What humidity should I keep my house at in winter?

Around 35% relative humidity is a good target. Much lower and wood shrinks noticeably; sustained levels above 50% in a Calgary winter risk condensation on cold windows. A whole-house humidifier on the furnace is the easiest way to hold it steady.

Should I fill the gaps that appear?

Generally not with a rigid filler, because the joint will close again in spring and the filler will be squeezed out or crack. A flexible paintable caulk tolerates the movement. For joints that open every year, the underlying fix is material and technique rather than repeated filling.

Is MDF trim really better than wood here?

For painted trim, yes. It is dead flat, takes paint beautifully, machines crisply and does not move seasonally. Use solid wood for stained trim and where impact resistance matters, and avoid MDF at floor level in bathrooms and mudrooms where it can get wet.

Materials

Why Trim Joints Open Every Winter in Calgary

Updated July 25, 20263 min read

The short answer

Indoor relative humidity in a heated Calgary house routinely falls to 15–25% in deep winter from around 40% in summer. Wood gives up moisture and shrinks across the grain, so mitres open, painted panel joints crack and wide shelves cup. It is usually wood behaving normally rather than poor workmanship — but the right material and technique choices largely prevent it.

What is actually happening

Wood is hygroscopic: it absorbs and releases moisture until it reaches equilibrium with the air around it, and it changes dimension as it does. The change is significant across the grain and negligible along it.

Calgary is one of the driest major cities in Canada, and running a furnace through a long winter drives indoor humidity far below what wood is comfortable with. Chinooks make it worse by cycling temperature and furnace demand rapidly, so the wood is repeatedly moving rather than settling.

The result is predictable: a 90-degree mitre in a shrinking piece of trim opens at the outside of the corner. A painted door panel contracts within its frame and the paint film cracks along the joint. A wide solid shelf finished only on its top face cups because the two faces exchange moisture at different rates.

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What prevents it

MDF for painted components: MDF does not respond to seasonal humidity the way solid wood does. This is precisely why it dominates painted trim and door panels in this climate — it is the more appropriate material, not a compromise.
Floating panels: Door panels should be free to move within their frames rather than glued in place. A panel that cannot move will either crack itself or crack the finish.
Coped inside corners: A coped joint closes as wood moves; a mitred inside corner opens. On profiled baseboard this single technique makes a visible difference by February.
Glued and pinned outside mitres: Glue holds a mitre closed under movement stress. Pins alone do not.
Acclimatisation: Material should sit inside the house for several days before being cut and fitted, rather than coming off a cold truck and going straight on the wall.
Sealing every face: Including the back, the underside and the ends. Uneven sealing means uneven moisture exchange, which is what causes cupping.
A humidifier: Holding indoor humidity near 35% through winter is the most effective single thing a homeowner can do — for millwork, hardwood floors and the people living there.

Answers

Frequently asked questions

What humidity should I keep my house at in winter?: Around 35% relative humidity is a good target. Much lower and wood shrinks noticeably; sustained levels above 50% in a Calgary winter risk condensation on cold windows. A whole-house humidifier on the furnace is the easiest way to hold it steady.
Should I fill the gaps that appear?: Generally not with a rigid filler, because the joint will close again in spring and the filler will be squeezed out or crack. A flexible paintable caulk tolerates the movement. For joints that open every year, the underlying fix is material and technique rather than repeated filling.
Is MDF trim really better than wood here?: For painted trim, yes. It is dead flat, takes paint beautifully, machines crisply and does not move seasonally. Use solid wood for stained trim and where impact resistance matters, and avoid MDF at floor level in bathrooms and mudrooms where it can get wet.

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