A potential source of confusion… (a bit techie)

Just as you can see your refection in a window, you can see it in a thermal image too. Thermal reflections in glass or metal surfaces  cause misleading temperature readings (as can their low emissivity).

If you’re not sure whether you’re seeing thermal reflections, try moving left and right, while watching the camera screen: reflections move differently from features seen through the glass.

At the infrared wavelengths used, glass is often more mirror-like than for visible light, depending if the glass has special coatings.  Low-e glass is coated to be more reflective (low emissivity) at these wavelengths. This makes these windows more thermally efficient, because radiant heat from warm things in a room (like you) is reflected back, rather than transmitted to the outside.

Reflections - me & house oppositeBe cautious interpreting thermal images of glass:

in this image you can see reflections in the window of my head & shoulders and more faintly, behind me, the warm windows of the houses across the street. How much of this window’s apparent warmth is real or due to reflected heat from houses across the street, etc? There’s no easy way to disentangle this – the image tells us little about whether heat is being lost to the outside through this window.


The glazed front door of the left-side house appears to be hot (7.1°C, Sp2) and so losing a lot of heat. Consider carefully if what you see in the glass could be reflected heat instead –  maybe reflections of hot parts of the houses on the camera-side of the street? In fact those houses were less hot.  I was standing opposite the door, but I can’t see my reflection in it. Either I was just off-centre or my reflection was flooded out by the heat radiation through the door.

So this single-glazed door does seem to be very poorly insulated, losing significant heat. This could be confirmed with PVC tape (see below). Double or secondary glazing, a curtain behind the door or a replacement insulated door are ways to reduce this heat loss.

The two upper floor windows on the left each have top-hung panes that are open, losing some warm air. The upper rooms seem to be fairly cool and well-ventilated. Maybe that’s what’s wanted. The open window panes, angled upwards, appear to be the coldest parts of the image (-5.5°C, Sp1), but this is because we’re seeing the cold sky reflected in them.

In Practice
Beware of reflections in thermal images of any glazing (or bare metal).

If you need to know glass temperature, stick on some PVC insulating tape and measure its temperature by thermal imaging after it’s equilibrated  (about 30 seconds).

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