What is Thermography?
Thermography is the use of thermal infrared energy emitted by various surfaces to create an image using a camera sensitive to thermal infrared photons. Depending on the camera, sensitivity and resolution, it shows the relative temperature of that surface. Since surfaces differ in the way they radiate thermal energy, it does require some understanding of physics, underlying structure, and device operation to meaningfully extract the information contained in a thermographic image.
Uses of Thermography
Thermography shows differences in apparent surface temperature. As such it can be used to detect sources of cold or heat, and presence of moisture. For example, the image above was taken inside an attic which had one or more unknown sources of heat leaking into the attic. Visual inspection did not reveal any obvious source, so thermography was used and it showed a section of old roof (this was inside an extension) that was supposedly insulated, but was obviously leaking heat into that section of the attic.
Thermography is often used by home energy auditors to detect weaknesses in insulation or air sealing. This is usually done in conjunction with a blower door that depressurized the house and thereby allows air leaks to be found. In some cases, it is possible to see the heat loss without the use of a blower door setup, as can be seen in the following image:
This image shows several hot spots where there are gaps between insulation bats, allowing heat to escape into the attic.
Thermography is also used by flat-roof inspectors to detect points of leakage (since the thermal mass of wet insulation is different from dry insulation). Other detection of leaks depend on the fact that evaporating water is always cooler than its surroundings, which allowed the detection of presence of water above a vapour barrier, from below the ceiling gyproc (see below):
This image showed that a leak found elsewhere, had also spread to a previously unsuspected area of the ceiling. This will allow the homeowner to address the potential damage to the insulation, etc., at the same time as they address the known leak area.
What does Thermography NOT show?
Thermography is not x-ray vision. It will not show structures behind solid surfaces, unless there is some thermal difference that ends up working its way to the surface. It does not show leaks directly – one has to confirm independently whether an apparent cold spot is due to evaporating moisture, or due to some other cause. It will not give much useful information if the surfaces are at an equilibrium temperature, which is why thermography requires a certain minimum difference in temperature to allow the heat characteristics of the materials to reveal themselves. Thermography does not distinguish between different sources of heat, such as a surface heated by the rays of the sun, compared to a portion of the surface in the shade. Sometimes the apparent differences in temperature are due to the type of surface and type of material from which the thermal infrared energy is radiated, and therefore the operator (or interpreter of the image) needs to know the emissivity of the material being examined.
In short, thermography in the hands of a capable operator is a useful tool. The same equipment and image can be used to give completely erroneous advice if the operator hasn’t taken the time to verify a number of conditions prior to interpreting the image. Details matter.
Why are we talking about thermography?
It’s another tool in the toolbox. Used in conjunction with a detailed inspection, it can give additional insight into the behaviour of a roofing system. It can collaborate (or contradict) certain observations, and it can give additional clues in diagnosing troublesome situations.
If you are using my consulting services, you will probably have seen me using the device to give me a better understanding of how the roof system is behaving. Better information should lead to better decisions.
(c) 2017 Paul Grizenko