Dry air in winter: why heating makes it worse
Below 30% is too dry. Target 40–45% — but drop to 30–35% when it is below about 20 °F outside, or you will get condensation on the glass.
Heating does not remove water from your house. It raises the temperature, and warm air can hold far more moisture, so the same water becomes a much smaller percentage.
Winter dryness catches people out because nothing in the house changed — the furnace came on, and suddenly there are static shocks and nosebleeds. The moisture did not go anywhere. The air's capacity went up.
Outdoor air at 20 °F holds very little water. Bring it in, heat it to 70 °F, and its relative humidity falls to somewhere around 15–20% before you have removed a single drop.
Confirming it is actually dry air
| What you see | What it means |
|---|---|
| Hygrometer reads below 30% and it is heating season | Straightforward winter dry air. Everything below applies. |
| Static shocks and dry skin, but the hygrometer reads 45% | Your hygrometer may be off. Check it with the salt test — a damp salt slurry sealed in a bag with the meter for eight hours should read 75%. |
| Dry upstairs, damp in the basement | Normal and worth leaving alone. Stack effect pulls dry cold air in low and pushes moist warm air out high. Treat the two spaces separately. |
| Very dry despite a running humidifier | Either the house leaks badly, the humidifier is undersized for the volume, or a wood stove or open fire is drawing enormous amounts of air through the house. |
What to do, in order
Free actions first. Do not buy anything until these have had two weeks.
Today
Drop the thermostat 1–2 °F. Relative humidity rises on its own, at no cost, and the room feels no different.
Leave the bathroom door open after showers instead of running the fan — in winter that moisture is useful.
Dry laundry indoors. The habit that causes damp in summer is a feature in January.
Stop over-ventilating. Short purge airings are still worth it for air quality, but leaving windows tilted all day in freezing weather is what dries a house out.
This week
Seal the obvious drafts — around window frames, under exterior doors, at attic hatches. Less air exchange means less dry air pulled in.
Put a hygrometer in the main living space and one in a bedroom, and check both morning and evening.
If you run a wood stove or open fire, understand it is drawing household air up the chimney continuously and replacing it with cold outdoor air.
Group houseplants together. The effect is modest but real, and unlike most "natural" remedies it works in the right direction.
If it has not improved
Run a humidifier set to 40–45%. Choose evaporative if you want it hard to overdo, steam for a nursery, ultrasonic only if you will use distilled water.
Consider a whole-house humidifier on the furnace if every room is dry rather than just one.
Clean whatever you use: rinse daily, disinfect weekly. A neglected tank aerosolizes bacteria into the room.
Watch the windows. Morning condensation on the glass means your setting is too high for the current outdoor temperature — that is your ceiling, not the humidifier's dial.
What dry air actually does
Below 30%, the mucous membranes in your nose and throat dry out, which reduces their ability to trap and clear viruses — one reason respiratory infections spread more readily in winter. It also causes the familiar cracked lips, dry eyes, itchy skin and nosebleeds.
The building suffers too. Wood floors, furniture joints, doors and musical instruments all shrink as they lose moisture, and gaps that open in a dry winter do not always close again.
The target is a band, not a maximum. Pushing past 50% in cold weather trades dry air for condensation and mold, which is a worse problem than the one you started with.
Common questions
Why is my house so dry in winter?
Cold outdoor air holds very little moisture. Once it enters your home and is heated, its relative humidity collapses — air at 20 °F and 80% outdoors ends up near 15–20% when warmed to 70 °F. Heating raises capacity rather than removing water.
What humidity should I keep my house at in winter?
40–45% in mild weather. When outdoor temperatures drop below about 20 °F, lower it to 30–35%, or condensation will form on windows and frames.
Does turning down the heat increase humidity?
Yes. Relative humidity is relative to temperature, so cooler air with the same water content reads higher. A 1–2 °F reduction is a free way to raise it.
Can dry air make you sick?
It does not cause infection, but below 30% it dries the mucous membranes that trap and clear viruses, which makes infection easier. It also causes nosebleeds, dry cough and irritated skin and eyes directly.