Moisture and Mould in Your Home


To avoid the growth of mould in your home you must control the humidity in the home.

Humidity in your house can come from the outside (like from basement leaks), or from the inside. For the purpose of this article, we will deal only with humidity in the home which is created from within the home.

All humans like a certain amount of humidity. With humidity we feel warm and comfortable, but too much of a good thing is this case is also bad. During the winter months our homes are sealed off from the outside and we trap moisture in our homes. This moisture comes from showers and baths, cooking, plants and aquariums, and breathing. All this humidity becomes trapped in the home and starts to level starts to elevate.

As highlighted in the CMCH publication “Moisture and Air – Homeowner’s Guide”, “during a typical heating season, lasting 200 days, when your home is typically closed up, 2,000 to 10,000 litres (400 to 200 gallons) of moisture can be trapped”.

During cold days the moisture begins to condense on windows, walls, attic spaces and with time mould can start to grow. To avoid most of the mould problems, keep materials dry! A way to do this is to control the humidity levels in the house.

Some things you can do if you notice that your windows are always sweating and moulds starting to appear in poorly ventilated areas are:

• If you have a humidifier (built into you heating system or individual room units), lower it’s setting. During very cold spells the relative humidity settings should be no more than 30% or lower if you can manage.

• When you cook, make sure that you turn on the exhaust fan. If you don’t have and exhaust fan consider installing on. Note: If the home is well sealed and the exhaust fan is a strong one, it can produce a negative pressure in the house and not allow the gas appliances (furnaces, fireplaces, water heaters) to vent to the outside properly. This can be dangerous to the occupants as Carbon Monoxide levels elevate. Have a home inspector or a heating contractor check for this if you are not sure!

• Always run your bathroom exhaust fans when you shower or take a bath and for some time after. This will make sure that the humid air is expelled.

• Ensure that aquariums have covers on them and that plant quantities are kept to a safe level. Both these items can contribute substantially to the humidity in the home.

• Don’t overwater or plants or have them sitting in pot saucers full of water.

There are many publications available for free which help you deal with high moisture levels and mould. Some of these can be downloaded CMHC website or which can be mailed to your home. A link to CMHC website is available on this website’s under the Resource” heading or contact the inspector at