Ideal Indoor Humidity in Winter in Ontario: Levels, Condensation and Fixes

The ideal indoor humidity in winter in Ontario is usually around 30–40% relative humidity (RH), dropping to about 25–30% when outdoor temperatures fall to −20 °C or below. That range is comfortable for most people, and low enough to avoid condensation on windows and moisture problems in walls and attics.

Humidity is one of the most misunderstood parts of winter comfort. Some Ontario homes are uncomfortably dry; others, especially newer and well-sealed ones, are too humid and grow mould on window frames. The fix depends on which problem you have.

Why does indoor humidity change in winter?

Cold outdoor air holds very little moisture. When it leaks into your house and warms up, its relative humidity drops sharply. Air at −10 °C and 80% RH, warmed to 21 °C, ends up at only about 8–10% RH. Leaky older homes therefore tend to feel very dry in winter.

Tight homes behave differently. Cooking, showers, breathing, plants and drying laundry add several litres of water vapour a day. Without enough ventilation, it builds up and condenses on the coldest surfaces: windows, exterior wall corners and closets on outside walls.

What is the ideal indoor humidity in winter?

The colder it is outside, the colder your window glass gets, and the lower indoor humidity must be to avoid condensation. This table is a common guideline for homes with typical double-glazed windows; very efficient triple-glazed windows tolerate higher humidity, and old single-glazed windows less.

Outdoor temperature	Suggested maximum indoor RH
Above 0 °C	~40–45%
0 °C to −10 °C	~35–40%
−10 °C to −20 °C	~30–35%
−20 °C to −30 °C	~25–30%
Below −30 °C	~20–25%

Treat these as starting points. If windows fog or frost at the bottom edge, you are too high for that day. If skin, eyes and sinuses are irritated and static shocks are frequent, you may be too low.

Tip: Buy a digital hygrometer for $15–$30 and put it in a main living area away from the kitchen and bathrooms. Check it daily for a week in cold weather before changing anything.

What if your house is too humid?

Signs include fogged or frosted windows, water pooling on sills, mould on window frames or in closet corners, musty smells, and frost on attic nails or roof sheathing.

Fixes, roughly in order of cost:

1. Use bathroom and kitchen fans during and for 20–30 minutes after showering and cooking. Check that they actually vent outdoors.
2. Turn down or off any humidifier. Many furnace humidifiers are left on a high setting all winter.
3. Run your HRV or ERV continuously on low, and adjust the dehumidistat. See our HRV and ERV guide.
4. Remove moisture sources: vent the dryer outdoors, cover sump pits, avoid drying laundry indoors, and limit firewood stored inside.
5. Open window coverings at night slightly, or leave a gap at the bottom, so warm room air reaches the glass.
6. Fix basement moisture: grading, downspouts, and sealing foundation cracks. A dehumidifier helps in damp basements, even in winter in some homes.
7. Upgrade the worst windows. Very cold glass or frames condense first. See our windows and doors guide.

Caution: Frost or moisture in the attic, ice dams, or rot at the eaves often mean warm, humid indoor air is leaking into the attic. That is an air sealing problem, not just a humidity problem. See our ice dams and attic ventilation guide.

What if your house is too dry?

Signs include RH below about 20–25% on a hygrometer, static shocks, dry skin and sinuses, wood floors gapping, and furniture joints loosening.

The first fix is often air sealing, not a humidifier. Every bit of cold air that leaks in dries the house out. Sealing the attic floor, rim joists and obvious gaps reduces the air exchange that strips moisture. See our air sealing guide.

If the home is still dry, a humidifier can help.

Humidifier type	How it works	Pros	Cons
Bypass (furnace-mounted)	Water trickles over a pad; furnace air passes through	Low cost, simple	Uses water down the drain; only works when furnace runs
Fan-powered (furnace-mounted)	Built-in fan pushes air through the pad	More output than bypass	Slightly higher cost and power use
Steam (furnace-mounted or standalone)	Electrically boils water	Precise, high output, works with heat pumps and low-temperature air	Highest electricity use and cost
Portable room humidifier	Ultrasonic, evaporative or warm mist	Cheap, targeted	Must be cleaned often; ultrasonic units can spread mineral dust if filled with tap water

With heat pumps, supply air is cooler than with a gas furnace, so evaporative humidifiers can be less effective. Steam humidifiers work regardless of air temperature, but they add electricity use.

How do you set a furnace humidifier?

Many furnace humidifiers have a manual dial or a controller with an outdoor sensor. If yours is manual:

1. Set it according to the outdoor temperature table above, and turn it down as cold weather arrives.
2. Check windows each morning during cold snaps.
3. Replace the evaporator pad at least once a season; mineral buildup reduces output and can grow bacteria.
4. Turn the water off and close the bypass damper (if fitted) for the summer.
5. Include the humidifier in your annual service; see our HVAC maintenance checklist.

An automatic controller with an outdoor sensor adjusts the target as temperatures change, and is worth considering if you often find yourself fighting condensation.

Does humidity affect your heating bill?

Slightly. Slightly more humid air can feel a little warmer, so some people are comfortable at a lower thermostat setting. However, humidifiers use energy too: evaporating water takes heat from the house, and steam units use electricity directly. The bigger savings come from air sealing, which both reduces heat loss and keeps humidity more stable.

Frequently asked questions

Is 50% humidity too high in an Ontario winter?

Usually, yes, at least during cold weather. At 50% RH, most double-glazed windows will condense when it is below freezing outside, and moisture can reach cold parts of walls and attics.

Why are my windows wet only in the morning?

Overnight, outdoor temperatures drop, curtains trap cool air against the glass, and bedrooms collect moisture from breathing. Lower the humidity slightly, keep bedroom doors open or use ventilation, and leave a gap under window coverings.

Should I run a dehumidifier in winter?

In most above-grade living spaces, ventilation is the better winter fix. A dehumidifier can still be useful in a damp basement. If you need one upstairs in winter, look for a moisture source or a ventilation problem.

My new home has heavy condensation. Is something wrong?

Not necessarily. New homes release moisture from concrete, lumber and drywall during the first one or two winters. Run the HRV or ERV as designed, use exhaust fans, and keep humidifiers off until levels settle.