How to measure indoor humidity at home
A humidity reading is only useful if it represents the room you are trying to understand. You do not need professional instruments for a useful first comparison: a basic digital hygrometer that shows temperature and relative humidity is enough. Consistency matters more than collecting lots of numbers.
For the calculator, collect these three things
- Room temperature from the hygrometer.
- Relative humidity (RH %) from the same device at the same time.
- The surface you are worried about — for example an older double-glazed window or a cold exterior wall. Surface temperature is optional.
1. Put the hygrometer where it represents the room
Use a shelf, table or other open position in the occupied part of the room. Avoid placing the sensor directly beside a radiator, heating vent, open window, humidifier, shower, cooking steam or strong direct sun. Those locations can describe a local microclimate rather than the room as a whole.
If you move the hygrometer from another room, do not treat the first changing number as the final reading. Let the device settle and follow its own instructions if the manufacturer specifies a response time.
2. Record temperature and RH together
Relative humidity changes when air temperature changes, so write down both values from the same moment. A bedroom at 19°C and 65% RH cannot be compared fairly with a living room at 23°C and 50% RH by looking at RH alone. Home Moisture Lab also calculates dew point so the actual vapor level is easier to compare.
3. Match the outdoor temperature to the same time
If you are using a window or wall preset, use the outdoor temperature from roughly the same time as the indoor reading. A morning indoor measurement paired with the warmest afternoon outdoor temperature can make the estimated surface look warmer than it was.
4. Decide which surface matters
Condensation usually appears at the coldest local surface, not necessarily in the middle of a wall or pane. Look at the glass edge, frame, reveal, exterior-wall corner, or a wall behind furniture if that is where the problem occurs.
If you have a trustworthy surface-temperature measurement, enter it. If not, leave the field blank and use a construction preset. The preset is intentionally treated as an estimate because real frames, spacers, corners, curtains, air movement and thermal bridges can be colder than a simplified model.
5. Repeat readings when the problem is most informative
| Room/problem | Useful comparison | What it can tell you |
|---|---|---|
| Bedroom | Before sleep vs. after waking | Whether overnight occupancy raised the dew point, or RH rose mainly because the room cooled. |
| Bathroom | Before shower, after shower, then while the room dries | How quickly the moisture spike clears rather than only how high the peak became. |
| Basement | Basement and upstairs at similar times | Whether the basement actually has a higher vapor level or simply runs cooler. |
| Window condensation | Milder weather vs. a colder morning/evening | How much the surface margin changes as outdoor temperature falls. |
Common measurement mistakes
- Reading directly beside the problem surface. That may be useful for a local sensor study, but it is not the same as room air conditions.
- Using RH without temperature. RH by itself can make a cooler room look much “wetter” than it really is.
- Taking one reading and treating it as permanent. Moisture conditions vary with people, showers, cooking, ventilation and weather.
- Mixing times. Keep indoor and outdoor readings reasonably aligned when you use a surface preset.
- Treating an estimated surface as measured. Presets are for comparison and planning, not a substitute for inspecting a real cold bridge.
A simple first measurement
Start with the room where you notice condensation or high humidity. Place the hygrometer in the room, let the reading settle, note temperature and RH, then choose the window or wall type that best matches the problem. Leave surface temperature blank unless you have measured it. Read the plain-English result first; only open the technical details if you want dew-point margin and saturation values.
Once that makes sense, add a second room. The comparison is often more useful than trying to decide whether one RH percentage is universally “good” or “bad.”
Turn the reading into a room-specific result
The calculator estimates surface RH, a suggested room-RH ceiling for that surface, and — for presets — the outdoor-temperature breakpoint where the modeled surface reaches the site's 80% caution level.
What this cannot diagnose
A hygrometer cannot tell you whether water is entering through a roof, pipe, foundation or wall cavity, and the calculator cannot identify mold or material damage. Persistent dampness, visible growth, recurring leaks or damaged finishes need source investigation beyond an air-humidity calculation. See the primary guidance sources and calculation methodology for the limits used by this site.