Ideal indoor humidity for an Ontario home

What relative humidity means

Relative humidity is the amount of water vapour in the air as a percentage of the most the air could hold at that temperature. Warm air can hold much more than cold air. That one fact explains most household moisture problems.

When warm, moist indoor air touches a cold surface, such as a window in January or a basement wall in July, the air next to the surface cools, its relative humidity rises to 100 percent, and water condenses. Mold does not need visible water. It can grow on surfaces where the humidity stays high for long enough.

The targets

Health Canada recommends keeping relative humidity in a home between 30 and 50 percent. In Ontario’s climate that works out as:

• Summer: no higher than about 50 percent
• Winter, or cold weather: about 30 percent

The winter figure is lower because surfaces are colder. In a very cold snap, even 30 percent can put water or ice on older windows, and the practical target becomes whatever keeps the glass clear.

Signs it is too high

• Condensation, frost or ice on windows
• Water pooling on window sills, and mold on frames
• Mold on outside walls, in corners and in closets
• A musty smell
• Damp or clammy air
• Peeling paint and wallpaper
• Frost in the attic in winter
• Mold or mildew in the basement in summer
• More dust mites, which matter to people with allergies and asthma

Signs it is too low

• Dry skin, lips and nasal passages, and nosebleeds
• Static shocks
• Gaps opening in hardwood floors, and cracks in wood furniture and trim
• Musical instruments going out of tune or cracking
• Houseplants struggling

Low humidity is uncomfortable. High humidity damages the house. When in doubt in winter, err on the dry side.

Measuring it

Buy a hygrometer. They are inexpensive and many also show temperature.

• Place it in a main living area, away from the kitchen, bathroom, humidifier and direct sun
• Use a second one in the basement, and one in a problem bedroom
• Check at different times of day
• Cheap units can be several points out. Look at the trend more than the exact figure.

Where indoor moisture comes from

A household adds litres of water to the air every day:

• Breathing and perspiration
• Showers and baths
• Cooking, especially boiling and gas burners
• Dishwashing
• Drying laundry indoors
• Houseplants and aquariums
• Humidifiers
• Damp basements and crawl spaces
• Firewood stored indoors
• New construction and renovation, as concrete, drywall compound and paint dry

More people in a smaller, tighter home means higher humidity.

Lowering humidity in winter

• Use exhaust fans. Run the bathroom fan during a shower and for about 20 minutes after. Use the range hood when cooking. Both must vent outdoors.
• Use the ventilation system. If the house has a heat recovery ventilator, run it. It swaps moist indoor air for dry outdoor air and recovers the heat.
• Turn the humidifier down or off. A furnace humidifier set for mild weather will soak the windows in a cold snap. Follow the setting chart, or lower it when the windows fog.
• Vent the dryer outdoors and do not hang laundry inside.
• Cover pots when cooking.
• Move air. Keep interior doors open, open blinds and curtains in the day, and leave a gap behind furniture on outside walls.
• Keep the house evenly heated. Unheated rooms have cold surfaces.
• Open a window briefly on a dry, cold day to exchange air, if you have no ventilation system.

Cold outdoor air is dry air once it warms up indoors, so ventilation lowers humidity in winter.

Lowering humidity in summer

In an Ontario summer the outdoor air is the humid air. Ventilation makes it worse.

• Run the air conditioning. It removes moisture as it cools.
• Run a dehumidifier in the basement, set to 45 to 50 percent and drained continuously. See do I need a dehumidifier in my basement.
• Keep basement windows closed in humid weather.
• Keep using bathroom and kitchen fans.
• Do not let the house get very cold with the air conditioning and then open the windows. Humid air condenses on the cool surfaces.

Raising humidity when it is too dry

Older, leakier houses can become very dry in winter.

• A portable or furnace-mounted humidifier, used with a hygrometer and kept clean
• Keep it at or below about 30 to 35 percent in cold weather
• Clean portable units as the manufacturer directs. A dirty humidifier spreads bacteria and mold.
• Replace the pad in a furnace humidifier each year, and shut its water and damper in spring
• Sealing air leaks reduces how much dry outdoor air comes in

Windows as your gauge

Windows are the coldest surfaces in most houses, so they show a humidity problem first.

• A little fog at the bottom of the glass on a very cold morning is normal for many windows
• Water running down the glass, wet sills or ice means the humidity is too high for those windows
• Fog between the panes is a failed seal, not a humidity problem

New, better windows tolerate higher humidity. But new windows and air sealing also make a house tighter, so humidity often rises after a retrofit and ventilation has to be improved to match.

Room by room

• Bathrooms: a working fan, vented outdoors, on a timer
• Kitchen: a range hood that vents outside, not a recirculating one
• Bedrooms: doors open or undercut, furniture away from outside walls, blinds open in the day
• Basement: dehumidifier in summer, nothing stored against the walls
• Attic: it should be cold and dry in winter. Frost means house air is leaking in. See attic condensation and frost.
• Crawl space: ground covered with a sealed vapour barrier

When humidity will not come down

Persistent high readings despite fans and good habits point to a source:

• A damp basement or crawl space
• A plumbing leak
• A disconnected dryer or bathroom fan duct
• An oversized humidifier, or one stuck on
• A ventilation system that is off, unbalanced or blocked
• A house that has been made airtight without adding ventilation

More: mold removal and remediation.