Diagnostic Tools-Thermal Imaging-Interpreting the Images

This blog post originally appeared on the Green Building Advisor’s website.  www.greenbuildingadvisor.com

Thermal imaging has numerous uses and benefits many different trades.  Everything from surveillance and industrial maintenance to checking someone’s temperature.  For this post, I’ll discuss interpreting images in the residential construction field. 

It’s important to have at least a basic understanding of how the camera works and adjustments that can be made to the different camera settings, I’ll briefly discuss a few of the settings.  It’s also helpful that you have somewhat of an expectation when viewing a thermal image.  Outside environmental conditions can affect an image taken inside a building, the conditions typically present in a predictable way, but not always.  Sometimes something unexpected shows up, usually this requires more investigation, possibly confirmed using other diagnostic tools or may even require the disassembling of a building component.

Camera Settings

Newer thermal imaging cameras have similar settings, regardless of the manufacturer.  Because I submit the photos I take to a homeowner or other interested party, I want the image to be easily interpreted.  I always set the camera to overlay the digital photo on the thermal image.  This setting is called MSX in Flir’s cameras and Hikmicro calls it Fusion.  This process can be manipulated in software provided by the manufacturers.  If not using the software, you’ll want to take the photo with this option selected in the camera’s settings menu.

The added details of the photo can help the thermographer and homeowner more easily identify the locations of problem areas in the image, the example photos above, we know the problem areas are above the curtain, this isn’t obvious without the digital photo overlay.

I choose to use the iron (flir) or ironbow (Hicmicro) color pallet.  This pallet shows the purples and blacks as cold areas and yellows and yellows and whites as warm areas, which in my option, is easiest for a homeowner to understand.

The images above show the iron or ironbow pallet compared to the rainbow pallet.  Whichever pallet is chosen, you want to make sure the customer understands what the different colors mean.  When starting a thermal imaging scan of a home, I typically show the customer the handprint on the wall trick.  By placing my hand on a wall for a second or two and then pointing the thermal image camera towards this area, my handprint becomes visible.  I use this example to explain what the colors they are seeing indicate.

Interpreting Images

Sometimes when looking at a thermal image, it can be difficult to determine what the subject matter is.  Take the image below, it appears there is a warm U-shaped object in the middle of an area that is cooler.

It’s difficult to discern exactly what we are looking at, even with the digital photo overlay.  But add in the actual digital photo, and the subject becomes clearer.

We are looking at a heating cable on a roof’s edge used to manage ice dams, and we know that the cable is currently or was recently operating by the heat it is producing.

It can also be important to understand how the sun shining on a surface can affect an image.

This exterior photo was taken with outdoor temperatures at -10°F.  The left side of the home is shaded with the right side in full sun.  It’s also interesting how the shadows from the trees in the yard also affect the temperatures seen on the home’s siding.  Depending on how well this home is insulated, we may see some temperature variations transferred to the interior side of these exterior walls as well.

Some of the best and most informative thermal images I’ve taken are when I’m using thermal imaging in conjunction with a blower door test.  The photo below shows an attic hatch with an air leak issue.

Now, compare that image with this home having the same issues.

Notice how the colors indicating the air leaks are opposite.  The upper photo was taken during warm weather, the air being drawn into the home with the negative pressure blower door test was warmer than the interior surfaces.  The second photo was taken during cooler temperatures.

Air leaks have a very distinctive look.  The thermal image above shows cold air leaking under a door threshold.  The floor was at a more constant temperature before the blower door was started, as the cold air was drawn under the door, it began cooling the floor’s surface shown by the fingers or rays of black and purple.  The longer a blower door runs, the more pronounced these rays or fingers become.

One piece of advice when using thermal imaging with blower door testing, conduct a thermal image scan of the home before starting the blower door.  There may be an issue that can be identified without the blower door that is not related to air movement cooling or warming a surface.  This issue becomes harder to determine if it’s air related or some other issue after the blower door is started.  The photo below shows an example.

This photo was taken during a blower door test, air was moving down the inside of this interior wall cavity during the negative pressure test.  The attic above was a vented attic, without a continuous air barrier.  Electrical and plumbing present in the wall moved up and into the vented attic space.  But what about the two dark spots on the right side of the image?  Could these also be related to cold air?

These areas turned out to be recently repaired drywall patches, the moisture in the drywall compound was still evaporating, this evaporation is making the patched areas a little cooler.  The moisture problem in this photo was easy to diagnose, but often damp and wet surfaces can be more difficult to interpret when using thermal imaging.

Here’s an example, is this surface wet or an area that is missing insulation?  Or is it something else?  The first clue is when the photo was taken, warm or cold outside temps.  If it was taken in a cold climate in January, it’s most likely missing insulation, but maybe not.  It could also be frost or ice that accumulated on a condensing surface that has now melted, causing a wet spot on the ceiling.  If it was taken during the warmer months, it’s probably a wet spot, but again, maybe not.  This could be a cool forced air duct above this area of the ceiling.  The presence of moisture will need to be confirmed by testing using a moisture meter.

How about the photo below, what is causing the dark, cooler surface temperature on the center of the window?

I get to see this condition once every couple of years.  A window where the seal between the panes of glass has broken, allowing the argon or krypton gas fill to escape.  The two panes of glass have moved closer together, causing there to be a lower insulation value between the two panes.  The space becomes cooler and can be detected by thermal imaging.  Typically, the condition isn’t this pronounced, usually it’s a more subtle light purple color, not the dark purple and black shown in this image.

Speaking of glass and other reflective surfaces, you have to be aware that these surfaces have a low rate of emissivity, meaning they reflect the heat of surrounding surfaces.  The temperatures indicated by the camera on these surfaces may not be accurate.  Typically, I’m not looking for an accurate temperature of a reflective surface when performing a thermal imaging scan on a building, but there may be an occasion where that becomes important, such as needing to know the temperature of a return and supply duct.  The easiest way to confirm the temperature is by using a black piece of tape stuck to the reflective surface.  Once the tape has had time to become the same temperature of the surface, thermal imaging will more accurately measure this temp.  The photo below shows my thermal image reflection in a metal forced air

One last image, this one was taken by Joe Burgett, Ph.D. at Clemson’s University, Department of Construction Science and Management.  Dr. Burgett teaches a class on thermal imaging using drone technology.  Looking at this image, I’d say the camera used to take this image is high end.  Notice the yellow blob on the roof.

Just looking at the image, it’s hard to determine what the warm spot might be.  It could be a water leak or moisture issue; it could also be warm air.  Luckily, Dr. Burgett filled me in.  It’s a louvered vent exhausting warm air, normal operation for this commercial building.  The photo was taken early in the morning with ambient temps around 30°F.  Thermal imaging can show problem areas, even when everything is operating normally.

This blog post doesn’t come close to covering everything that can be detected in a residential thermal imaging scan.  The capabilities of this tool have saved me from needlessly taking building components apart trying to identify a problem and has also found issues that would have gone undetected without the technology.  I’m often delaying building diagnostic work until there is enough temperature difference between inside and outside when thermal imaging can be used, it’s that big of a game changer in finding problems.  Given the decrease in cost and improvements in the technology, no contractor should be without one.

Bonus content: I recently purchased a new thermal imaging camera made by a company called Hikmicro.  The Pocket 2 resembles my Flir C5 but has higher resolution in both the thermal and digital images and adds the ability to shoot video.  The camera is also about a $100 cheaper than the Flir.  The videos below were taken during my first blower door test after purchasing the camera.

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