Infrared thermometers: how to use one to find cold walls

What is an infrared thermometer for at home?

An infrared thermometer measures the temperature of a surface without touching it: aim, pull the trigger and read. It costs ~€15–20 and, together with a ~€10 hygrometer, answers the most important question in a mould case: are there places in the home cold enough for moisture in the air to condense?

Condensation and mould don't appear where the home is damp, they appear where it is cold. In an uninsulated 1970s building, the outside wall can be 4–5 °C below the bedroom air in a corner or along the ceiling slab. That is where mould starts, and that is what the thermometer shows.

The instrument doesn't measure humidity and doesn't «see» water inside the wall. It measures surface temperature. The value only means something when you compare it with the dew point of the air, which you get from the hygrometer and the dew point calculator.

Emissivity: the setting almost nobody adjusts

Infrared thermometers work out temperature from the radiation a surface emits. Each material emits differently, and this property is called emissivity, on a scale from 0 to 1. Most instruments come set to 0.95, which works well for the surfaces in a home.

The case that matters most in Portugal is aluminium window frames without a thermal break: measured directly, they can look lukewarm when they are ice-cold. Stick a strip of paper masking tape on the frame, wait a few minutes for it to reach the same temperature and measure over the tape. The guide on aluminium windows without a thermal break explains why those frames condense.

Distance and spot size: what 12:1 means

The thermometer doesn't measure the laser dot: it measures the average of a circle that gets bigger the further away you are. The distance-to-spot ratio, given on the box or in the manual, tells you the size of that circle. On a 12:1 instrument, every 12 cm of distance corresponds to 1 cm of measured diameter.

Measuring a ceiling corner from the floor, 2–3 m away, averages the cold part of the corner with the warmer wall around it. The result looks better than reality. Climb a stable stepladder or hold the instrument 30–50 cm from the surface. The laser is only a sight; the measured circle is always larger than the red dot.

When to measure: 7 am, in winter, with the home as usual

The best time is early morning on a cold winter day, before opening windows and before switching on heaters. That is when outside walls are coldest and bedroom humidity is highest, after a night of breathing with the door shut.

• Don't measure walls in direct sun or that have had sun in the last few hours: they are warmer than for the rest of the day.
• Don't measure straight after heating the room or opening windows: surfaces take time to settle.
• Read the hygrometer at the same moment, in the middle of the room, to work out the dew point.
• Repeat on 3–4 different mornings, including the coldest morning of the week.

Note the outdoor temperature that day. In Lisbon, with 8 °C outside, an uninsulated wall behaves very differently from a 14 °C day. Measuring on mild days underestimates the problem.

Where to aim: corners, behind furniture and window reveals

Start with a reference reading: the middle of an interior wall, away from windows. Then go through the spots where condensation usually appears:

1. Upper corners of outside walls, especially where two outside walls meet.
2. The line between the outside wall and the ceiling, where the concrete slab forms a thermal bridge.
3. Behind wardrobes, beds and sofas against outside walls — move them a few centimetres to measure.
4. Window reveals, lintels and sills, and the roller-shutter box.
5. Concrete columns and beams built into the façade.
6. Lower corners of outside walls, next to the floor.

A difference of 3–4 °C between the corner and the middle of the same wall is a clear sign of a thermal bridge. Draw a simple plan of the room, note the values and repeat after changing habits: that is how you see whether moving a wardrobe or heating a bedroom more steadily makes a difference. The guide mould in the wardrobe uses this method.

Comparing with the dew point: what the values mean

The dew point is the temperature at which water vapour in the air turns into water. It is calculated from the air's temperature and relative humidity. Example: at 18 °C and 75% RH in the bedroom, the dew point is ≈13.5 °C. But mould doesn't need droplets: it grows when humidity at the surface stays above ~80% for days, which in that bedroom happens on any surface below ~17 °C.

The numbers also show where to act. You can lower the dew point by reducing vapour — airing, extractor fans, drying laundry outside — or raise the wall temperature by heating more steadily and moving furniture away. Both routes are in the guide on condensation.

Limits of the thermometer, and which one to buy

A wall wet from rising damp or water ingress also shows up cold, because evaporating water cools the surface. So a cold area near the floor of a ground-floor flat, with salts, isn't necessarily condensation. Combine the reading with the other signs — shape of the stain, season, floor — or use the condensation or rising damp tests and the free diagnosis.

For home use, look for a distance-to-spot ratio of at least 12:1, 0.1 °C resolution and, if possible, adjustable emissivity. You don't need more. The humidade.pt shop has a kit with a thermometer and a hygrometer, but any model with these features will do. Some links on the site are affiliate links and are always marked — see how we make money.