Where damp is present in a home it is usually as a result of high humidity in the air. Humidity depends upon water content of the air but also air temperature. If the air temperature is higher it can hold more water compared with when it is colder, and a lot of that water needs to be removed from the home to prevent it dropping out of the air as the air cools (e.g. at night when heating is reduced) or onto cold surfaces such as walls, windows (seen as condensation) and exposed cold water piping.
Where does the water come from? We breathe it out but also sweat to vapour surprisingly quickly - an interesting exercise is to place your hand into a plastic bag and seal it around your wrist, completely enclosing your hand. Within minutes your hand will feel warmer and the bag will start to 'steam up' as water vapour from your hand condenses on the inside of the bag. Other major activities that send litres of water into our indoor air are clothes washing and drying in the house, showering, cooking.
If all that water vapour isn't released into the outside air it will build up resulting in the appearence of damp in your home. The first clue is often windows dripping with condensed water. Most of us open windows to release the excess water vapour, closing them again once all is dry but under some circumstances that is impossible e.g. in below ground-level basement areas of your home. If ventilation cannot be improved then a dehumidifier is often used in an attempt to reduce air moisture but how well does that work?
This technical paper asked questions about the effectivness of a dehumidifier in reducing humidity in 30 basements all through the year in Canada and concluded that they were effective at preventing peaks of humidity in the summer months. However the dehumifiers used for the study were powerful with a water removal capacity of 31 litres per day (1.3 litres an hour). At peak use they were removing up to 750ml of water per hour from the air. To cut energy use (>250-300W ~ 60-75p per day in the UK) each unit was controlled by a humidistat that was set at 50% Relative Humidity (RH) - once reached the unit turned itself off, saving part opf the daily cost.
Many of the dehumidifiers available for sale in the UK are cheap and have very low capacity which we could not expect to perform as well as the units used in this study. If you have a problem with damp that a dehumidifier could help with then the purchase of a unit that is powerful enough to do the job is important. Those with the capacity and features mentioned in the study cost around £250 for use in a single basement. Smaller units are cheaper to buy as they store less water and have to be emptied out more frequently but as a rule of thumb should still have the same rate of water removal - at least 20 litres per day at 30C and 80%RH.
NB other small devices use absorbent materials to collect water. These are generally also of low capacity and suitable only for small areas of the home.
NOTE: as with any device that attracts water, dehumidifiers need to be cleaned regularly to avoid fungal growth.
Dehumidifiers will work well to reduce damp as long as they have a high enough capacity to perform well in the area required. Expect to pay >£100 for a small unit. They are expensive to run (~£20 - 30 per month) even for quite small parts of your home though most of the most powerful units switch themselves off automatically when ideal humidity is reached, reducing costs somewhat.
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