An expert explains how to get high quality and helpful thermographs when conditions are not ideal.
Conditions for the InspectionIf you understand what is necessary to do a good inspection, you can choose the right time and circumstances for the inspection and proceed with confidence that you will many, if not all, problems in the building you are examining. Without this basic knowledge and experience, however, you may not see any indications of problems. You won’t know if there are, in fact, problems in the building. The best place to begin is by following industry guidelines; with experience, you learn when to push the limits—carefully.
The standards for building inspections all call for a minimum, stable temperature difference of 18°F (10°C). This is not simply the difference between indoor and outdoor temperature. Rather, it is the difference between inside wall surface and outside wall surface temperature. Most infrared (IR) systems are sensitive enough that you can get good images with a temperature difference of less than 18°F (10°C), but in any case the exact temperature difference needed to image insulation-related problems depends on the building, and on the ambient conditions during, and several hours previous to, the inspection. If you can’t see the framing, you won’t see insulation problems.
You can adjust interior temperatures to enhance patterns (like framing), but this can be tricky if heat flow is not uniform. Some envelope surfaces may grow warmer or cooler when they are flooded with warm or cool air while others remain unchanged—for instance, where furniture is pushed up against the wall. Often, however, simply pulsing the HVAC system for 15 or 20 minutes and then waiting another 15 or 20 minutes can yield remarkable results as the interior temperatures change. I turn the furnace or the A/C on full blast, and, depending on the system and the home, I get an increase or decrease of up to 5°F (5°C). Interior temperature will still not be uniform, but it will be more uniform than it was before.
The swing seasons, fall and spring, are particularly challenging. However, they offer a tremendous opportunity to inspect twice a day if you know what you are doing, and if you can manipulate the home just a bit. Arrive early in the morning, during fall or spring, at a home that has been closed up all night and you will usually have good winter imaging. You may need to turn the heat on briefly. Arrive in the late afternoon or early evening, again with the home closed up all day, and you will often have an excellent round of summer imaging. What you are trying to do here is to catch the thermal flywheel, either inside or outside, and use it to help establish a temperature difference.
The sun can raise havoc with an otherwise good inspection, inside or outside, day or night, in any kind of weather. Strong sunlight will quickly make exterior inspection work impossible for a thermographer, masking over the patterns you want to see. Retreating inside may help, but the effect of the sun will show up there, too, with potentially very confusing results, even two to six hours after the sun is no longer on the exterior of the building. This can happen even well into the evening—when all reason would seem to suggest that you should be safe from solar loading—especially during summertime inspections. Walls you know to be insulated suddenly appear uninsulated! Voids miraculously disappear! This is just a trick of the sun playing with the direction of heat flow. I’ve measured temperatures in excess of 150°F (65°C) on dark siding in a corner out of the wind when the air temperatures were below 0°F (-18°C)—the net flow of heat is into the building at that point.
Thermally thick envelopes are also problematic. Brick and stone facades and exterior sheet insulation all make exterior work very difficult. Even vinyl siding or aluminum siding with backer board can make for a long day. In these cases inside work is usually much more productive, though you must account for the moderating affect of the siding, especially brick or stone. Many homes now have some sort of sheet insulation on the interior face of the framing, typically strapped out for Sheetrock, with an air space between Sheetrock and insulation. The result is that it is nearly impossible to inspect for insulation from the inside, though you are often rewarded with compelling and extensive patterns of air leakage into the air space.
Wet exteriors will also prove confusing, as they can result in evaporative cooling or increased conduction. Even when the moisture is not obvious, it may be playing a role in shaping the thermal pattern that you see with the IR camera, so keep a moisture meter handy and compare what you see to known dry areas.
The wind too affects inspections, for better or for worse. A wind in excess of 10–15 mph will make good thermography work difficult; on the exterior, upwind side, it’s impossible. On the interior and the downwind exterior views of the envelope, expect that air leakage will show up and either enhance or confuse the conduction patterns, depending on the type of insulation, the velocity of the wind, and the temperature of the air.
Exterior work is also difficult in extremely cold weather. Thermography results when air temperatures are 10°F (-12°C)to -30°F (-34°C) are often marginal compared to those when temperatures are 32°F (0°C) or higher. While some IR imagers may do better than others, most of today’s systems simply don’t have the sensitivity to work well at these low temperatures. The amount of radiation given off by very cold exterior surfaces is limited. Inside offers better imaging and is also more comfortable.
The best way to really master understanding the influence of ambient conditions is to be aware of them at all times. Keep careful notes and compare your results and the conditions under which you worked. Compare all the homes you look at, and learn from the exercise. The exact relationship between conditions and results is not always obvious, but over time, studying the interaction will help you know when to inspect with confidence, when to bail, and when to pull out a bag of tricks that may improve your chance of getting better images.
Mastering the IR Imaging SystemToday’s imaging systems are remarkably easy to operate. Some are entirely automatic, and others can be operated automatically with good success most of the time—especially if you understand the conditions under which they will not do the job well automatically. The trouble for many new thermographers comes when they rely solely on this automatic functionality when what is needed is a manual adjustment—essential to getting the best image in at least half of the time.
