Why does the recommended ∆T for air leak investigations differ from the recommended ∆T for insulation inspections? This can be better understood if we think on a molecular level. The 2nd law of thermodynamics states that hot goes to cold. The recommendation for insulation inspections is a minimum ∆T of 18°F from surface to surface for a period of 4 hours (ASTM C 1060 p.6). A bigger ∆T is preferred, of course. The change in temperature from the outside surface to the inside surface (or vice versa) depends on the results of conductive heat flow from the warm side to the cool side. It takes time to transfer that heat and the speed of the movement is affected by the insulation values in the cavity. Picture the movement of heat on a molecular level through the various building materials (exterior siding, insulation, interior sheet rock, etc). Is the heat moving rapidly or slowly? Heat moves rather quickly through materials that are highly conductive. But the building envelope is usually constructed with materials that have a poor conductivity factor, with good reason. With an 18° surface to surface ∆T and the time it takes the heat to travel through the materials the resulting effect on the inside surface will most likely be very slight. You might see only a few degrees difference between a fully insulated bay verses the wood framing. A variable to consider is the sensitivity of your infrared camera. The more sensitive the infrared camera detector system, the more detail it will pick up. Notice the temperature difference between the wood framing and the bay in the images below (82.1°F for the bay. 83.9°F for the framing). It’s less than two degrees and yet the temperature on the other side of the ceiling, on the roof, is well over 120°F.
Now, how about the ∆T during an air leak inspection? And are we talking about surface to surface or ambient to ambient temperatures? We’re talking about ambient to ambient temperature difference. Some publications say we need a minimum ∆T of 9°F. Some say 3°F. Why is the requirement lower when dealing with air leaks opposed to insulation issues? Air leaks are not a result of conductive heat movement. In an air leak issue air moves through an opening and the temperature of the air changes the temperature of the surface that it flows over if there is enough ∆T. To enhance this type of inspection depressurization is recommended, when doing interior inspections, so that you can speed up the movement of the air over exposed surfaces. Notice the ‘before depressurization’ and the ‘after depressurization’ images below.
How’s your comfort level in thermography?
(The above comments represent my opinion).
Rod Hoff / Restoration Consultants Inc
Thermography Instructor / IR camera sales
3284 Ramos Circle, Sacramento CA 95827
toll free 888-617-3266 ext 301
Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.
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