Winter Humidity
Window condensation is the most useful instrument in the house. When the windows run, the humidity is too high for the weather.
Short answer
Aim for roughly thirty to forty percent relative humidity in a Toronto winter, reducing toward the lower end in a cold snap. Too low is a comfort issue; too high puts water into window frames, wall cavities and the attic, which is a building damage issue. Measure it rather than guessing.
Why there is an upper limit
A cold surface in a humid room condenses, and in January the coldest surfaces in a Toronto house are the windows, the window frames and the exterior wall corners.
That water runs into the sill, the frame and the wall below, which is how rot and mould start at windows.
Warm moist indoor air also leaks into the attic, where it condenses on the cold sheathing as frost and then stains in spring.
The colder it is outside, the lower the indoor humidity has to be to avoid condensation, which is why a fixed humidistat setting is wrong for part of the winter.
Closets on exterior walls and furniture against them are the other common locations.
And in a tighter modern house the moisture has nowhere to go, which is why ventilation matters more than it used to.
The window test
Persistent condensation on the inside of the windows in cold weather means the indoor humidity is too high for the outdoor temperature. Turn the humidifier down, or run the HRV more. It is a free, continuous instrument and it is more useful than the humidistat on the duct.
Where the moisture comes from
People: breathing and perspiring, which in a full house is a surprising amount.
Showers and baths, which is why the bathroom fan matters and why it has to run long enough.
Cooking, particularly boiling and simmering, which is why a range hood ducted outside matters.
Drying laundry indoors, which puts litres into the air per load.
A furnace humidifier set too high, which is the deliberate source and the easiest to adjust.
And a damp basement or crawlspace, which contributes continuously and which is frequently the hidden source.
Controlling it
A hygrometer in two or three rooms, so you are managing a number.
Run the bathroom fan during and for twenty minutes after a shower, which a timer switch makes achievable.
Use the range hood, and make sure it actually exhausts outside rather than recirculating.
Run the HRV, which is the ventilation in a tight modern house and which is frequently switched off.
Turn the humidifier down, and down further in a cold snap.
And deal with a damp basement or crawlspace, because that is a continuous source that no amount of ventilation upstairs fully offsets.
Common questions
What humidity should a Toronto house be in winter?
Roughly thirty to forty percent, lower in a cold snap.
My windows are running with water. What now?
The humidity is too high for the weather. Turn the humidifier down and ventilate more.
Where is the hidden moisture source?
Frequently a damp basement or crawlspace contributing continuously.