30–40% — Sliding: 40% in mild weather, 25–30% in a hard freeze
Cold-climate humidity is the one case where a single target number is actively misleading. Hold 45% through a January cold snap and you will find water running down the windows, ice on the inside of the frames, and frost on the underside of the roof sheathing.
The reason is that your coldest surface follows the outdoor temperature, and the colder it gets, the lower your indoor humidity has to be to stay above its dew point.
Why the target slides with the weather
Condensation happens when air meets a surface below its dew point. Raise indoor humidity and the dew point rises; drop the outdoor temperature and the glass temperature falls. In a cold climate both move a long way, so the safe indoor humidity is a moving target rather than a fixed setting.
A working table for a double-glazed house: above 40 °F outdoors, 45% indoors; 20–40 °F, 40%; 10–20 °F, 35%; 0–10 °F, 30%; −10 to 0 °F, 25%; below −10 °F, 20%. Single glazing or metal frames need lower still.
The same physics produces the region's two signature building problems. Attic frost forms when moist indoor air leaks through ceiling penetrations onto sheathing that is below freezing — then melts in a thaw and appears as ceiling stains. Ice dams form when heat escaping into the attic melts snow that refreezes at the cold eaves. Both are air-leakage problems dressed up as roof problems.
What is your reading telling you?
- Dew point
- 53 °F
- Mold risk, cold walls
- Moderate
Still inside the accepted range, but this is where cold surfaces begin to sweat.
What to do here
Ranked by what it costs you, and ordered for this climate rather than in general.
Let the windows set your ceiling: morning condensation on the glass means the humidifier is 5% too high for last night.
Open curtains and blinds during the day so room air can reach the glass.
Keep low background heat in every room rather than shutting cold rooms off.
Weatherstrip and insulate the attic hatch — a major air-leakage path straight into the coldest part of the house.
Air-seal around recessed lights, plumbing stacks and wiring penetrations before adding attic insulation.
Run a humidifier on a humidistat and lower the target as it gets colder outside.
Consider an HRV — in a cold dry climate it is usually better value than an ERV.
Secondary glazing or replacement units raise the coldest surface temperature, which raises the humidity you can safely hold.
What will not work here
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One fixed humidity setting all winter
What is comfortable at 35 °F outside puts water on the glass at 0 °F.
-
Adding attic insulation without air sealing first
More insulation makes the sheathing colder, so leaking moist air condenses on it more readily. Seal, then insulate.
-
A powered attic fan to fix frost
It can pull more conditioned moist air up through ceiling leaks. Fix the leaks.
-
Treating ice dams as a gutter problem
They are caused by heat escaping into the attic. The repair is air sealing and insulation.
Through the year
Winter
Peak risk in both directions: dry air upstairs, condensation on every cold surface. Lower the humidifier as the temperature drops.
Spring
Thaw reveals what winter hid — ceiling stains from attic frost, damp in the basement from snowmelt.
Summer
Basements become the problem as warm humid air meets cool below-grade walls.
Fall
Air-seal the attic and check ventilation before the first freeze. This is the useful window.
Common questions
What should indoor humidity be in winter in a cold climate?
It slides with the outdoor temperature: about 40% when it is 20–40 °F outside, 35% at 10–20 °F, 30% at 0–10 °F and 25% below that. If the windows are wet in the morning, you are 5% too high.
Is attic frost a roof problem or a humidity problem?
A humidity problem wearing a roof problem’s clothes. Warm moist household air escapes through ceiling penetrations and freezes on sheathing that is below freezing. Sealing those leaks fixes it; roofing work does not.
Do ice dams have anything to do with humidity?
They share a cause. Heat and moist air escaping into the attic melt snow that refreezes at the cold eaves. Air sealing addresses both the ice dam and the attic frost.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions