Thermal imaging: what infrared can and cannot see

Thermal imaging is the most misunderstood tool in leak detection, mostly because of how it looks. The camera does not see water, it does not see pipes, and it does not see through anything. It measures the temperature of the surface pointed at it and colours the picture accordingly. Every useful thing it does for us follows from that one sentence.

What it is actually reading

A leak changes the temperature of the surface above it in one of two ways. Hot water warms the slab and then the tile over it, giving a warm plume with a shape. Or water evaporating from a damp surface cools it below the surrounding material, giving a cool patch on drywall or on a ceiling. In both cases you are seeing the consequence of the leak on a surface, several steps removed from the pipe.

Where the camera earns its place

Hot-water lines under a slab

This is the best case. A hot line losing water under a tile floor produces a readable warm trail, and because the line follows a route rather than a point, the shape of the plume often shows the direction of the run. It is the reason a warm patch of floor is such a useful thing to report when you call about a slab leak.

Mapping a run before anything is cut

Run the hot water and the supply line warms. On a readable surface, the camera shows where it goes. That is a genuinely valuable few minutes before anybody marks a floor.

The extent of a wet area

Once a leak is known, infrared is good at showing how far the water has actually travelled behind a finish — useful for deciding what has to come up, and for documenting the loss if there is a claim. See documenting a claim.

Why Phoenix sun is the biggest obstacle

Infrared measures small temperature differences, and sun swamps small differences. A west-facing wall in July, a paved deck at four in the afternoon, a tile floor with a patch of sunlight across it — every one of those produces a thermal pattern far stronger than any leak. Even after the sun has gone, surfaces release stored heat at different rates depending on what they are made of, and that alone will draw shapes on a picture.

The practical answer is timing and interpretation: work early, or work on interior surfaces that have not been sun-loaded, and never read a single image without knowing what is behind the surface.

What a warm or cool spot is not

• A heating or cooling duct running under or above the surface.
• A recessed light, an appliance or a dryer duct on the other side of the wall.
• A grout line, a mortar bed, a change of tile or an expansion joint, all of which hold heat differently from what is around them.
• A patch of sun, or the place a patch of sun was an hour ago.
• Cool masonry against warm drywall — a structural difference, not a wet one.

Which is why a thermal reading is a pointer and never the verdict. It gets confirmed with isolation, a pressure gauge and acoustic listening before anyone opens anything.

Where thermal is the wrong tool

A cold-water leak under a slab in a house sitting at the same temperature as the ground may show nothing at all. Carpet and pad insulate the floor from what is happening beneath it. Thick tile over a mortar bed blurs the plume until it is not worth much. A pool deck is sun-loaded across its whole area, so infrared rarely adds anything at a pool — the equipment pad and the spa lines are the exception. And the camera cannot give a depth or identify a pipe, ever.

How it fits with everything else

Thermal is fast and non-invasive, which makes it a good early pass: it can save an hour of listening by suggesting where to listen. It cannot prove a line is leaking — only a gauge does that — and it cannot place a leak to the inch. Used as one of three agreeing pieces of evidence it is excellent. Used alone it puts holes in floors, which is why no leak repair starts on a thermal image by itself.