What should the humidity in your house be?
Foggy windows, a musty corner, static shocks, a cough that will not shift — most of it traces back to one number on a cheap hygrometer. Here is the number you want, and what to do when yours is wrong.
The short answer: keep indoor relative humidity between 30% and 50%, and aim for 40–50% if you can. Below 30% the air dries out your skin, airways and woodwork. Above 60%, water condenses on cold surfaces and mold and dust mites move in.
The catch: that single number behaves very differently at 65 °F than at 75 °F, and mold responds to the humidity at your walls, not in the middle of the room. The checker below works both out.
Check your reading, get a plan
Enter what your hygrometer and thermometer say. You get the dew point, the moisture your air is actually carrying, the mold risk on cold surfaces in that room — and a ranked list of what to do about it, cheapest first.
Ideal indoor humidity — 45% RH
You are in the sweet spot.
- Dew point
- 47 °F
- Moisture in the air
- 48 gr/lb
- Mold risk on cold walls
- Very low
What to do about it
The mold estimate assumes cold surfaces run about 5 °F below room air, which is typical of an uninsulated exterior wall or a window reveal. To check a specific spot, measure it with an infrared thermometer and use the mold risk calculator.
The ideal range, room by room
Health authorities and building scientists converge on the same window: 30–50%. It is not arbitrary — it is the band where dust mites, mold, bacteria, viruses and chemical off-gassing are all near their minimum at once.
| Room | Target | Why it differs |
|---|---|---|
| Living room | 40–50% | Comfort baseline — warm air here carries more moisture without condensing. |
| Bedroom | 40–50% | A sleeping adult adds up to a pint of water a night; air it out in the morning. |
| Bathroom | Under 60% after 20 min | Spikes past 90% during a shower. What matters is how fast it falls back. |
| Kitchen | Under 60% | Cooking is the second-largest moisture source in most homes. |
| Basement | 50% or below | Coldest surfaces in the house, so mold appears here at lower room readings. |
| Nursery | 40–50% | Dry air irritates small airways; damp air feeds mite allergens. |
| Wine, books, instruments | 45–55% | Wood, paper and cork move with humidity — stability matters more than the exact value. |
Winter exception: when it is freezing outside, holding 50% indoors will put water on your windows. If the glass beads up in the morning, drop toward 30–40% until the cold snap passes.
Find the fix that matches your cause
Damp has four completely different causes that look identical on a wall, and each needs a different repair. Start with the mechanism, not the product.
Dehumidifiers
Pulls 20–50 pints a day out of the air. The strongest tool you can buy — and the one most often bought for a problem it cannot fix.
Ventilation
Free to nearly free, and the correct first move in almost every damp house. The catch is that on a muggy summer day it backfires.
Moisture absorbers
Real but tiny. A calcium chloride tub pulls about an ounce a day; a dehumidifier pulls forty. Right for a closet, useless for a room.
Insulation & heat
The answer when mold keeps returning to one specific spot while the room reads a healthy 50%.
Structural damp
Four different problems that look identical on the wall. Getting the diagnosis wrong means paying for the wrong repair.
Removing mold
Cleaning removes the growth. If you do not fix the moisture, it returns in the same spot within a season.
Humidifiers
The winter problem. Straightforward to fix, easy to overdo — and overdoing it puts condensation on your windows.
How to measure it properly
Get one honest reading
A digital hygrometer costs very little and is the only way to know. Place it at chest height, away from vents, windows, exterior walls and doorways, and give it two hours to settle before you trust it.
Cheap units drift. Check yours with the salt test: seal a damp salt slurry and the hygrometer in a zip bag for eight hours — it should read 75%. Note the offset and subtract it from then on.
Measure the room that is failing
Humidity is not one number for a whole house. The coldest, least-heated room is where condensation and mold appear first — usually a north-facing bedroom, a spare room with the door shut, or the basement.
If you are chasing a damp problem, put a hygrometer in that room and read it early in the morning, when surfaces are coldest and the reading is at its worst.
Common questions
What is the ideal humidity level in a house?
Between 30% and 50% relative humidity, with 40–50% the most comfortable band for most homes. Below 30% the air is drying; above 60% you risk condensation, dust mites and mold.
Is 70% humidity too high in a house?
Yes. Sustained readings above 60% let mold colonize cold surfaces, and at 70% dust mites thrive and moisture begins to damage plaster, paint and framing. Ventilate, find the moisture source, and run a dehumidifier.
Why is the humidity in my house so low in winter?
Cold outdoor air holds very little moisture. When it comes inside and is heated, its relative humidity collapses — air at 20 °F and 80% outdoors ends up near 15–20% once warmed to 70 °F. Your heating is not removing water; it is raising how much the air could hold.
How do I measure the humidity in my house?
Use a digital hygrometer at roughly chest height, away from vents, windows, exterior walls and doorways, and let it settle for a couple of hours. Measure the coldest room as well — that is where condensation and mold show up first.
How do I lower humidity without buying anything?
Air the house out for five to ten minutes twice a day in cool weather, run the bathroom fan during and after showers, stop drying laundry indoors, vent the dryer outside, and pull furniture two inches off exterior walls. Those five habits fix a large share of damp houses at no cost. Check the window calculator first in warm weather — ventilating on a muggy day adds moisture.
What humidity level causes mold?
Mold needs around 80% relative humidity at the surface it grows on. Cold walls, window reveals and corners sit several degrees below room temperature, so a room reading of 55–65% can already mean 80% against the coldest wall. That is why mold appears in corners while the room feels only slightly damp.
Dehumidifier or better ventilation — which should I fix first?
Ventilation, every time. A dehumidifier treats the symptom and costs electricity for as long as you run it; ventilation and removing the moisture source treat the cause. Use a dehumidifier when ventilation is not enough on its own — a cold basement, a humid climate, or a room you cannot air properly.