How to measure humidity at home without misleading yourself

The inexpensive instrument is the easy part

A hygrometer measures relative humidity. Many inexpensive indoor thermometers include one, and they are useful for understanding patterns even when they are not laboratory instruments. The challenge is obtaining readings that represent the room rather than the nearest radiator, window or shower.

If two new sensors disagree, place them side by side for a day before deciding either is faulty. Note the difference and use the same unit for comparisons. Consistency is usually more useful for household troubleshooting than treating a single display as perfectly calibrated.

Choose a representative position

Place the hygrometer at roughly breathing height in the occupied part of the room. Avoid exterior windowsills, direct sun, heaters, air-conditioner outlets and the immediate path of an extractor fan. In a bathroom, keep electronics outside wet zones and follow the manufacturer's safety instructions.

When investigating a cold corner or wardrobe, take a second reading there rather than moving your only sensor back and forth. Air trapped behind furniture can behave differently from the rest of the room.

• Label each sensor so the room history stays clear.
• Allow a moved sensor time to settle before recording it.
• Record temperature with RH because the two readings are connected.

Build a useful record

Record at consistent times—after waking, mid-afternoon and before bed is a workable starting schedule. Add event notes for showers, cooking, wet weather, open windows, heating and indoor laundry. A sensor with history logging can simplify this, but a notebook or spreadsheet is enough.

A temporary peak followed by a steady return tells a different story from humidity that stays elevated. Also document what the room does: condensation, damp patches, odour and surface mould are evidence that should not be dismissed because the display happens to look normal later.

Know when measurement is not the answer

A hygrometer cannot distinguish condensation from a plumbing leak or rain penetration. If damp is localised, worsening after rain, accompanied by damaged plaster, or returning despite normal indoor RH, investigate the building rather than buying successively larger appliances.

Where mould is extensive, anyone in the home has concerning symptoms, or the cause is unclear, seek qualified help. The purpose of a home log is to support better decisions—not to replace professional building or medical advice.

Check two sensors before trusting a decimal point

Consumer displays often show whole percentages or decimal places that imply more precision than the underlying sensor can deliver. Read the stated accuracy range in the manual. If the manufacturer gives no humidity accuracy at all, treat the product as a trend indicator rather than a reference instrument.

Place two sensors together in a stable room for 12–24 hours. Do not compare one beside a radiator with another on a cold sill. If one consistently reads several percentage points higher, label the offset rather than changing devices during the experiment. A known, repeatable difference is manageable; unexplained drift is not.

A salt-based calibration check is sometimes suggested online, but executing it poorly can expose electronics to condensation and does not replace traceable calibration. For ordinary home decisions, side-by-side comparison and consistent placement are usually the safer starting point.

Use a log that can answer a question

Do not collect numbers without deciding what you want to learn. A useful question might be: does the bathroom recover within an hour of a shower, does opening the bedroom door change the overnight peak, or does the dehumidifier reduce laundry-drying time at a lower total energy cost?

Record the intervention as carefully as the reading. Otherwise a lower result might be caused by warmer weather, fewer occupants or no laundry rather than the change you intended to test.

Interpret recovery time, not just the peak

The highest number attracts attention, but recovery time is often the more useful measurement. A bathroom may peak sharply during a shower; effective extraction should then move conditions back toward the surrounding baseline. A lower peak that remains elevated for many hours can represent more total moisture exposure.

Compare like with like. Use similar shower duration, door position and starting temperature when testing an extractor change. For laundry, record the wet load, room volume, machine setting, start and finish time, water collected and plug-level electricity use. This turns a vague impression into evidence you can use for buying decisions.

Create a room map when the problem is local

For recurring condensation or mould, sketch the room and mark external walls, windows, radiators, vents and large furniture. Take readings at the occupied centre and at the problem location without pressing the sensor against a wet surface. Note whether doors are open and whether air can circulate behind furniture.

A room map helps distinguish a whole-room moisture problem from a cold bridge or stagnant pocket. If the local area remains materially different while the room centre is stable, buying a larger dehumidifier may be less effective than addressing insulation, airflow or the building fabric.