Aim for a relative humidity of about 30 to 50 percent, and keep it under 60. That’s the range the U.S. Environmental Protection Agency gives, and it holds up well in a house that is muggy for part of the year and bone dry for another part. Which end of that range you sit at depends mostly on the season.
The short answer: what humidity to aim for
The EPA’s wording is worth reading as written: indoor relative humidity “should be kept below 60 percent, ideally between 30 percent and 50 percent, if possible.” (EPA, Mold Course chapter 2). Read it as a ceiling of 60 with a comfortable band underneath, 30 to 50, where you’d like the house to sit most days.
Why a ceiling at 60? Mold and dust mites both do well in damp air, and dust mites are a known asthma trigger. The EPA also notes that lower humidity can discourage dust mites and pests such as cockroaches. So the upper number is the one with health reasons behind it, and it’s the one to take more seriously.
The lower number is softer. Below 30 percent, the complaints are mostly about comfort, which we’ll get to.
A quick word on “relative.” Relative humidity is how much moisture the air holds compared with the most it could hold at that temperature. Warm air can hold far more than cold air. That’s why the same outdoor air that feels fine on a January afternoon turns desert-dry once your furnace heats it to 70 degrees: the moisture didn’t go anywhere, the ceiling above it just got a lot higher. It also explains why a cold window pane in a comfortable room can still run with water.
Is 40 percent humidity too low for a home?
No. Forty is comfortably inside the EPA’s 30 to 50 range. In winter it’s a perfectly good reading, and on the coldest nights you may want to be a little under it.
Is 70 percent humidity too high for a bedroom?
Yes. Seventy is well past the 60 percent line, and a bedroom that sits there night after night is the kind of room where a musty smell shows up in the closet. One high reading after a hot shower down the hall is nothing to worry about. A week of them is.
What too much and too little humidity look like
Too damp usually announces itself. Water beading on the inside of windows, a musty smell that comes back no matter how often you clean, towels that never quite dry, spots of mold on bathroom ceilings or behind furniture pushed against an outside wall. In the basement you might see sweating on cold water pipes.
Too dry is quieter. Static shocks off the light switches, dry skin and lips, a scratchy throat first thing in the morning. People also mention wooden floors and furniture opening small gaps in winter, though plenty of other things move wood around too. None of these prove the air is dry on their own. A gauge will settle it.
How to measure humidity at home
Buy a hygrometer. It’s a small, inexpensive gauge (the EPA calls it a “moisture or humidity gauge” and notes you can find one at most hardware stores), and most models show the temperature too. Nothing else in this article works without one, because your skin is a poor judge of humidity. A room can feel perfectly normal at readings well apart from each other.
Some habits that make the reading worth trusting:
- Keep it away from windows, outside doors, supply vents, the kitchen and the bathroom. You want the room’s air, not a draft or a burst of steam.
- Give it a few days before you act. One reading is weather. A week of readings is your house.
- Move it around, or buy two. The living room and the basement can read very differently in August, and the basement is usually the one that matters.
Two cheap gauges set side by side will often disagree by a few points. Don’t lose sleep over it. You are looking for “comfortably in range” or “clearly out of it”, and any working gauge can tell you which.
Summer in New Jersey: bringing humidity down
Plenty of houses in this state are old enough to have a basement of concrete block or fieldstone that nobody ever meant to finish. Those basements stay cool right through summer. That’s pleasant on a hot afternoon, and it’s also exactly why muggy July air, drifting down the stairs or seeping in around the sill, ends up as water on the walls and the cold pipes. Summer, not winter, is usually the season with the harder humidity problem.
Your air conditioner is the first line of defense, and it helps to know how it does it. As warm air passes over the cold indoor coil, moisture condenses out of it and drains away. Cooling and drying happen together. The catch is that the drying only happens while the system is running. The Center for Energy and Environment puts the problem plainly: “If your air conditioner is old, undersized, or struggling to keep up, it may not be removing enough moisture even if the temperature feels fine” (CEE, Managing humidity levels). An oversized system can cause a quieter version of the same thing. It cools the house quickly, shuts off, and never runs long enough to take much water out. The result is a house that hits the thermostat setting and still feels clammy. If that sounds like your summers, the gauge will usually confirm it, and the cooling guides go into how systems are sized and why it matters.
Then there’s the basement. Even a house with a healthy air conditioner can have a basement that sits well above the rest, because the basement often gets little of the cooled air. A basement dehumidifier is the usual answer. CEE suggests setting one to 55 percent. That’s a sensible number: low enough to stay clear of the 60 line, not so low that the machine runs all day for little gain. Empty it or give it a drain, clean the filter, and check the gauge after a week to see whether it’s keeping up.
About opening windows. The EPA’s advice is to open them to lower humidity only “if it is not humid outdoors.” On a muggy July afternoon, airing out a damp basement lets in more moisture than it lets out, since outdoor air in summer is often wetter than what’s already in the room. Wait for a dry, breezy day instead.
A few smaller things help at the margins: run the bathroom fan during showers and for a while after, use the range hood when you boil pasta, and make sure the clothes dryer vents outdoors. None of this fixes a basement at 70 percent. All of it helps a house that’s hovering around 58.
Winter: keeping dry air in range
Heating dries the air. Cold outdoor air holds little moisture to begin with, and once it leaks in and the furnace warms it, its relative humidity drops a long way. That’s the reason indoor humidity in winter can slide into the 20s without anyone doing anything wrong. Older, draftier houses tend to get there first, simply because more cold air finds its way in.
The fix is a humidifier, either a portable one in the rooms you use most or one mounted on the furnace that humidifies the air as it goes through the ducts (the kind people usually mean by a whole house humidifier). How to choose between them is a topic of its own. What matters here is the setting.
In winter, the ideal indoor humidity is lower than the textbook 30 to 50, and it drops further as it gets colder outside. The reason is your windows. The colder the glass, the less moisture the room can carry before water starts condensing on it. CEE suggests running humidifiers with low settings of 30 to 40 percent, for short periods, and adds: “If you add humid air to your home and see evidence of condensation, be sure to lower your RH levels.” In practice: pick a setting in the low 30s, watch the bottom corners of your windows, and back off when they fog. If you still have original single-pane glass or tired old double-hungs, those panes get cold fast, and the first real January cold snap is usually when they fog up first. Treat them as your early warning.
CEE also publishes a table of targets by outdoor temperature. It’s written for Minnesota winters, so we wouldn’t copy its numbers here. The principle carries over fine: colder outside, lower target inside.
One more thing about humidifiers: keep them clean. Standing water in a neglected tank is a good way to grow the very things you were trying to keep out of the air. Follow the maker’s cleaning schedule, and empty and dry a portable unit if you stop using it for a while.
Spring and fall: the in-between weeks
The switch between seasons is rarely clean here. A warm, damp spell can turn up in April, or again in late September, while the furnace is still on its last cycles and nobody has thought about the air conditioning yet. Nothing is drying the house during those weeks, so the basement is the room to watch.
Check the gauge once a week or so. Turn the humidifier off when the furnace stops running for the season. If the basement starts creeping toward 60 before the AC is on, start the dehumidifier; don’t wait for the calendar.
If your readings don’t match anything described here, or the gauge says one thing and the house feels like another, that’s worth a closer look. The indoor air quality guides cover the rest of what affects the air in your home, and if you’d like to talk it through with our team, you can get in touch.