What Humidity Should My House Be in Winter?

The first time I saw the number 40 percent on a weather report, I thought it was about the outside air. It was not. It was about inside. The reporter was telling renters like me what the indoor humidity should sit at during heating season for comfort and health, and I had been living in this apartment for two winters with no idea that there was a number to aim for. My baseline was somewhere around 18 percent. That is closer to desert air than to a healthy home. My skin knew it. My piano knew it. My sinuses knew it. I just did not have a tool to measure it until I bought a $10 hygrometer.

What humidity should my house be in winter is one of those questions with a boring, specific, science-backed answer. The number is between 30 and 50 percent relative humidity for most occupied homes, with 40 to 45 percent as the comfortable middle. The EPA, ASHRAE, and most building science textbooks agree on this range. Below 30 percent you get dry sinuses, cracked wood, and static shocks. Above 50 percent you start feeding dust mites and seeing condensation on the coldest windows.

The setup to actually hit that number in a rental takes about 30 minutes and costs less than a takeout dinner. Here's what to buy, where to put it, and what actually moved the needle over three winters.

The Target Number, and How It Changes With Outdoor Temperature

Indoor relative humidity in winter should sit between 30 and 50 percent for most homes. The exact target depends on the outdoor temperature, because cold outdoor air cannot hold much moisture and the colder it gets outside, the drier your indoor air becomes when you heat it. The rule of thumb is to keep indoor humidity low enough that you do not see condensation on your windows, and high enough that your skin does not feel tight an hour after a shower.

A practical target for a renter in a heating climate is 40 percent on days when outdoor temperature is above 20 degrees Fahrenheit, 35 percent on days between 0 and 20, and 30 percent on days below zero. The colder the outdoor air, the lower your indoor target needs to be, because the dew point of cold outdoor air is so low that even moderate indoor humidity will condense on the glass. That condensation, if it sits long enough, grows mold and rots the window frame.

If you only want to set one number and forget it, 40 percent is the right setting for most heating-season days in most climates. Adjust down by 5 points for every 20 degrees the outdoor temperature drops below freezing. Adjust up by 5 points for shoulder seasons in the spring and fall when outdoor humidity is naturally higher and your furnace is not running as much.

What You Need to Buy, and What to Skip

The only tool you need is a hygrometer. The $10 analog dial kind from the hardware store is accurate to within 5 percent. The $15 digital kind with a min-max memory is accurate to within 3 to 5 percent and is what I would buy again. Skip anything with Bluetooth, app connectivity, or a screen larger than 2 inches. The $40 smart versions do the same job, they just show you the number on your phone instead of on the device, and the radio inside them fails after about a year.

Place the hygrometer on a shelf or table in the room where you spend the most time, away from the kitchen, away from the bathroom, and out of direct sunlight. Sunlight on the sensor will give you a falsely low reading. Kitchen and bathroom air will give you a falsely high reading during cooking and showering. The middle of the living room or bedroom is right.

After one week of watching the dial, you will know your baseline. If the dial sits between 30 and 50 percent, you do not need to buy anything else. If it sits below 30, you need a humidifier. If it sits above 55, you need a dehumidifier. Most renters in heating climates fall into the first group with a working furnace, and into the second group only if they have a basement apartment or a known moisture problem.

When to Add Moisture, and When You Are Adding Too Much

If your baseline is below 30 percent and your skin is cracking and your wood trim is shrinking, add a humidifier. The three types are evaporative, steam, and ultrasonic. For a renter in a 600 to 900 square foot apartment, a 4 to 6 gallon evaporative unit in the main living area will raise baseline humidity by 5 to 10 percentage points. Expect to spend $15 to $25 a month on electricity and wick replacements—wicks need replacing every 1-2 months at $10-15 each. A steam unit will do the same job faster and cleaner, but it is the most expensive to run—expect $25 to $35 a month in electricity, depending on your utility rates. An ultrasonic unit is the quietest but puts mineral dust on everything if you have hard water.

Set the humidifier to its built-in humidistat, not to its highest setting. Set it to your target number, in my case 42 percent. The humidifier will turn itself on and off to maintain that number. You do not need to babysit it. Refill the tank every morning or hook a garden hose to the inlet if your model supports continuous fill. Clean it once a week according to the manual to prevent bacterial growth in the tank.

If your baseline is above 55 percent and you see condensation on the windows in the morning, you have the opposite problem and you need a dehumidifier. A 35-pint Energy Star dehumidifier will pull about 6 pints a day in normal conditions and will cost about $8 a month in electricity. Set it to 45 percent. Empty the bucket daily or hook it to a sink drain. The wet basement smell that goes with high humidity usually clears in two to three days after you start running a dehumidifier.

The Three Things That Move the Needle Without Buying Anything

Before you spend money on a humidifier, try three free things first. The first is to lower your thermostat by 2 degrees. Warmer air holds more moisture, but it also dries your sinuses faster because the temperature differential between your skin and the air is bigger. Dropping the setpoint from 72 to 70 usually lets your baseline humidity rise by 3 to 4 percentage points without any device.

The second is to stop running the bathroom exhaust fan longer than needed. Ten minutes is usually enough to clear the mirror and remove the bulk of the moisture. If the mirror is still fogged after 10 minutes, run it longer—mold risk outweighs the humidity savings. Anything beyond that is pulling warm moist air out and replacing it with cold dry outdoor air. Same for the kitchen hood. Run it while cooking and for 5 minutes after. Turn it off before bed.

The third is to add a few large houseplants to the room you spend the most time in. A single large plant transpires about a cup of water a day. Five large plants in a 200 square foot room will add maybe 1 to 2 percentage points to the baseline humidity over time, depending on ventilation and plant size. They help, but they are not a substitute for a humidifier. They will not solve a serious dryness problem, but they help, and they look better than a plastic humidifier on a shelf.

The Three Things That Did Not Work for Me

I tried a $30 essential oil diffuser and it was decorative, not functional. The reservoir held 4 hours of water and put the mist on my desk, not in the air. The hygrometer did not move. I tried a $25 indoor plant collection marketed as a natural humidifier and the 6 plants I bought did not move the dial either, but they did make me feel better about the room. I tried running the dishwasher with the door closed overnight to let the steam leak out, and the result was exactly enough moisture to make the kitchen feel damp for an hour and not enough to move the hygrometer in the next room.

These are not failures of the products. They are just the wrong tools for the job. A $50 evaporative humidifier made a measurable difference in the first week—something none of the smaller gadgets did in a month.

What to Check If Your Number Will Not Stay Put

If you set your humidifier to 42 percent and the hygrometer in the living room reads 28 percent an hour later, you have an air leak. Cold dry outdoor air is sneaking in through window frames, electrical outlets on exterior walls, the gap under the front door, or a missing furnace filter. Fix the air leaks with weatherstripping, outlet gaskets, and a new furnace filter before you spend more money on humidification. The leak is pulling moisture out faster than the humidifier can add it.

If you set your dehumidifier to 45 percent and the reading in the basement stays above 60 percent, you have a moisture source. The usual suspects are a sump pump that is not pumping, a foundation crack letting groundwater seep in, or a dryer venting into the basement. Call your landlord if you rent, or a basement waterproofing company if you own. A dehumidifier alone will not fix a foundation leak. It will just run 24 hours a day and run your electric bill up by $40 a month.

The number to aim for is 40 percent relative humidity in the rooms where you spend the most time. A $10 hygrometer, a $50 humidifier for heating season, and a $150 dehumidifier for the shoulder seasons will get you there. The annual cost is around $150 to $200, depending on your climate. The comfort difference is worth it.