Hygrometers: which one to buy and how to measure correctly

Which hygrometer should you buy? The short answer

For an ordinary home, a simple digital hygrometer costing about €10, with ±2–3% relative humidity (RH) accuracy and min/max memory, is enough. You don't need professional equipment to find out whether you have a condensation problem: you need to measure in the right place, at the right time, for a few days.

In Portugal, 32,1% of residents live in homes with damp or leaks (INE, 2025), and many spend money before measuring anything. The hygrometer is the first step before any bigger purchase. It tells you whether you need a dehumidifier, whether airing is working and whether the damp you see on the wall could be coming from the air. With those numbers, the free diagnosis becomes much more precise.

It measures one thing: relative humidity, the percentage of water vapour the air holds compared with the maximum it could hold at that temperature. That's why the same amount of water in the air reads 60% in a living room at 21 °C and about 80% in a bedroom at 16 °C. Temperature matters as much as water.

What to check before buying a hygrometer

Avoid analogue dial hygrometers, especially decorative ones: they don't store readings and their accuracy is rarely stated. Be wary, too, of accuracy claims without a range — a "±1%" that doesn't say between which RH values is advertising, not a specification.

In practice, two cheap instruments side by side can differ by a few points. That's normal, and the salt test described below settles the question at home, for free.

Is a Bluetooth hygrometer worth it?

Bluetooth or Wi-Fi loggers store a reading every few minutes and show a graph on your phone. They cost more than a simple hygrometer, but they have one real advantage: they show what happens at 4 am, after a shower or while laundry is drying.

They're useful if:

• you want to show your senhorio (landlord) or condomínio (building management) that humidity stays high for weeks — an exported graph says more than a photo (see the guide for tenants);
• you're testing a change, such as leaving the window on trickle vent, running the extractor fan longer or using a dehumidifier;
• you have several rooms and want to compare the north-facing bedroom with the living room at the same time.

For a first assessment, a simple instrument with min/max memory gives the same essential information. Note the readings on paper or on your phone every morning for a week and reset after each reading.

How to calibrate a hygrometer with the salt test

A paste of table salt and a little water — a saturated solution — keeps the enclosed air around it close to 75% RH at normal household temperatures. It's the simplest calibration test there is.

1. Put a tablespoon of fine salt in a bottle cap or small cup and add a few drops of water until you have a damp paste. Some salt must remain undissolved.
2. Place the cap and the hygrometer inside a well-sealed freezer bag or airtight box, without touching each other.
3. Leave for 8–12 hours somewhere with a stable temperature, away from sunlight and heaters.
4. Read the value without opening the bag. If it shows 72%, the instrument reads 3 points low; if it shows 79%, it reads 4 points high.
5. Add or subtract that difference in future readings, or use the calibration function if the instrument has one.

Where and when to measure humidity at home

The reading that matters isn't the afternoon one, with the house open and heated. It's the early-morning one, when the temperature drops and the vapour from sleepers builds up. Two people sleeping release about 0,5–1 litre of water per night.

• Time: early morning, around 7 am, before opening windows. The max memory captures the overnight peak.
• Height: mid-wall, 1–1,5 m above the floor, at least 1 m from windows, heaters and the bed.
• Rooms: the bedroom with mould, the bathroom after a shower, the kitchen while cooking dinner.
• Duration: at least 24 hours in each room, ideally a week in winter.
• Avoid: holding it in your hand, placing it near someone's breath, in direct sun or inside a wardrobe (unless you want to measure the wardrobe).

How to read the numbers: 40–60% is the recommended range; 60–65% calls for attention; above 65–70% every morning for weeks, the risk of condensation and mould is real in homes with cold walls. More detail in ideal humidity at home.

Keep a simple log with the date, time, room, humidity, temperature and the weather outside. After a week a pattern emerges: if humidity rises on nights with the door closed and two people asleep, the likely problem is ventilation; if it rises on rainy days and a wall darkens, think water ingress.

Hygrometer plus infrared thermometer: the dew point

Air humidity tells half the story. The other half is the surface temperature of the wall. Mould grows when the humidity right at the surface stays above ~80% for days, even without visible droplets — and that happens at the coldest spots: corners, near the ceiling, behind wardrobes, around aluminium window frames without a thermal break.

An infrared thermometer, about €15–20, measures that temperature without touching. You then compare it with the dew point, the temperature at which water vapour turns into liquid. Example: at 18 °C and 75% RH in the bedroom, the dew point is ≈13,5 °C. If the corner behind the wardrobe is at 13 °C, water condenses there.

You don't need to do the maths: enter the temperature and humidity in the dew point calculator and compare with the thermometer reading. A wall about 3 °C or less above the dew point is already in the zone where mould can grow.

humidade.pt's shop sells a kit with both instruments, but any hygrometer and thermometer with these features will do. Some links on the site may be affiliate links and are always marked — see how we make money.

The readings are high. Now what?

If the bedroom spends its mornings above 65–70%, start with the sources of vapour: laundry drying indoors, showers without an extractor fan, gas or paraffin heaters, pans without lids. Then ventilation: 5–10 minutes of cross-draught in the morning and in the evening. The condensation guide has the steps in order.

If after two weeks of good habits the readings don't come down, a dehumidifier may make sense — see which dehumidifier to buy.