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There is no single correct reading, which is why the question has no one number answer. A hygrometer should read whatever keeps the thing you are protecting safe, and for occupied rooms that is usually a band of 40 to 60 percent relative humidity. The second half of the answer matters just as much: whatever band you choose has to be checked against the coldest surface in the room, because relative humidity that is comfortable in the middle of a room can still be at saturation against a cold window reveal or an uninsulated external corner.
There is no single correct humidity
Humidity targets exist to prevent specific outcomes, and the outcomes pull in opposite directions. Low humidity dries mucous membranes, raises static and shrinks timber and glued joints. High humidity feeds dust mites and mould, slows evaporative cooling so a warm room feels warmer, and raises the risk of condensation on anything cool. Between those two sets of problems sits a band, and the band moves depending on which problem you are more worried about.
That is why a general purpose number is less useful than it looks. A reading of 65 percent is a problem in a room with a cold external wall, a non event in a cellar with uniform temperature, and deliberately maintained in a humidor. The reading has not changed. The thing being protected has.
Target bands by what you are protecting
The ranges below are the bands in common use in general practice for each purpose, not figures from a published standard, and they are starting points for your own rooms rather than limits anyone enforces.
| What you are protecting | Commonly used band | What goes wrong outside it |
|---|---|---|
| Occupied rooms, comfort and airways | 40 to 60 percent | Dry airways and static below, mite and mould growth above |
| Occupied rooms in a cold climate in winter | 30 to 45 percent | Condensation on windows and cold corners at the upper end |
| Surfaces at risk of mould | Below about 80 percent at the surface | Sustained surface humidity above that supports germination |
| Books, paper and photographs | 35 to 55 percent | Brittleness below, foxing and cockling above |
| Wooden instruments and joinery | 45 to 55 percent | Shrinkage and cracking below, swelling and glue failure above |
| Electronics storage | 30 to 60 percent | Static discharge risk below, corrosion and leakage above |
| Cigars | 65 to 72 percent | Drying and splitting below, mould above |
| Wine storage | 50 to 70 percent | Cork shrinkage below, label damage and mould above |
Notice the overlaps and the conflicts. A room storing a guitar and a book collection has a workable joint target. A room storing cigars and electronics does not, and the answer is a sealed enclosure for one of them rather than a compromise that fails both.
Dew point decides whether a surface gets wet
Relative humidity tells you how close the air is to saturating at its current temperature. Dew point tells you the temperature at which it would actually saturate, and it is the figure that predicts condensation. Any surface colder than the dew point of the air touching it collects water, regardless of what the room reading says.
| Reading at 21 degrees C | Dew point | Coldest surface that stays dry |
|---|---|---|
| 30 percent | About 2.8 degrees C | Almost anything in a heated building |
| 40 percent | About 6.9 degrees C | Single glazing survives a mild night |
| 50 percent | About 10.2 degrees C | Window frames and cold corners start to be marginal |
| 60 percent | About 12.9 degrees C | Uninsulated external walls will wet in winter |
| 70 percent | About 15.3 degrees C | Most cool surfaces in the room will wet |
This is why the same 60 percent reading is benign in a well insulated room and a damp problem in a solid walled one. The reading did not change; the coldest surface did. If you can measure a surface temperature as well as the air, you have everything you need: keep the room dew point a few degrees below the coldest surface you can find and condensation stops being a question.
Plus or minus three percent near a limit
Every band above assumes you know the actual humidity, and the instrument has a tolerance. A typical consumer figure is plus or minus 3 percent relative humidity, which the Govee H5179 thermometer hygrometer states alongside a temperature tolerance of plus or minus 0.3 degrees C and a refresh every two seconds. That tolerance means a displayed 58 percent describes a true value somewhere between 55 and 61. Against a 60 percent threshold, the reading cannot answer the question it was bought to answer.
Three habits deal with this honestly. Treat a limit as a band and act when the reading is clearly across it rather than touching it. Establish your instrument’s offset with a salt check so you can correct in the direction you know it leans. And watch the trend rather than the instant, because a tolerance that is mostly a fixed offset cancels out when you are comparing this week to last week. The procedure in how to calibrate a hygrometer covers the salt method, and reading a calibration certificate explains what a stated tolerance includes and what it leaves out.
