Interpreting IR Images

A Bunch of B.S. (Building Science, of course):

Worth A Thousand Words: An IR Image With a Lot to Say

Nate Gusakov, BECxP

Take a look at the IR image that goes along with this article (and its similar but visible light image). Both were taken at the same time (about 11:30AM) by different lenses on the same camera. When I took the pictures, I was in an office building in Middlebury, VT looking up at a vaulted ceiling with skylights on both sides of the ridge. Obviously, there’s more going on than meets the visible-spectrum eye. Let’s explain it.

First, what exactly is an IR image? IR stands for infra-red. ‘Infra’ means below or beyond, and ‘red’ means, um, red. So, it’s a ‘below-red’ image? Exactly. Electromagnetic radiation is detectable by different receptors at different wavelengths. For example, UV light’s wavelengths are just barely too short for us humans to see, but as we move down the vibrational spectrum the waves grow in length, vibrations get slower, and our eyes perceive a backwards rainbow of visible light: violet, blue, green, yellow, orange, red…hmmm, what’s below the frequency of red light? Infrared, indeed.

Now, what is the most common way we perceive electromagnetic radiation in the IR spectrum? As heat. Our nervous system can perceive IR radiation directly, right through our skin. It’s helpful enough to keep us a safe distance from a fire or know when it’s too cold to be outside without a jacket, but our own IR perception isn’t nearly sensitive enough to map out detailed, subtle differences in radiation patterns on a surface 20 feet away. For that you need an IR camera, which has a non-glass lens (usually made of Germanium, because glass will block heat and would be about as good as a lens cap in this case) and a crazy system of micro-mirrored ‘pits’ behind it. These pits direct wave/photons of incoming IR radiation down towards a very special receiving element at the bottom (one ‘pit’ equals one pixel). This receiving element (usually vanadium oxide or amorphous silicon, if you care) generates an electrical charge relative to the intensity of the IR radiation it’s receiving, and a ‘picture’ of the surface temperature of things can be created.

OK good, now that we’re all up to speed on how this picture gets made, let’s dive into what it tells us. For reference, the narrow color bar on the left side of the IR image shows how the image colors relate to temperature, going from coldest at the bottom (dark blue, ~52F in this image) to warmest at the top (red/white, ~68F). So, what can we see?

1. It’s at least somewhat cold outside, and there is no thermal break above or below the roof rafters. The rafters (the green rib-like pattern in the IR image) are acting as thermal bridges, drawing heat from the room directly through the sheetrock and across their wooden bodies to the roof, which must be relatively cold if it’s continuing to cool the room via the rafters at mid-day. Even a fairly thin layer of continuous insulation (often rigid foam, in an application like this) on the top or bottom side of the rafters would create a thermal break and make the rafter pattern disappear from the thermal image (which would mean that less heat is being lost to the outside).

2. The spaces between the rafters do a better job of insulating than the rafters themselves. The ceiling surface is warmer between the rafters, so there must be something in there that has a higher R-value than wood framing. This is good.

3. The ridge of the building runs north-south. The difference in temperature between the surfaces of the skylight wells on the right (bright red) versus the left (deep blue) is striking! In this case, it’s all about angle of incidence. This is the angle at which sunlight strikes a surface. If the sunlight is striking more or less perpendicular to a surface (high angle of incidence), the full force of its radiation is absorbed as directly as possible. In this picture, the skylight wells on the right slope towards the east and at that time of day are receiving much more heat energy than they are releasing. If the sunlight strikes at a slant or askew against a surface, the amount of IR radiation absorbed by that surface is significantly less. In this case, the sun is still shining on the left (west-facing) skylights, but it is striking them at a very low angle of incidence, so they are gaining less IR radiation than they lose.

There it is—much can be learned from an IR image. Maybe not quite 1000 whole words for this particular picture, but pretty close anyhow. Now I have to find some fresh germanium; my stash is running low.

Nate Gusakov is a building enclosure consultant at Zone 6 Energy and Silver Maple Construction. He aspires to be like Friar Tuck in the Sherwood Forest of modern building science.