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Blog #004 – Your THERMOGRAPHY COMFORT LEVEL – Caution: ∆T for insulation investigations?

Thermal imaging cameras can be used very effectively in finding missing or compromised insulation.  This is important for anyone involved in inspection of buildings for building envelope integrity and efficiency.   How does the infrared camera help locate these issues?  And what caution is in order?  Most publications and training classes seem to suggest a standard ∆T of 18°F or greater.  In fact, the ASTM C 1060 – 90 “Standard Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings” suggests a minimum ∆T of 18°F.   What does that mean?  Does it mean a ∆T (or temperature differential) from inside ambient to outside ambient (inside air to outside air)?   This ASTM standard goes on to say:  “The preferred measurement of ∆T is surface to surface, because this minimizes problems with accounting for solar and wind effects.  Air-to-air measurements are also permitted under this practice.” If there is no influence or impact from the sun on a wall or if there is no wind issue it could very well be that a ∆T of 18°F or greater ambient to ambient will work just fine.  However, what if the sun is hitting the south wall in the winter time while the outside ambient temperature is 30°F?  Under those circumstances the south wall and north wall could have very confusing results.  I use the Fluke Ti32 which is a high resolution camera with a 50mK detector system.  It is very sensitive.  Most of the time I use the blue / red palette as my dominant palette.  When the sky is clear and the sun is shining on a wall, you will need to look for a surface to surface ∆T of 18°F or greater because of the sun’s influence.  While the outside ambient temperature might be 30°F, the surface temperature on the South wall might be 120°F and the surface temperature on the north wall might be 30°F.  This is important to you! Why?  The 2nd law of thermodynamics establishes the direction of heat flow.  Heat flows from hot to cold.  So, under these circumstances the heat flow on the south wall is from outside to inside and the heat flow on the north wall is inside to outside.  Missing insulation on the south wall will appear warm on the IR camera whereas missing insulation on the north wall will appear cool.

Another caution: Since wind can lower the temperature of the surface, we have to verify the surface to surface ∆T.  What if the outside ambient temperature is 88°F and the inside ambient temperature is 70°F.  We have an 18°F ∆T, right?  But what if you have a strong wind against the outside surface?  Might it lower the surface temperature to a point that interferes with the investigation?  It’s something to take into consideration before coming to any definite conclusions.

Do you always need a minimum ∆T of 18°F?  The ASTM standard says: “The criterion for satisfactory thermal conditions is the ability to distinguish framing members from cavities.”  The NETD (noise equivalent temperature differential) can vary from camera to camera.  This is specifying the “sensitivity”, or the ability of the detector system to pick up thermal differences (the lower the number the more sensitive the camera).  My Fluke Ti32 has a 50mK (or better) detector array system.  That’s very sensitive for building diagnostics.  That means I can work with smaller ∆Ts (outside to inside), which means I can work more hours of the day and more days of the year.

Before inspecting an interior wall for insulation issues always go outside and look at the outside wall surface.

South wall as seen with a Fluke infrared camera

South wall as seen with a Fluke infrared camera

Ambient 50°F. Notice the effect of the sun on dark paint verses the white paint.  The dark painted area registers a 129 degrees F.  The white painted area at the bottom registers 81 degrees F.  The shaded area at the top registers 94 degrees F.  This image was taken with a Fluke FlexCam Ti45 infrared camera, 160×120 resolution, 70mK detector system.


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
fax 916-736-1134

Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.


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About infrarod

Rod Hoff is a Thermographer and instructor with Restoration Consultants, Inc. He teaches a two-day IR class in moisture and building envelope investigations. A graduate from Florida State University, with a degree in education, he received his formal training in Thermography from Snell Infrared and Restoration Consultants, Inc.

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