Indoor Humidity in Winter: The Right Range

Indoor humidity in winter is one of those numbers that quietly decides whether your house feels comfortable or miserable. Too low and you get static shocks, cracked wood trim, a sore throat every morning and doors that suddenly stick in reverse. Too high and you get water running down the windows, black spots on the frames and, in bad cases, damp sheathing in the attic.

Most advice stops at a single number, usually somewhere around 40 percent. That is not wrong, but it is incomplete, because the right indoor humidity in winter depends on how cold it is outside. Here is the range to aim for, the adjustment nobody mentions, and what to do when your house is on the wrong side of it.

The range to aim for

The baseline target comes from the EPA, which advises that indoor relative humidity should be kept below 60 percent, and ideally between 30 and 50 percent.

So the working range for a home is 30 to 50 percent, with 60 percent as a hard ceiling you should not sit above for long. In winter specifically, most houses land comfortably somewhere between 30 and 40 percent, and the lower half of the range is often the safer place to be.

That last point is the part people get backwards. In summer, high humidity is the enemy and you are usually trying to bring the number down. In winter, the instinct is to crank a humidifier because the air feels dry, and that is exactly how condensation problems start.

Why winter dries a house out

Relative humidity is relative to temperature, and that is the whole explanation.

Cold air physically cannot hold much moisture. When outdoor air at 20 degrees and 70 percent relative humidity leaks into your house and gets heated to 70 degrees, the amount of water in it does not change, but the air’s capacity goes way up. The same moisture now represents a much smaller percentage, so the relative humidity falls through the floor.

That is why a leaky house in a cold climate can drift down to 15 or 20 percent indoors without anybody doing anything wrong. It is also why a tight, well sealed modern house often has the opposite problem, because the moisture from cooking, showers and breathing has nowhere to go.

Which problem you have depends less on your climate than on how airtight your house is. Older drafty homes run dry. Newer or recently air sealed homes run wet.

The adjustment most advice leaves out

Here is the part that makes the difference. The colder it gets outside, the lower your indoor humidity has to be to avoid condensation, because your window glass gets colder and condensation forms on the coldest surface in the room.

The Center for Energy and Environment publishes a ceiling for indoor humidity at each outdoor temperature:

Outdoor 20 to 40 degrees: keep indoor humidity under 40 percent
Outdoor 10 to 20 degrees: under 35 percent
Outdoor 0 to 10 degrees: under 30 percent
Outdoor minus 10 to 0 degrees: under 25 percent
Outdoor minus 20 to minus 10 degrees: under 20 percent
Outdoor below minus 20 degrees: under 15 percent

Read that as a sliding ceiling rather than a target. If you live somewhere that rarely drops below freezing, you can comfortably sit at 40 to 45 percent all winter. If you get a week at 5 degrees, a humidifier set to 45 percent will fog and then freeze your windows, and the water running off the glass will sit on the sill and rot it.

The practical takeaway is to treat your humidifier setting as something you change with the weather, not something you set in November and forget.

What your windows are telling you

Before buying a meter, look at the glass on the coldest morning of the week. Windows are a free humidity gauge because they are almost always the coldest interior surface.

Clear and dry: you are in a sensible range for the current weather.
A thin fog at the bottom edge that clears by mid morning: borderline. Fine occasionally, worth watching if it happens daily.
Water pooling on the sill, or a wet frame: too humid for the temperature. Turn the humidifier down now, before the wood stains.
Ice on the inside of the glass: well over the limit. This is where frames rot and mold starts.
Static shocks, cracking furniture joints, gaps opening in hardwood floors: too dry.

One caveat. Single pane or old aluminum framed windows will condense at humidity levels that are perfectly safe elsewhere in the house, because the glass is so much colder. If that is what you have, the condensation is telling you about the window, not necessarily about the whole house. Our guide to standard window sizes and rough openings is useful if replacement is on the table.

How to measure it properly

A basic digital hygrometer costs very little and is worth having, but where you put it changes the reading more than people expect.

Measure in at least two rooms. A bedroom with the door closed overnight and a living room with the heat running can differ by 10 points or more.
Keep it away from the obvious distortions: bathrooms, the kitchen, exterior walls, direct sun and heating vents.
Put it at about seated or standing height, not on the floor and not on top of a bookshelf.
Give it half an hour to settle after moving it.
Expect inexpensive sensors to drift by a few points. Two cheap meters in the same room disagreeing by 3 or 4 percent is normal, so treat the reading as a range.

