Interpreting Thermal Images

Pretty pictures aren’t enough…

Gaps in insulation show as cold areas when seen from inside – like this missing loft insulation, viewed from the room below. Loft insulation gap The temperature markers (added with software afterwards) show the poorly insulated area (15.8°C) is over 5° cooler than a normally-insulated part of the ceiling (21.5°).  This is significant cooling, well worth fixing, whereas if it was only 1 or 2° cooler, that’s not normally serious. But of course the larger the area of poor insulation, the more heat is wasted.
Behind, there’s a loft hatch, deserving closer investigation. They often need better insulation or draught-proofing.
Draught under door
Draughts show as feathery-edged areas, as at the bottom of this door. Cold air currents, which we can’t see, are cooling the floor next to the door. The heat wasted depends on how much cold air is seeping or howling through the gap, but draught-proofing is usually an easy and worthwhile job to improve comfort / save energy.
Radiator needing bleeding



This radiator needs bleeding: the top part is full of air, rather than hot water and so is cooler than the bottom. Gently open the valve at the top of the radiator, which should let out a hiss of air, until the radiator fills with water. Close the valve and have a cloth handy to catch any water that escapes. This improves the efficiency of your central heating. See fuller instructions.

Radiator, no foil


Here’s another radiator, visible from outside the house! Some of its heat is being wasted through the wall. Don’t heat the street!  Fit reflective foil behind radiators that are mounted on external walls – unless those walls are already very well insulated.
The windows at the top appear to be draughty and need checking.

It’s good to thermal image buildings from inside and out, as extra problems often show up, like this radiator problem which couldn’t be seen from the inside.

Outside IR_0503


Cavity wall insulation:
The home owner paid a contractor to insulate the cavity in this wall, but the room still felt cold.  Thermal imaging showed that the lower part of the wall was well insulated, but not the top.  The cavity hadn’t been fully filled with insulation or it had slumped.

Thermal images enabled the home-owner to get the job redone properly without charge.



Terraced Houses: 20 sec video comparing solid and cavity walls, plus doors and windows:

Stone Threshold inside 2

Cold Bridges are where something conductive compromises the insulation. For example this image from inside an eco-renovated house, shows a well-insulated front door with a stone threshold under it from the outside. It’s about 10°C cooler than the door.  The home-owners have now made an insulating draught excluder.




External Corners of rooms are often cooler, because for that area there are more conductive paths through the walls to the outside. Also convected heat from radiators may not reach so well into the corners.

The colder area to the left is caused by the lintel over the window, whose curtain you can see. Such structural beams, usually of steel or reinforced concrete, often cause cold bridges. The problems here are hard to fix, without internal or external wall insulation.


Wasted Electricity is often caused by items left on standby or by forgotten power supplies. Electrical consumption causes heat and so is visible with thermal imaging. eg: the middle power supply is warmest and consuming the most power internally.
You can check the total power consumption of any device that has a mains plug with a plug-in energy monitor.



This house has solid brick walls that have been externally insulated, which is effective, as shown by the blue areas.

The sloping roof, showing 10.1°C is less well insulated. Improving the loft insulation would be very cost-effective, but a lot is stored up there, which the householder finds hard to deal with. You can see our reflections in the sliding glass patio door.

Also see these pages on Thermal imaging:
WhyBasics, Auto & Manual, Emissivity

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