What Humidity Should a Basement Be in Ontario?
The ideal basement humidity in Ontario is between 30% and 50% relative humidity, and it should not stay above 60%. Below 50% mold has a hard time getting started. Above 60% for days at a time, it will grow on drywall, wood, cardboard and carpet. In most homes summer is the season that pushes the number too high.
What is the ideal basement humidity in Ontario by season
The target range stays the same all year. What changes is which end of it you are fighting.
- Summer: aim for 45% to 50%. Outdoor air in southwestern Ontario is humid from June into September, and the basement is where that moisture settles. Without a dehumidifier, readings of 70% or more are common.
- Winter: 30% to 40% is typical and fine. Cold outdoor air is dry, and the furnace dries it further. A basement rarely needs dehumidifying in January. If yours reads high in winter, look for a water source.
- Spring and fall: watch the gauge. Snowmelt and long rains raise moisture in the soil around the foundation, and a mild, wet week can lift indoor humidity before you think to turn the dehumidifier on.
Why basements run humid in summer
Relative humidity depends on temperature. Warm air holds more water vapour than cool air. When warm, humid outdoor air drifts into a cool basement, it cools down, and its relative humidity climbs even though no water was added.
Basement walls and floors sit against soil that stays cool all summer. Air touching those surfaces is cooler still, so the humidity right at the wall is higher than the reading in the middle of the room. That is why mold shows up first in corners, behind furniture and at the bottom of outside walls. If the surface is cool enough, the vapour turns to liquid and you see sweating on the concrete and on cold water pipes.
This is also why opening basement windows on a muggy day makes things worse. It feels like airing the place out. What it does is bring in more wet air to condense on cool surfaces. Keep basement windows closed in humid weather.
How to measure basement humidity
A digital hygrometer costs about $15 to $20 at a hardware store and is the most useful tool you can own for a basement. A few habits make the readings worth trusting.
- Set it at about table height, away from the dehumidifier, the dryer and any supply vent.
- Leave it a few hours before reading it. Then check it over several days, since one reading tells you little.
- Take a second reading in the worst spot, such as a closet on an outside wall, a storage room or the cold room. Closed rooms often run ten points higher than the open area.
- Do not rely on the display on the dehumidifier. It measures the air at the machine, which is the driest air in the room.
- Pick a gauge that records the high and low, so you can see what happened while you were not looking.
How to keep basement humidity under 50%
Start with the sources of moisture, then add equipment.
- Keep water away from the foundation. Clean the eavestroughs, extend downspouts and make sure the ground slopes away from the house.
- Vent the dryer outdoors and check that the duct is connected and not crushed. Do not hang laundry to dry in the basement in summer.
- Run the bathroom fan during and after showers if there is a basement bathroom.
- Run central air conditioning if you have it. It removes moisture as it cools, and keeping basement vents open helps the air circulate.
- Run a dehumidifier sized for the space from late spring to early fall. Set it to 45% or 50%. Connect a hose to a floor drain or sump so it does not shut off with a full bucket while you are away.
- Keep doors to storage rooms and closets open, or cut in a grille, so dry air reaches them.
- Keep furniture and stored items a few inches off outside walls so air can move behind them.
A dehumidifier controls moisture in the air. It will not keep up with water coming through a wall or floor. If the machine runs constantly and the reading stays above 60%, there is a leak or seepage to find.
Can a basement be too dry?
For mold, no. For comfort and for the house, very dry air has drawbacks. Below about 30% people notice dry skin, static and irritated sinuses, and wood floors and trim can shrink and gap. This is mostly a winter issue, and it affects the upper floors more than the basement. If you run a furnace humidifier in winter, keep it modest. Too much added humidity condenses on cold windows and inside outside walls.
Buy a hygrometer, put it in the basement and write down the reading morning and evening for a week. If it stays under 50%, you are in good shape. If it sits above 60%, start at the top of the list above and work down.
Common questions
Is 60% humidity too high for a basement?
It is the upper limit, not a target. A few hours at 60% does no harm, but a basement that sits there for days gives mold what it needs, especially at cool wall surfaces where the humidity is higher than the room reading.
Should I run a dehumidifier in the basement in winter?
Usually not. Winter air in Ontario is dry and most basements read under 40% without help. Use the hygrometer and only run the machine if the reading climbs.
Does an HRV lower basement humidity?
In winter it helps by swapping damp indoor air for dry outdoor air. In humid summer weather it brings moist air in, so it will not replace a dehumidifier.
Send photos, get a firm quote in 2 hours
Three photos from your phone is enough for most jobs. No obligation.