Are thermal imaging cameras worth it for finding heat loss?

Thermal images are seductive because they look like evidence. They are also very easy to misread: what the camera measures is the infrared emitted by a surface, which depends on its temperature and on how emissive the material is, so shiny metal, glass and wet patches all lie. Used in the right conditions with a bit of care, it is a superb tool. Used in July, it shows almost nothing.

What you'll need

• Infrared spot thermometer (optional)
• Thermal camera or a phone attachment (optional)
• A cold day and a warm house

Step by step

1. Decide which question you are asking.If it is 'is this specific surface below dew point', a spot thermometer answers it directly and costs very little. If it is 'where are the problems I have not found yet', that is what a camera is for.
2. Get the conditions right.You need a substantial temperature difference between inside and outside — a cold day, ideally below about 5 degrees outside, with the heating having run for several hours. In mild weather the image is meaningless.
3. Do it before sunrise or after dark.Solar gain on a wall persists for hours and swamps the differences you are looking for. Early morning in winter is the best time.
4. Survey from inside.Internal surveys are far more informative than external ones for a homeowner, because you see the actual internal surface temperatures, which is what condensation and comfort depend on.
5. Learn the common patterns.Cold rectangles in a wall are missing or slumped cavity insulation. Cold stripes on a ceiling are joists showing through insulation. Cold plumes at skirtings, ceiling edges and around sockets are air leakage. Cold frames around openings are cold bridging at the reveals and lintel.
6. Beware the false readings.Glass, polished metal, radiators and any shiny surface reflect infrared and read wrongly. A wet or recently washed surface reads cold because of evaporation. Never conclude from a single image of a reflective surface.
7. Photograph both the thermal and the normal view.A thermal image on its own is very hard to interpret weeks later. Pair each one with an ordinary photograph of the same view.
8. Follow up with a spot thermometer.Once the camera has found the cold areas, measure them with a spot reading and compare against the dew point calculated from your room temperature and humidity. That converts a pretty picture into a decision.
9. Consider borrowing rather than buying.Some local libraries and community energy groups lend thermal cameras, and an independent energy assessor's survey delivers both the images and the interpretation. Either is cheaper than owning one you will use for one winter.

Tips

• Phone-attachment cameras are far better than nothing and adequate for finding missing insulation and air leaks. Their absolute temperature accuracy is limited, so use them to find patterns and a spot thermometer to measure.
• An external thermal survey of a whole house looks impressive and tells you less than an internal one, because you are seeing the outer skin rather than the surfaces that condense.
• The candle-and-hand survey still works and costs nothing: a hand run round a window frame and a candle flame held near a skirting find most draughts.

Common mistakes

• Surveying in mild weather — With only a few degrees of difference between inside and outside there is nothing for the camera to show. The image will be dominated by reflections and surface finishes rather than by heat loss.
• Reading a thermal image of a window as a fault — Glass reads cold because it is cold and because it reflects. That is normal and expected; the useful information is in the frame, the reveal and the junction with the wall.

Questions people ask