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Part IX: Tools, Calculators, and TestingChapter 14. Measure It: Testing Your Builds

Using a Thermal Camera Well

Reflections, emissivity, and the other ways a thermal image can lie about heat loss.

Difficulty
Intermediate
Cost
Free if you have the camera
published

A thermal camera sees infrared radiation and paints a picture of apparent temperature, which is not always the real surface temperature. Shiny metal and glass reflect the room, the sky, or you, and the camera will happily call that reflection a surface temperature. A real leak shows up when the house is much warmer or much colder than the outdoors, on a surface that is not a mirror, and it stays put when you move.

Use the camera as a way to decide where to look harder. Confirm with the smoke pencil, a hand on a safe surface, or a contact thermometer. The picture is not a heat-loss calculation and not an electrical inspection.

Difficulty

Level
Intermediate
Time
A cold evening indoors, plus the time to confirm what the colors meant
Skills
Moving your body to see whether a spot is a reflection

Cost estimate

Free if you have the camera

Borrowing one is enough for a leak hunt. Buying one is a tool purchase this page will not price, because the models change and the skill matters more than the badge. A contact thermometer is the cheap confirmation.

Materials

ItemQtyNotes
A thermal camera and a large indoor-outdoor temperature difference1A mild afternoon hides leaks. A cold night or a hot day earns the picture.
A contact thermometer or the smoke pencil1For the spots you might act on. The camera nominates them. Another tool confirms them.

Emissivity and reflections

Start near 0.95 for painted drywall, wood, and other dull surfaces, and do not believe a number the camera prints on bare metal, foil, or glass. Those surfaces reflect. Low emissivity is why a selective solar absorber can get hot in the sun and still radiate less than a flat black paint. The camera is the wrong instrument to "read the absorber temperature" off a shiny fin through the glazing. It is mostly seeing a reflection of the sky and of you.

If a cold or hot spot moves when you step sideways, it is a reflection. If it stays on the wall while you move, it might be a real temperature difference: a missing batt, a thermal bridge, a damp patch, or air leaking at a trim line. The image alone does not say which. Air leaks often show as a feather or a streak at a joint. Missing insulation looks broader. Water can look warm or cold depending on evaporation and on whether the sun has been on the wall. Do not condemn a roof from one bright blob.

Windows are mirrors in the infrared. From indoors you are often photographing the reflection of the room, not the outdoor temperature of the pane. A more useful window check is the edge, the sash, and the trim, confirmed with smoke or a hand, not the center of the glass number.

Apparent temperature can lieA wall with a real cold leak and a shiny pipe that only looks cold because it reflects.Real cold spotShiny pipereflects the roomIf the spot moves when you move, it is a reflection.
A real cold spot stays on the wall. A shiny pipe shows a reflection that moves when you do.

A useful evening

Scan from indoors on a cold, reasonably calm night, with the heat on and the sun long down so you are not seeing solar gain. Walk slowly. Note the room temperature and the outdoor temperature in the notebook so the picture has a scale later. Look at outlets on exterior walls, attic hatches, rim areas you can see, and the corners where insulation is hard to fit. Then confirm. A camera that costs real money can still be wrong, and a draft you can feel is the leak the picture was hinting at.

Do not scan an open electrical panel, a bus bar, or a meter socket as a homeowner project. Those temperatures are an electrician's, taken under rules this page will not stretch. Do not walk a roof at night because the camera made you curious. The ladder guide does not have a night exception.

Composite: the cold window number

Invented image. The center of a window reads very cold. The household prices new glass before anyone steps sideways and watches their own reflection slide across the pane. The sash, which is the actual leak, is a thin streak they did not confirm with smoke. The composite lesson is that the dramatic color is often the mirror, and the dull edge is often the job. It is not a real audit, and it names no camera brand.

  1. Wait for a real temperature difference

    Cold night or a clearly hot day. Write both temperatures down.

  2. Distrust metal, foil, and glass

    Start emissivity near 0.95 only for dull painted surfaces. Move, and see what moves with you.

  3. Confirm before you buy anything

    Smoke, a safe hand, or a contact thermometer. The picture is a short list, not a scope of work.

  4. Stay out of panels and off the roof

    Electrical gear and night roofs are outside this tool's job.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.