Damp house have been proved beyond any doubt to cause respiratory infections, allergies and asthma.

Visible mould in the home is a problem, however most of the toxic mould floating around our air is invisible to the naked eye.

When we understand what causes it to occur, we can work out how to prevent it.

To begin, lets understand humidity and water vapourĀ in relation to the home.

Water vapour is simply water that is ‘diffused’ or ‘suspended’ in the air. Humidity is ‘the amount’ of water vapour in the air.

Mould LOVES humidity.

Older houses may have changed A LOT since construction. They may not be as well ventilated as they used to be.

For example. the original fireplace may be bricked up now, or an electric fire may be installed in it’s place. Double glazed and Upvc windows and doors may have replaced old drafty wooden ones. Coal fires were used to heat water and keep the house warm. The fire would burn all the old breathed air, along with any water vapour and suck it up the chimney. Fresh air would replace it, being sucked in through the original poorly fitted and drafty windows and doors.

Below: Condensation on a window means the house is humid, there is a high level of water vapour in the air.


Lets also consider the social change we have seen in recent decades.

In the old days when water had to be heated with coal fires, it was a chore. People had less baths and often shared the water, The amount of water vapour being produced via bathing was low. By the same token laundry day was a similar trial. Most people dried their clothes outside because there were no tumble dryers or radiators to dry them indoors. The amount of water vapour produced from drying clothes was massive, but it was not produced in the home. Even when it was, it went up the chimney. Nowadays people shower every day and never wear the same clothes twice without them being washed. Hot water is available instantly; the kettle, the shower, the steamer, fryer etc…

These two revolutionary changes in housing and living standards combine to result in dwellings with very high humidity. This creates a tendency towards condensation and mould growth. There are of course other issues but those mentioned are the most important.

No ventilation + much more water vapour = Mould City.

Below: Visible mould around a window frame, additional thermal image (via Intec Analysis)

What can we do?

Understand that excessive humidity is the cause of mould growth. Then identify how we humidity can be lowered.

Kitchen, ensure the fan/cooker hood is working correctly and use it frequently. Fans extract air (and the water in it). When working in the kitchen make sure the door is closed and the fan/cooker hood is on to remove the water vapour and prevent it travelling through the home.

Bathroom, when showering or taking a bath, keep the door closed and use the extractor fan. If there is not one installed, open the window to get that water vapour out of the house. A hot shower or bath will produce trillions of water molecules. These are warm (energetic) so they skip from liquid state into air and form water vapour. Extractor fans in bathrooms need to stay on longer to shift huge amounts of water vapour. Consider installing an automated one.

Windows, check the trickle vents, a small horizontal vent at the top of each frame. These should always be open as they provide a route in for ‘make up’ air. When the extractor fans are working around the house these small vents are handy for pulling in fresh, dry air to replace the old air being pulled out by the fans. A lack of vents or having them closed all the time will encourage drafts at lower levels (where you will feel it). This will reduce the efficiency of the extraction fan. Open them all.

Radiators, do not use all the radiators around the house to dry clothes. Wet clothes feel heavy because they are wet. When they are on a radiator the water evaporates into the air then travels to the coldest spot in a room and condensate back to water causing unnecessary problems. If you must dry clothes on radiators use the kitchen or bathroom and use the extractor fans.

Below: Invsible mould around a window frame, additional thermal image (via Intec Analysis)

A cold house is a damp house!

Mould is almost exclusively a problem of the internal environment. Heating, or more precisely lack of heating raises humidity because the ability of air to comfortably hold water vapour is entirely dependent on heat. Warm air can hold a lot more water vapour than cold air. In effect if we cool the air, then we raise its humidity relative to its temperature. Mould will thrive in a house with a high relative humidity.

Thanks for reading.