Check it on a cold morning rather than a mild afternoon. The problem conditions show up when the outside temperature is at its lowest.

If your house is too dry

Below about 30 percent, start adding moisture. Before you buy anything, remember that a dry house in winter is often a draft problem wearing a humidity costume.

Air seal first. Every cubic foot of cold air that leaks in has to be heated, and heating it drives the relative humidity down. Weatherstripping, outlet gaskets on exterior walls and sealing the attic hatch all raise indoor humidity for free.
Stop over ventilating. Running a bath fan for an hour after a shower in January is pulling dry air in and heated moist air out.
Use a portable humidifier in the rooms that matter, usually bedrooms, rather than trying to lift the whole house. Clean the tank regularly, because a dirty humidifier is a genuine air quality problem.
Let moisture stay where it is produced. Air dry laundry indoors, leave the bathroom door open after a shower, simmer rather than boiling with a lid off.
Plants help a little, not a lot. They are pleasant and they contribute, but a few pots will not move a whole house. They do raise humidity in a small closed room. Our guide to overwintering potted plants covers which ones can come inside.

Whole house humidifiers tied into the furnace work well in genuinely dry climates, but they need a humidistat you actually adjust through the winter, or they create the opposite problem during cold snaps.

If your house is too humid

Above the ceiling for your current outdoor temperature, the fix is almost always ventilation at the source rather than a dehumidifier.

Three activities produce most of the moisture in a home, and all three have a mechanical answer:

Showers. Run the bath fan during the shower and for 15 to 20 minutes after, and make sure it is actually sized for the room. An undersized fan moves the steam around without removing it. See how to size a bathroom fan by CFM.
Cooking. Use the range hood every time you boil or simmer, and confirm it vents outside rather than recirculating. A recirculating hood does nothing for moisture. Our range hood sizing guide covers the difference.
Laundry and drying. A dryer vent that has come loose inside the house dumps an enormous amount of water into the air. Check the connection.

Also look for sources you have not thought of. An unsealed crawlspace with a dirt floor, a basement with a slow leak, a firewood stack indoors and a large aquarium all add moisture continuously. If humidity stays high after addressing showers, cooking and laundry, the source is probably below the living space.

And turn off the humidifier before anything else. It is startling how often the cause of winter window condensation is a humidifier running on a schedule nobody has revisited since the mild weather.

Frequently asked questions

What should indoor humidity be in winter?

Between 30 and 50 percent as a general range, following EPA guidance, with most homes sitting best at 30 to 40 percent in cold weather. The colder it is outside, the lower you need to go, down toward 25 or 30 percent when outdoor temperatures approach zero.

Why do my windows sweat in winter?

Because the indoor air holds more moisture than the cold glass can tolerate, so water condenses on the coldest surface in the room. Either the indoor humidity is too high for the current outdoor temperature, or the window itself is unusually cold, as with single pane or metal framed glass.

Is 30 percent humidity too low for a house?

It sits at the bottom of the recommended range, so it is acceptable rather than ideal. If you are getting static shocks, dry skin and visible gaps in wood floors or trim, adding a little moisture makes sense. During a severe cold snap, 30 percent may be the highest level your windows can handle anyway.

Should I run a humidifier all winter?

Only if your measured humidity is actually low, and only with the setting adjusted as the weather changes. A humidifier left at a fixed setting through a cold snap is one of the most common causes of window condensation, frozen glass and mold on window frames.

Can high indoor humidity damage a house?

Yes. Repeated condensation rots window sills and frames, stains drywall, and supports mold growth. Moist indoor air that reaches a cold attic can condense on the roof sheathing, which is a more expensive problem. Keeping humidity below 60 percent, and lower in cold weather, is the preventive measure.

The short version

Aim for 30 to 50 percent, lean toward the low end in winter, and drop your ceiling as the outdoor temperature drops. Buy a cheap hygrometer, put it in a bedroom and a living room, and check it on the coldest morning of the week. If the windows are dry and nobody is getting static shocks, you are in the right place and you can leave it alone.

Indoor Humidity in Winter: The Range That Protects Your Home