Your THERMOGRAPHY COMFORT LEVEL – Exercise #5 – insulation detection!
“Are you comfortable using your infrared camera to investigate insulation issues? I’m going to give you a method to determine if the wall is fully insulated or not using your infrared camera. You start with two basic steps: Step #1-Identify the heat source and thus the direction of the heat flow! Step #2-Identify the path of least resistance, the lowest R-factor. Inspecting walls for insulation issues with an infrared camera requires an understanding of the 2nd Law of Thermodynamics. That law defines the direction of heat flow and is simple: hot goes to cold. The speed with which “hot goes to cold” is affected by at least a couple of factors: (1) the ∆T from surface to surface (the greater the temperature difference, the greater the conductive speed) and (2) the R-factor (the greater the R-factor or R-value, the slower the conductive speed). In a fully insulated wall the framing members would be the path of least resistance and the bay would be the path of most resistance to heat flow. In a wall that does not have insulation the bay would have the lower R-value and thus be the path of least resistance and the framing members would have the higher R-value and thus be the path of most resistance. Try to see this on a molecular level because that’s the way energy (heat) flows. If you go back and read Blog #004 (Jan 10, 2011) you’ll see a brief discussion on the recommended ∆T ( outside surface to inside surface ∆T of the wall being inspected). Sometimes the difference in ambient temperatures (inside to outside) is sufficient. But the influence of the sun and wind can change the outside surface temperature dramatically from the ambient temperature. For example, the outside ambient temperature could be 30°F and the outside wall surface could be 120°F due to solar heat. So, it’s a good idea to be aware of what’s happening on the outside of the wall that you are inspecting. The recommended ∆T (temperature differential surface to surface) is 18°F or greater. This is perhaps due to the nature of the R-value of materials. R-values can be expressed as the temperature difference needed to transfer 1 BTU/hr/ft². For example, R-19 value would require a 19° F temperature difference to transfer 1 BTU/hr/ ft². 1 BTU is not of lot of heat transfer, is it? The effect on the target wall that you are looking at is very small. I was looking at an uninsulated wall with a 73°F inside surface temperature and a 47°F outside surface temperature. That is a 26°F ∆T, well above the 18°F minimum suggested. The area on the wall where the wood framing is located read 74.3°F. The bay read 73°F. That is only a 1.3°F temperature difference. That’s why it’s important to have a “sensitive” infrared camera for this type of work. The lower the ∆T from outside surface to inside surface, the less thermal effect you’ll see on the wall you are inspecting. As mentioned, some infrared cameras are more sensitive than others and will pick up more detail. (Get the most sensitive camera you can afford)”
I’ve given you 8 scenarios in previous blogs (see blogs #22, #23, #24, #25) to practice detection of insulation issues. In this blog #031 and the following blog #032 I want to give you a few more scenarios so that you can practice identifying insulation issues. If you think your way through these exercises you’ll raise “your Thermography comfort level”.
As you view the wall with your infrared camera in the blue/red palette you see the following image. Warm wall cavity areas (lighter color) and wood framing (darker color).
#9 scenario: viewed from the inside during the heating season (winter; solar loading 120°F) – Are the bays insulated?
#10 scenario: viewed from the outside during the heating season (winter; solar loading 120°F) – Are the bays insulated?
Scenario #9 – no insulation
Scenario #10 – Insulated