What a salt test should read
There is one situation where a hygrometer has exactly one correct reading, and it is the check itself. A sealed container holding a saturated slurry of ordinary table salt maintains 75 percent relative humidity in the air above it, across the normal room temperature range and with only a small temperature dependence. A hygrometer sealed in with it should settle at 75 percent, give or take its own tolerance, after several hours at a stable temperature.
Saturated magnesium chloride gives roughly 33 percent and is worth doing as a second point, because two points reveal whether the error is a fixed offset or a slope. A unit reading 69 at the salt point and 29 at the magnesium chloride point has a consistent offset of about five or six points that you can simply subtract. A unit reading 75 and 24 has a scale problem instead, which no single correction fixes.
Most consumer units have no adjustment, so the output of the exercise is a number you keep rather than a change you make. That is still worth having, and it is the difference between a reading and a measurement. For a worked example of what this looks like on an inexpensive unit, see the Jedew hygrometer and Allsun thermometer hygrometer pages.
Reading your own rooms instead of a chart
The practical version of all of this is to stop looking for a number and start building a picture of your own building.
- Put a sensor in each space that has a different purpose or a different temperature, and give each one its own threshold. The GoveeLife 2.0 hygrometer thermometer set states a gateway that pairs up to ten sensors with a stated tolerance of plus or minus 3 percent relative humidity and app and email alerts, which is the shape of tool this calls for.
- Find the coldest surface in each room with an infrared thermometer or the back of your hand on a cold morning, and set the target from the dew point table above rather than from a general band.
- Log for a fortnight before changing anything. Humidity swings daily with occupancy, cooking and washing, and a single reading tells you almost nothing about whether a room has a problem.
- Treat short excursions differently from sustained ones. A bathroom at 85 percent for twenty minutes is fine if it clears. A bedroom corner at 80 percent for weeks is not.
- Watch particulate and carbon dioxide alongside humidity if ventilation is the lever you are pulling, since all three respond to the same window. The Temtop S1plus air quality monitor states temperature and humidity alongside particulate and an air quality index on one display with a stated sixty day battery life, which saves placing two devices in the same spot. Its humidity tolerance is not stated in the listing bullets, so treat its percentage as a trend indicator until you have salt checked it.
- When a reading finally crosses a threshold you care about, check the sensor before acting on it. Most surprising humidity readings are placement or self heating rather than the room.
Answered that way, the question changes usefully. Not what should a hygrometer read, but what does this room need to stay below, and is my instrument good enough to tell me when it does not.
Frequently Asked Questions
Is 40 to 60 percent the right answer for a house?
It is the usual band for occupied rooms, because it sits above the range where dry air irritates airways and below the range where dust mites and mould thrive. It is guidance from general practice rather than a standard, and in a cold climate the lower half of it is safer because the limiting factor becomes condensation on cold surfaces.
Why does my bedroom read higher at night?
Two people breathing add water to the air while the room cools, and relative humidity rises for both reasons at once. The same water content reads as a higher percentage as the temperature falls, so an overnight climb of ten or fifteen points is normal rather than a sign of damp.
What should a hygrometer read in a salt test?
Seventy five percent over a saturated sodium chloride slurry, once the sealed container has equilibrated at a stable temperature for several hours. Saturated magnesium chloride gives roughly thirty three percent and is a useful second point because it brackets the normal indoor range from below.
How do I know whether 58 percent is actually under 60?
You do not. With a typical plus or minus 3 percent tolerance, a displayed 58 could be anywhere from 55 to 61, so a reading that close to a limit cannot settle the question. Either tighten the instrument with a salt check and a known offset, or treat the limit as a band rather than a line.
Should every room have the same target?
No, and that is the main practical consequence. A bathroom tolerates short excursions and needs extraction rather than a target. An unheated store room is governed by its coldest surface. A room with an instrument or a collection in it is governed by that object. Sensors in each space with their own thresholds beat one number for the house.