Every recommendation on this site depends on one number, and that number is easy to get wrong. Cheap hygrometers drift by five to ten points, placement changes the reading more than the season does, and the time of day you look decides whether you see the problem at all.
None of that requires better equipment. It requires putting the meter in the right place and checking it once against something known.
Where to put it
Chest height, on an interior wall or a piece of furniture away from the wall, in the middle of the room. Keep it clear of the things that produce a local microclimate rather than a room reading.
- At least 3 feet from any heating vent, radiator or air conditioner
- Away from windows and exterior walls, which read colder and damper than the room
- Not in direct sunlight, which heats the sensor and drops the reading
- Not in a bathroom or kitchen if you want a whole-house number — those spike by design
- Not on the floor and not on a high shelf; both stratify away from where people breathe
When to read it
Give a hygrometer at least two hours in a new position before you believe it, and longer if you moved it between very different rooms.
Then read it first thing in the morning, before opening any windows or doors. Overnight is when moisture accumulates and surfaces are coldest, so the morning reading is the worst-case one — which is the one that decides whether mold grows.
An evening reading in a ventilated house looks reassuring and tells you very little.
Check whether yours is lying: the salt test
Cheap sensors drift, and there is a simple way to find out by how much. A saturated salt solution holds the air above it at 75% relative humidity at room temperature, reliably enough to calibrate against.
Half-fill a bottle cap with table salt and add water until it is damp but not dissolved — a slurry, not a solution. Put the cap and the hygrometer together in a sealed zip bag, leave it somewhere at a stable room temperature for eight hours, then read the meter without opening the bag.
It should read 75%. If it reads 68%, your meter runs seven points low and every reading you take needs seven added. Some units have a calibration adjustment; most do not, and knowing the offset is just as good.
Measure the room that is failing
Humidity is not one number for a house. The room that matters is the coldest, least-heated, least-used one — a north-facing bedroom, a spare room with the door closed, the basement.
That room reaches mold conditions while the living room reads a comfortable 45%, which is exactly why so many people are surprised by growth in a house whose hygrometer says everything is fine.
If you are chasing a specific damp problem, put a second hygrometer in that room and leave it there. Two cheap meters in the right places beat one accurate meter in the wrong one.
The reading you cannot get from a hygrometer
Mold responds to humidity at a surface, and no room hygrometer measures that. To get it you need the surface temperature, which an inexpensive infrared thermometer gives you in a second.
Point it at the corner that keeps growing mold, the window reveal, the wall behind the dresser. Put that temperature and your room reading into the mold risk calculator and you get the number that actually decides whether growth starts.
This one addition turns a hygrometer from a comfort gauge into a diagnostic tool.
Getting a reading you can act on
Most disagreements about a damp house come down to two people measuring different things and both being right. These are the practices that make a reading comparable with itself over time.
Measure at a fixed time, and make it early morning. Indoor humidity swings substantially over a day — a bedroom can rise fifteen or twenty points overnight with people in it and fall again within minutes of a door opening. A reading taken at 6 am before anything has been opened is the honest worst case, and it is the one that predicts whether surfaces will condense. An afternoon reading in a ventilated house flatters everything.
Measure in the room that is failing. A hygrometer in the living room of a well-heated house tells you almost nothing about a north-facing bedroom or a basement, and the gap between them is routinely fifteen points or more. If you are chasing a problem, the reading that matters is in the problem room; a second cheap unit costs about as much as takeout and settles most household arguments.
Give the instrument time and the right position. Chest height on an interior wall, away from vents, radiators, direct sun, exterior walls and doorways, and left for at least two hours before you trust it. Sensors moved from a much wetter or drier place take longer than people expect to equilibrate, and a reading taken five minutes after unpacking is meaningless.
Check the calibration once, with the salt test. Half fill a bottle cap with table salt, add a few drops of water to make a slurry without dissolving it, seal the cap and the hygrometer together in a zip bag, and leave it eight hours at a stable temperature. Saturated salt holds the air above it at 75% relative humidity, so whatever your unit reads is its offset. Note the difference and subtract it from then on; a cheap unit that is consistently five points high is perfectly usable once you know that.
Finally, record what you read. A single number tells you very little; two weeks of morning readings tell you whether a change worked, and it is the only way to separate the effect of something you did from the effect of the weather. A note on a phone is enough — the discipline matters more than the format.
Common questions
Where should I put a hygrometer?
At chest height in the middle of the room, at least 3 feet from vents and radiators, away from windows and exterior walls, and out of direct sunlight. Give it two hours to settle before trusting it.
How do I know if my hygrometer is accurate?
The salt test. Seal a damp salt slurry and the meter in a zip bag for eight hours at stable room temperature — it should read 75%. Whatever it reads instead is your offset.
What time of day should I check humidity?
First thing in the morning, before opening windows or doors. That is when moisture has accumulated overnight and surfaces are coldest, which is the worst-case condition that matters.
Do I need an expensive hygrometer?
No. Two cheap ones in the right rooms are more useful than one accurate one in the wrong room. Check each with the salt test and note the offset.
Are the humidity readings from a smart thermostat or weather station accurate?
They are usually accurate enough, with one important caveat: position. A thermostat is mounted where it is convenient for the heating system, which is typically a hallway on an interior wall — a location that reads more favorably than any of the rooms that actually have problems. Use it as a house baseline and put a separate unit in the room you are worried about.
How often should I check the humidity once things are under control?
Weekly is plenty in a stable house, plus a check whenever the season turns or a cold snap is forecast. The two moments worth being deliberate about are the first hard freeze of winter, when the ceiling drops sharply, and the first properly humid week of summer, when a basement or crawl space starts to climb. Both arrive before anyone thinks to look.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions