I'm wondering if there's a reliable method of accounting for, say, a ten-degree drop during the progress of a whole-house scan after sunset. Thanks for any light you can shed!  

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Lorna Fear
(650) 520 4869
San Francisco Bay Area
Visual Cue Thermal Imaging
http://www.infraredmagic.com

Where is the temp. drop you are talking about. Looking at a house from the outside I would expect that to happen. If you are talking about the inside you might have to give us some more info. Do you have any pics?

Regards

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Jeremy Kays Reliability Technician
Level II IR, MCA / UE Level I, In Compliance with the ASNT guidelines
X-Ray Eyes
xrayeyesjl@hotmail.com
www.xrayeyesonline.com

Be aware too that the issue is not just a drop in AIR temperature but also, if skies are clear, that the building surfaces open to the sky can radiate enough energy to that clear sky to drop by an even greater margin. As an example, you can find frost on surfaces when air temperatures are ABOVE freezing.

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Thermally Yours,

John
ASNT NDT Level III #48166
The Snell Group
www.thesnellgroup.com
www.thermalsolutions.org
800-636-9820

I did an exterior  scan last fall, early October. The ambient exterior temperature was about 10 degrees C at 8:00 in the morning. I was shocked when the camera was telling me that the stucco was actually frozen! It showed -4 to -6 C! I remember hearing about this phenomenon during my Level I training, but didn't realy believe it until I experienced it.

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Darrell Paul
Building Envelope Specialist - Wood Frame
Certified Energy Advisor
Certified Moisture Control Technician
Level I Thermographer
403-586-1950
www.qualistat.com

You asked for a reliable method for accounting for temperature change during an inspection. This brings up quite a few issues that need to be addressed before answering. What is(was) the indoor and outdoor surface  temperatures.  According to ASTM 1060, we need stable surface temperature differences in order to create the thermal contrast needed to see framing and insulation. Stable means at least 3 hours. So if you are arriving at the building with little or no temperature difference and then over the course of an hour or so the temperature drops 10 degrees, in all likely hood the building will not have time to react to this temperature difference to allow a thorough IR inspection. If on the other hand the outside temperature at sunset was 85F (70F inside) and during the inspection the temperature dropped to 75F you will most likely be in good shape because the building mass will not have cooled during the inspection time.

 Give us some specifics and we'll be able to respond with more detail.

 Rob

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Rob Spring, P.E.
ASNT NDT LIII #65375
The Snell Group
rspring@thesnellgroup.com
800-636-9820