Dry air is good air


“Put an end to desert like air in your home” says the flyer containing an ad from a well known maker of heating and ventilating products. Reading further, “The average humidity in the Sahara Desert is 25 percent. In wintertime the average humidity inside some homes can be even lower….”

Yes it can, does that mean you should rush out and buy a humidifier? Not necessarily. Some of The biggest problems in homes are the result of excess humidity. CMHC recommends that humidity levels in a home in very cold weather (ie Edmonton) never exceed 30%, pretty close to that desert like air. Desert air is not only healthy, but it also preserves things, like the building materials in your home.

Excess indoor humidity results in condensation, and condensation results in, mold growth, wet insulation, wet wood, and the destruction of all building materials that need to be kept dry.

Excessively dry air on the other hand, can shrink warp or crack  solid wood furniture or flooring, dry out paper, and some textiles, make you feel colder, dry your skin,  or even create health problems for some people.

Health experts and those who sell or install hardwood flooring will recommend humidity levels of between 35 and 60%, but those levels are too high for homes in the extremely cold winters we have in Edmonton.

To understand why the ideal of 50% relative humidity is not so ideal for cold winter homes, the dew point for air containing 50% relative humidity at 20 degrees C is 10 C. Dew point is when water vapor becomes condensation.  In other words if you keep your home at the usual indoor temperature of 20 C, and your indoor humidity at 50%, condensation will form on anything that has a temperature of 10 C or less.  Like the surface of your double glazed or double pane windows when the temperature outdoors is around -18 C.  If you see condensation or frost inside your house the humidity is too high.

Next;  Keeping it dry

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