Cold Spots, and What a Thermometer Actually Reads

The infrared thermometer is the most photogenic instrument in the hobby and the most misunderstood. It is a good instrument. It is answering a question. The question is not the one being asked of it.

What the gun is doing

An infrared thermometer collects long-wave infrared radiation from a cone in front of it, focuses it onto a thermopile, and computes the temperature the emitting surface must be to radiate that much. Three things follow.

It measures surfaces, not air. Air at ordinary humidity is close to transparent in the band these instruments use. Point the gun into the middle of a room and the beam travels through the air and lands on the far wall, and the number you get is the far wall.

The spot grows with distance. Every one of these has a distance-to-spot ratio, typically between 8:1 and 12:1 on consumer units. At 12:1 and three metres, the instrument is averaging a circle a quarter of a metre across. A "cold spot the size of a person" measured from across a room is an average over an area much larger than a person.

Emissivity is a setting, and it is wrong. The conversion from radiation to temperature depends on how efficiently the surface radiates. Most cheap instruments assume 0.95, which is about right for matt paint, plaster, wood and skin, and badly wrong for glass, polished metal, foil, gloss paint, mirrors and stainless steel. Point one at a chrome handle and it will report a temperature dramatically below the room, every time, in any building. That is not a cold spot. That is a shiny thing.

What to use instead

A contact or immersion probe: a thermocouple or thermistor on a thin lead, with a small enough thermal mass to respond in seconds rather than minutes. Hold it still, in the air, away from your own body, and watch it settle.

The important discipline is giving it time. A probe moved into a new position is reading its own thermal history for the first twenty seconds or so. If you sweep a probe through the air and watch the number fall, you have measured the probe, not the room. Almost every dramatic handheld temperature drop on the internet is this.

Where cold spots actually come from

The Survey's list, in rough order of frequency:

• Draughts, and specifically the stack effect. A building warmer inside than out behaves like a chimney: air leaves at the top and is drawn in at the bottom. Every gap at low level — under a door, a letterbox, a floorboard, an unsealed pipe penetration — becomes an inlet, and cold air moves along the floor and up staircases in ways that feel entirely purposeful. Cold spots in halls and on stairs are this until proven otherwise.
• Chimneys. An open, unblocked flue in an unused room is a hole in the roof.
• Thermal mass. A stone or brick wall in an unheated building runs several degrees below air temperature and cools everything near it by radiation. Stand next to one and you will feel cold on that side, correctly, because you are radiating to it.
• Cold bridging. Lintels, joist ends, steel and uninsulated reveals conduct heat out. A thermal camera finds these instantly and it is the best reason to own one.
• Evaporation. Damp surfaces, and damp skin. A person who has been moving and stops in a cool room will feel a distinct chill that is entirely their own evaporation.
• You. Turning through 180° puts a different side of your body toward the coldest surface in the room. This is a real temperature change at your skin and it correlates with facing a direction, which is exactly the pattern people report.

How to measure one properly

1. Fix positions and mark them. A cold spot that cannot be returned to is not a measurement.
2. Two probes minimum: one in the suspected spot, one a metre away, logging together. A single probe cannot distinguish a room cooling from a spot appearing.
3. Let everything equilibrate for at least a minute before you record anything.
4. Record air temperature, surface temperature and air movement together. A smoke pencil or a strip of tissue on a thread costs nothing and answers the question in about four seconds.
5. Note the outside temperature and wind. The stack effect scales with the difference, and a building that was quiet on a mild evening will be full of draughts when it drops to freezing.

What survives all that

Not much, and that is the point. When the Survey has run this properly — fixed positions, logging probes, air movement measured, outside conditions recorded — the great majority of reported cold spots turn out to be air moving, thermal mass, or an instrument reading itself.

What is left over is a small number of files in which two logging probes a metre apart diverged by more than a degree, with no measurable air movement, for a bounded period, and came back together. Those are worth something precisely because everything else was excluded first. A cold spot found with a thermal gun and a torch is worth nothing at all, not because it was not real, but because there is now no way to tell.