Humidity in the home – controlling condensation

Some Options

In the British climate humidity and condensation are common problems in our homes. A clammy atmosphere feels unpleasant and also encourages the spread of house dust mites causing respiratory problems through allergic reaction. In serious cases mildew can make an appearance too. The traditional solution of throwing open a window, for example after taking a shower, is still a good one but may be wasteful of heating energy if you forget and leave it open too long. Many of us are concerned about security and are reluctant to open windows at all.

An electric dehumidifier is another solution. It uses a mechanism very similar to a domestic refrigerator to chill the air and condense out excess moisture. The dry air is restored to room temperature before leaving the machine. Distilled water is collected in a reservoir so that you can see how it is performing but this does have to be emptied periodically. The water may be of use for a steam iron for example but it is NOT sterile and should not be used for washing contact lenses. Domestic scale dehumidifier machines have been available for many years priced at around £250 and have been useful in the wetter parts of the country and as an expedient measure in properties with more serious dampness problems.

Recently, much more affordable machines have become available, priced at around £100 and this makes them a useful option in all parts of the country. They are quite noisy machines, like a combination of a fan heater and a fridge and are no substitute for trickle ventilation in dealing with air exhaled in breathing. But for moist events like cooking, clothes drying and showering the dehumidifier makes a lot of sense. (Check on wiring regulations for use in bathrooms). If the outdoor temperature is more than five degrees below the indoor temperature it is more environmentally friendly to use a dehumidifier than to ventilate and turn up the gas heating. If you use green electricity the dehumidifier is always preferable during the heating season.

Like refrigerators, these devices contain liquid refrigerants (freons) which could have climate changing effects if released into the atmosphere. The worst offenders in this context (CFCs) have been banned by the Montreal Protocol. HCFCs are better but, purists might argue, still too risky. But, just as the fridge is accepted as the most practical means of food storage, the dehumidifier may become the most rational means of climate control. Some explanation is required.

In the nineteenth century Lord Rayleigh devised a means for heating houses using a heat pump. He knew that the air outdoors contains plenty of energy but at a temperature too low to be useful to us. By compressing the air it can be heated (as in a bicycle pump) and deliver heat to a house many times more efficiently than gas central heating – theoretically. In practice frictional losses in the machinery severely reduce the advantage and the system can be very bulky because large volumes of air are needed. Also as the outside temperature drops efficiency is further compromised yet this is exactly when we need the heat most. However, Lord Rayleigh’s scheme is starting to be used in the UK as the technology improves.

The dehumidifier is also a heat pump but it uses moist indoor air as its source. This is at a more consistent temperature than outside air and the water vapour is a more concentrated source of heat. So the dehumidifier is also a heating system and the combination of these two services makes the heat pump a viable technology after all. Don’t expect it to set the world on fire – a domestic size machine has a heat output of about 400 Watts – much more than a heated towel rail for example. It can provide a useful heat contribution in a well insulated dwelling with a low rate of ventilation.

For examples of the inexpensive products now available look at Sust-it, they also have a useful calculator for the running costs of Dehumidifiers.

Peter Pope

Peter is an Environmental Engineer and Permaculture Advocate living in Cambridge. He works in housing refurbishment, renewable energy and composting.