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Blog #057 – Your THERMOGRAPHY COMFORT LEVEL – Using IR – Flat roofs verses slanted roofs?

Are you comfortable when you are asked to perform a moisture investigation on a “roof”?  Roofs are notorious for water leaks and highly expensive to repair.  An infrared camera is a great tool for pointing us in the right direction to locate the failing membrane.  Remember, though, infrared cameras are not moisture meters and therefore are not used to “verify” the presence or absence of moisture.  I’ll refer to two kinds of roofs:  (1) flat roofs and (2) slanted roofs.  There is a greater application for thermography in finding moisture issues in flat roofs than slanted roofs.  Why?  Water ponds or settles right beneath the failing membrane when the roof is flat.  The water doesn’t usually run off to the side (unless there is a slant – thus the problem with slanted roofs).  There is an advantage to having the water settle right beneath the failing membrane, which is what should happen on a flat roof.  Using the principle of “thermal capacitance” a thermographer can identify “areas of investigation” where a moisture meter can be used to verify the presence of moisture.  How does this work?  During a sunny day the energy from the sun cooks the roof membrane and any water that is beneath it in the insulation.  So, we obviously need a “sunny day” as a prerequisite for flat roof moisture investigations using infrared cameras.  Water has a higher thermal capacitance than the roof membrane.  So, after the sun goes down, the roof membrane starts cooling down.  The water starts cooling down, too, but at a much slower rate.  The hot water will transfer heat to the membrane above it and keep it warmer than the adjacent dry areas.  When viewed with a thermal imaging camera you’ll easily see a warm area which is begging for more attention.  That’s when you pull out your moisture meter and verify and enjoy the fruits of your labor.

Caution!!!  Moisture investigations on flat roofs demand certain prerequisites.   The investigation needs to be done at night after the sun has gone down and the roof has had time to cool down.  And, as already mentioned, there is the need for heat from the sun, a sunny day.  So, cloudy, overcast days most often hinder the investigation.  And since wind cools things down, you need to make sure that your scan is done in a “no wind” situation.  Although the principle of thermal capacitance is simple and straight forward, flat roofs vary from building to building and can be very complicated.  Understanding the makeup of the roof layer by layer is very important.  Conductive heat transfer is a part of the equation in order to keep the roof membrane warm.  The success of conductive heat transfer is affected by the “path of least resistance” from the water to the membrane.  The  point is that the principles are easy, the roofs can be complex.  Know your roofs!!!

Image of moisture signature in flat roof

Image of moisture signature in flat roof

Due to the high thermal capacitance of water we can see a heat signature after the membrane has cooled off.
Can you use an infrared camera for slanted roof moisture investigations?  Yes, but not using the principle of thermal capacitance.  Water travels according to the direction of the slant and doesn’t have the time needed to heat up.  So, we use the principle of evaporative cooling and scan the roof from the inside.  If moisture reaches the underside surface of the roof you will usually experience evaporative cooling, which will drop the temperature of the wet area a few degrees F.  I hope these comments have raised your “comfort level”.

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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