. These cost around £120, which whilst being significantly more expensive than a standard model, are a lot cheaper than a full MVHR system. Combining this with undercutting the doors allows the whole house to benefit from this continuous background ventilation.
18 – Dehumidifier. Dehumidifiers take the moisture out of the air by passing the air over a cold element so that the water condenses on it, and the heat is expelled back into the room. As you can imagine, these are expensive to run because they use a similar amount of electricity to an air conditioner. In actual fact, the only difference between an air conditioner and a dehumidifier is that the warm air from an air conditioner is discharged outside to cool the room down, whereas a dehumidifier discharges the heat back into the room so that there is no change in air temperature. In this way they are a short term fix, suitable for use on a building site where you have a large amount of air to get rid of at one point in time, but if the moisture build-up is constant, something more fundamental needs to be done.
19 – MVHR (Mechanical Ventilation with Heat Recovery). The most expensive solution is a whole house ventilation system, costing £3-4,000. This provides extraction and supply and a heat exchanger makes a very efficient heat transfer from the outgoing to the incoming air, so that they can be a great part of a sustainability strategy for a house, but they rely on the house being highly airtight and insulated before it would deliver energy savings (an air tightness level of 3 air changes per hour would be sufficient). It definitely doesn’t make sense to install one primarily as a means to deal with condensation.
20 – Last resort. Sell one of your children. This will reduce the humidity levels and help with the finances as well!