Most automatic image adjustment routines build the image around the hottest and coldest points. For example, in an IR image that includes a hot radiator and a cold window, the relatively small temperature difference between sections of a nearby wall with insulation and sections without it won’t show up on the image. The big contrast between the radiator and cold window wipes out the smaller contrast between the insulated and uninsulated parts of the wall. When working outside, the classic problem is looking up at the building with your IR camera and having the cold, clear sky (summer or winter, day or night) and an exterior light in the same image. Again, the automatic adjustment builds the image between these two extremes, and the information you typically want is lost in the middle. We see so many thermographers who simply don’t understand this phenomenon. If they did, they could either move to a different position or manually adjust the image and be successful.
Some skill is required to learn to focus, even when the temperature differences in the frame are not extreme. The key is to focus on a sharp line of thermal contrast, in the same way our eyes seek out sharp lines of visual contrast—for example, black letters against a white page. Much of what we look at in a building—framing, for example—can be quite diffuse, and therefore difficult to pop into sharp focus. Then too, thermal imaging systems simply don’t have the same level of resolution that we’ve come to expect from even inexpensive visual cameras.
I also see people make mistakes during image capture. Just as with a visual camera, you need to push the freeze button or pull the capture trigger gently, or the image, even if it is well focused, will be blurred as it is stored. Again, isolating this skill and practicing it will quickly yield dramatic improvements.
Another common pitfall for new thermographers is settling for a blurry image. The source of this problem may be a focus-free camera; these can give good results in a range of focus distances but often don’t work well outside of that range. If you have this type of system, work at learning what it will and won’t do.
Thermographers often ask me, “What palette should I use?” The answer is simple: Use the one that shows the most detail in the specific image. In reality the answer varies from one brand of imaging system to another. Generally, but not always, a gray or monochromatic color image is best, but again, the actual results are often unpredictable. Become familiar with the palettes in your camera and try several when results are less than spectacular, in order to find the best.
Image InterpretationWith the right conditions and a mastery of the camera, you should have no trouble capturing an image full of information for building diagnostics—except if you fail to understand how the building is constructed and how it is behaving thermally. This is, of course, often the case, because despite the generalizations we can make about buildings, each one is unique, and the process of gaining diagnostic skills is a lengthy one. The more buildings I inspect, the more appreciative I’ve become of their complexity and the more humble about my knowledge.
Many of my comments thus far would seem to suggest that results are often better on the interior. Generally this is true, but working inside brings its own set of challenges. Dealing with furniture, bookshelves, posters, closets, small rooms, and inaccessible areas is all part of working on the interior. Not being able to see the big picture is also challenging. Trying to understand a pattern that continues between two rooms or fades into an inaccessible closet will try your patience and your diagnostic skills. A larger-scale view from the exterior, even if it isn’t perfect, may help in such situations.
Another challenge when you work inside a house or apartment is simply putting yourself in the right place. Using a wide-angle lens is wonderful, but many IR systems don’t allow for that option. Because of the challenges associated with working in small inside spaces, I often work from the opposite side of the room and end up with several images of a wall.
Many new thermographers make big mistakes when trying to understand air leakage using their IR camera. This work is usually best done from the interior, using a blower door to depressurize the building. Mistakes and omissions are almost guaranteed if you don’t use IR and a blower door together. Working on the exterior with interior pressurization often leads to confusing results.
Even when using the blower door (or the HVAC system) to enhance imaging conditions by controlling air movement, it is easy to get cocky and forget that some of the air comes quite a distance thermally before it reaches the surface we are imaging. Natural plenums for air, such as floor cavities and chases, can end up stealing all the thermal difference from us, delivering air at virtually the same temperature as the room, and leaving us with little or nothing to see. On the other extreme, a blower door left running at 30 Pa can, over a short period of time, reduce the signatures on an envelope to thermal mush by drawing in so much outside air that the envelope approaches the temperature of the ambient outside air.
When Does Thermography Work Well?Thermography works well whenever you have an imaging system in your hands that you are skilled at operating and can use with whatever conditions are at hand or that can be enhanced—and whenever you are willing to say, “I’m not sure what’s going on!” The inability to know when you don’t know what you are doing is probably the greatest problem for a thermographer. We are, by definition, experts, and we don’t get paid for saying we don’t know. The temptation, then, is to overextend ourselves. In the end, we need to be able to understand all the variables—ambient conditions, our equipment, and our knowledge—so that we can understand the building and say whether or not there are problems, or know that we need to come back when conditions are better.
John Snell’s background in building-related training and educational services began in 1977 in DOE’s Weatherization Assistance Program and continued from 1979 to 1983 in the Residential Conservation Service (RCS) Program. It was in this capacity that he first used thermography for building diagnostics. In 1986 he founded Snell Infrared, which later became The Snell Group. Since then, he has trained thousands of people to use this remarkable technology, and he remains committed to the profession by serving on a number of standards committees.
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