Can infrared really see through walls?
The answer to this question depends on what we mean by the word ‘see.’ The way a good infrared camera works does give us information about what is going on behind certain materials, particularly drywall. We know that framing material should be behind drywall; and when we want to hang something on the wall, we use a stud finder to find the denser parts of the wall which gives us an idea of where the stud is located. Infrared is similar in that it gives us an idea of the framing behind the wall, in particular, where framing members are relative to each other.
A picture of a bedroom and its thermogram showing the framing members behind the drywall. The thermal image can also show missing insulation behind walls or above ceilings in the attic. This is produced by detecting differential temperatures created there is a differential temperature between the exterior and interior.
To understand this, one needs to know how infrared thermal imaging works. Infrared light is part of the invisible light spectrum. Humans can only see the tiny visible portion of the light spectrum. The colors we see everyday are between the UV and IR on the spectrum, outside that tiny portion of the light spectrum the light is invisible to use. We cannot see UV or IR light but you know it there when UV burns your skin on a sunny day. And; you know IR light is there when you come out of the shade and into the sun, you can feel the warm of the IR rays.
An infrared camera works by detecting IR rays. Different materials absorbs and gives off heat differently. For example, metal objects are good a conducting heat and thus heat transfer through it fast thus it is often cool to the touch. Wood on the other hand absorbs and releases heat much slower compared to metal. We refer to this relative ability as emissivity. To put it another way, emissivity is the relative ability of an objective’s surface emit radiation. It is compared to a black body, which is and idealized physical object that absorbs all incident electromagnetic radiation. A black body is an ideal emitter and diffuses energy isotropically (independent of direction). When it comes to building materials, different materials have different emissivity values, indicating its relative ability to emit radiation.
So how does an infrared camera see behind drywall? Wood studs in contact with drywall will produce a different emissivity value compared to just drywall. Drywall in contact with insulation will also produce different emissivity value compared to just drywall. Given the 3 emissivity values so far, we should see 3 different color bands in the thermogram produced by the infrared camera. Keep in mind that in building inspection applications, the pattern of the thermogram what we are interested in seeing.
Read more about infrared thermal imaging.
