Keeping it dry

Windows have the coldest surfaces in most houses, and will usually be the first place condensation is seen.

If your house has too much humidity in winter time, and it is in a place with very cold winters, it is easy to to lower your indoor humidity.  If you see condensation other than in the bathroom a few minutes after a shower, turn off any humidifiers, including turning off the water line.  Bathroom exhaust fans should be kept running until all signs of condensation in the bathroom are gone, at least 30 minutes.

If your humidifier is already turned off, or there isn’t one, and you see persistent signs of too much humidity, bring in outside air.  Condensation means the air has reached 100% relative humidity where you see condensation has formed. Relative humidity is the measure of how much moisture air can hold at a given temperature, and unlike sales talk or motivational speeches, there is no such thing as 110% relative humidity, anything over 100% becomes a drop of water or ice.

Relative humidity means cold air holds less water than warm air, warming air increases its ability to hold water, so cold air warmed up will have a lower relative humidity and less likely to form condensation.  A cubic meter of outdoor air with a relative humidity near 100% at -20 C, holds about .9 gm water.  When heated to +20 C, that same cubic meter of air with .9 gm water vapor has a relative humidity of about 5%.  Even a small amount of cold outside air will reduce indoor relative humidity significantly.

Always use exhaust fans ducted outdoors when taking showers or cooking meals.  Canadian homes built after 2000 and many built before may have some form of whole house ventilation system that exhausts stale and humid interior air while replacing it with fresh dry air from outdoors.  Use it, the most basic are controlled by a switch, usually located in a hallway, that turns on an exhaust fan and the furnace fan.

Rusty hardware in this bathroom is the result of not using the exhaust fan and excess humidity


If there is condensation on the windows or bathroom walls turn the switch on and leave it on till the condensation is gone.  More sophisticated systems have air handlers that capture the heat from exhausted air and use it to heat incoming air.  These efficient heat recovery systems are best  left on when the house is occupied.  CMHC recommends that homes exchange inside  air with outside air about once every three hours.  If you have heat a recovery system (HRV), it is most likely there because your house has been built ‘tight’ and you can’t expect inside and outside air to exchange naturally through air leakage as it would in older homes.

If your home does not have a ventilation system to bring in outside air, and you are seeing signs of high humidity, use any exhaust fans you may have, and open a door or a window for a few minutes.

Make sure that all parts of the house are more or less the same temperature, especially rooms that are colder than the temperature setting of the thermostat.  Try to keep all rooms, even unused rooms at the same temperature.  Do not close heat registers to save money, it does not work, and may damage your home, cold areas are the first place condensation forms.  Do open or close heat registers to maintain a consistent temperature throughout the house.

Use hygrometers to keep track of relative humidity, keep one in the basement and one on the main floor.

Try to eliminate or control the factors that can create high humidity, too many people, too many house plants, too much cooking, too long hot showers.

Old school hygrometer, a nice find at value village
Electronic hygrometer about $20 or less