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Thirty to forty percent relative humidity. That is the winter target for most US homes, and it sits deliberately at the lower end of the EPA recommended indoor range of 30 to 50 percent. The reason winter is different is condensation: warm humid indoor air meeting cold window glass drops below its dew point and deposits water on the pane. So the practical ceiling slides down as it gets colder outside, from about 40 percent when it is freezing to nearer 25 percent when it is below zero. Below 30 percent, dry throats, static and shrinking hardwood become the trade.
We do not buy or physically handle the units we rank; our picks come from aggregated owner reviews and expert consensus, explained in how we rank. Model shortlists are in best humidifier, inside the air quality hub.
How low should it go as it gets colder outside?
Slide the target down roughly 5 percentage points for every 10 degrees the outdoor temperature drops below freezing. This guidance originates with window and glazing manufacturers, because the coldest surface in a typical room is the glass, and it is the surface that tells you first when you have gone too far. The numbers below assume an indoor air temperature of 70 degrees and older double glazing; a house with triple-pane or high-performance windows tolerates several points more, and a house with single panes and storm windows several points less.
| Outdoor temperature | Target indoor RH | Approximate indoor dew point |
|---|---|---|
| 40 to 50 °F | 40 to 45% | 45 to 48 °F |
| 30 to 40 °F | 40% | 45 °F |
| 20 to 30 °F | 35 to 40% | 42 to 45 °F |
| 10 to 20 °F | 35% | 42 °F |
| 0 to 10 °F | 30% | 37 °F |
| Minus 10 to 0 °F | 25% | 33 °F |
| Below minus 10 °F | 20 to 25% | 28 to 33 °F |
The windows are a better instrument than the table. If glass is fogging at the edges in the morning, the setting is too high for that house on that night, whatever the number says. If the air makes your throat sore and door handles are shocking you, it is too low. Adjust to the house rather than to the chart.
Why does winter air get so dry indoors?
Because heating cold outdoor air multiplies its capacity to hold water without adding any. Cold air physically cannot carry much moisture, so even air that is at 80 percent relative humidity outdoors at 20 degrees contains very little water by weight. Every time that air leaks into the house through gaps, vents and door openings and gets heated to 70 degrees, its capacity rises roughly fourfold while its water content stays fixed, so the relative humidity falls into the teens. This is why the leakiest, draftiest homes are the driest ones in January, and why a tightly sealed modern house often needs no humidification at all and sometimes needs the opposite.
Cooking, showering, laundry and human respiration add moisture back, which is why an occupied house is more humid than an empty one and why a small apartment often sits comfortably without any equipment. If your problem is too much moisture rather than too little, the winter dehumidifier setting is covered in what should I set my dehumidifier to.
What actually goes wrong below 30 percent?
Three separate categories, and they arrive at different thresholds. Comfort goes first: below roughly 30 percent, nasal passages and throat tissue dry out, which shows up as morning sore throats, nosebleeds and worsened snoring. Static electricity becomes noticeable in the same band and is a genuine hazard to electronics. Materials go last but cost the most, since wood flooring manufacturers commonly specify 30 to 50 percent as the service range for warranty purposes, and sustained air below 25 percent produces visible gapping between boards, cracked furniture joints and separated trim miters. Musical instruments and antique furniture are the most sensitive items in a typical house.
There is also a modest respiratory-transmission argument for the middle of the range, since several studies find airborne virus survival is lowest between roughly 40 and 60 percent. That is worth knowing but should not push you past what your windows tolerate, because chronic condensation and the mold that follows is a more certain problem than the marginal benefit.
How do I hold the number reliably?
Measure in the room that matters, not at the humidifier. Onboard sensors read the air immediately at the intake, which on a running humidifier is the most humid air in the house, so they routinely read several points high and shut off early. Put an independent hygrometer at seated or sleeping height on the far side of the room and set the machine to whatever gets that reading right. Two or three inexpensive meters around the house will show you how much variation there is between a warm upstairs bedroom and a cold north-facing room, which is often 10 points or more.
For placement in a bedroom, see where to place a humidifier in a bedroom. For the technology choice, evaporative vs ultrasonic humidifier matters more than it sounds, because an evaporative wick unit is self-limiting and will not over-humidify a room the way an ultrasonic one can. Whatever you run, clean it on schedule; a humidifier is the one appliance that can actively worsen indoor air if neglected, as covered in how often to clean a humidifier and why does my humidifier smell. If dry skin is the reason you are here, do humidifiers help with dry skin covers what to expect.