While many building materials typically have high emissivities, there are some that are in fact quite the opposite (i.e. thermally reflective). The most obvious, and typically most reflective, might be unpainted flashing on the exterior or chrome surfaces on the interior. It is important for building thermographers to never forget this and keep an eye out for low emissivity surfaces that can potentially confuse thermal patterns or even mask the signatures of missing insulation or air leakage. Also reflective are some types of wallpaper. Wallpaper? Absolutely. It can be quite reflective as I saw firsthand during a recent weatherization training course in southern Vermont.

The class was held at a circa 1900 country inn that, besides having a number of expected insulation problems in the exterior walls, also had a variety of wallpaper styles throughout each guest room. In one of the first rooms we looked at there was this ornate floral pattern on the wall that did not show up thermally (Image 1, picture-in-picture, above right).

Besides the fact that an infrared camera does not see visible light, the emissivity of a material does not vary by color. As such, it was not terribly surprising that this pattern was not seen thermally in the first image. Outside, in direct sunshine, surface color does matter and darker areas will look different (i.e. warmer), but that is because they absorb more of the broader spectrum of radiation given off by the sun. This being absent on the inside, no differences are seen; however, as we found out in the next room, this is not always true.

Upon entering the second guestroom we immediately saw that the visual patterns on the wall surface matched what was seen thermally! So stunning was the contrast that I even first double-checked the camera to see if it was set to a visual/thermal "fusion" mode.  It was not. Image 2, above right, shows how clearly this floral pattern was seen thermally. This was true across all wall surfaces throughout the room. 

A closer visual examination of the wall showed that the base, gray, surface color of the paper was actually a silver, foil-like, material. This finish was also somewhat visually reflective too as seen in the light from the lamp in image 3 (right). In contrast, the surface of the flower pattern was quite different and more paper-like in appearance. 

Additionally, these apparent emissivity differences made it initially difficult to detect thermal surface patterns of missing insulation on the exterior wall. It was only when we ran the blower door that any anomalies began to pop out sufficiently in the camera (Image 4, right).

This wallpaper that we encountered certainly was not just plain “paper” and is a great reminder as to why one should always keep an eye out for low emissivity materials while conducting an infrared building inspection.