Successful Warm-Weather Infrared Inspections
While infrared cameras are relatively easy to operate, it is the task of interpreting the image that turns out to be the most challenging aspect of this technology.
As a thermography instructor, I have, on occasion, overheard other thermographers questioning the merits of using infrared (IR) thermography for warm-weather inspections. There seems to be a perception that thermal imaging can only be used successfully for cold-weather work. Many of these discussions, understandably, tended to come up during courses in more northern climates.
The reality is that thermal imaging can be used practically year-round as a supplemental tool for energy audits in many climates, given the right conditions and equipment. Since I live in central Vermont, though, much of my IR experience in buildings (like that of my northern colleagues) has come from cold-weather work. So it was last summer, during a series of field trips throughout Vermont, that I set out to see firsthand how successfully this technology could be applied to warm-weather inspections. I was quite pleased with the results.
IR for Building Applications
All objects radiate some amount of infrared energy. Typically, the warmer something is, the more it radiates. An IR camera detects this radiation and converts it into a thermal image, displaying the various apparent surface temperatures as different colors or shades of gray to indicate varying intensities of heat energy (brighter = hotter, darker = colder on most palettes).
Many IR cameras can discern temperature differences of as little as 0.05 °C (0.09°F), sensitive enough to detect thermal patterns on the surface of a wall that can reveal subsurface details, such as framing members or insulated cavities. Although most cameras display temperature values, this feature is not usually required for buildings work. Rather, what we as building thermographers are most often interested in is the qualitative differences in thermal patterns on the surfaces of walls that reveal subsurface problems, including missing insulation or areas of air leakage.
While infrared cameras are relatively easy to operate, it is the task of interpreting the image, to determine whether an anomaly really exists in the building system, that turns out to be the most challenging aspect of this technology. Inspecting buildings with thermal imaging is arguably one of the more complicated applications of IR, requiring qualified operators who have proper IR training, a solid knowledge of building systems, and a keen understanding of thermodynamics.
With a few exceptions, pretty much any IR camera sold for building applications can be used for warm-weather inspections. For best results, though, it is recommended that the camera meet at least the following minimum specifications for conducting a warm-weather building inspection:
- a thermal sensitivity of 100 milliKelvin (mK) (0.1°C/0.18°F) or better (70mK—0.07°C/0.13oF—or better can be very helpful);
- a 120 x 120 or better detector array;
- a wide-angle or telephoto lens (need varies);
- rugged, user-friendly construction; and
- digital image storage with postprocessing and report software.
The most important parameters to consider are those of thermal sensitivity and detector array size. For thermal sensitivity, the lower value indicates a more sensitive camera that can detect smaller temperature differences.
For example, a 70mK thermal sensitivity translates into the camera’s ability to see a 0.07°C (0.13°F) temperature difference on a surface. When large temperature differences across a building envelope exist—say during very cold midwinter conditions up north—this is less of a concern, as most anomalies in a wall will jump right out at you when you are inspecting from inside a home. However, when the inside-to-outside temperature differences are closer together, having a system with a lower (that is, better) thermal sensitivity value can really pay off. This greater thermal sensitivity allows the thermographer to work in more marginal conditions and ultimately to inspect during more hours and days in a given year, even when smaller temperature differences make it more challenging to find anomalies.
As for the size of the detector array, any system between 120 x 120 and 320 x 240 is fine for residential work (see Photo 1). Larger buildings might require a larger detector array (or the option of a telephoto lens). Generally speaking, for most residential applications, anything above 320 x 240 is not necessary, but would certainly work well. Low-resolution systems—those with an array of less than 120 x 120—are simply not sufficient. In fact, the new RESNET infrared guideline sets a thermal sensitivity ceiling of 100mK and a minimum detector array size requirement of 120 x 120. Many cameras meet these requirements, but be sure that you understand what you are getting before you buy.
Conditions for Warm-Weather Inspections
Just as with cold-weather inspections, you need a temperature difference across the building envelope to drive heat transfer and generate thermal patterns in a wall system. Industry standards such as the American Society of Testing and Materials (ASTM) C 1060 and the new RESNET Interim Guidelines for Thermographic Inspections of Buildings specify at least an 18 °F/10°C (inside surface-to-outside surface) temperature difference for conducting insulation inspections.
It is also important to remember that during warm-weather inspections, where it is hotter outside and colder inside, the direction of heat flow (assuming steady-state conditions) is the opposite of what one encounters in cold weather. The temperature of a cavity with missing insulation that appeared cooler during a wintertime inspection will appear hotter during warm weather, again assuming steady-state conditions (see Photo 2).
To check for air leakage, ASTM E 1186 says a 9°F (5°C) difference is needed, while the RESNET IR guideline says a minimum of 3°F (1.7°C) is sufficient. In my experience, the new RESNET standard better reflects the true capabilities of modern IR systems, especially those with 70 mK or lower thermal sensitivities.
Obviously, the greater the temperature difference you are working with, the better you will be able to see both insulation and air leakage problems at any time of year. Success, of course, still depends on many environmental conditions, as well as on the construction of the building itself.
Inspect Inside or Outside?
During warm-weather inspections, it is usually best to focus your efforts on the interior. Working from the exterior can be very difficult, unless you do it first thing in the morning or late at night. Daytime inspections from the outside during warm-weather conditions seldom reveal much, as the entire exterior is saturated by solar loading. Using a narrow span setting and a high-contrast color palette can help define thermal patterns on the surface. If your particular IR camera lets you manually adjust level and span in the field, this can be very helpful, since the auto adjust may not be adequate in some situations.
This is especially true if you are working with a masonry building (see Photo 3). And because most building IR cameras now have only LCD displays (and not viewfinders), seeing what is on your screen outside in broad daylight can be a challenge as well.
Regardless of the time of day, always remember to keep the exterior in mind, noting where the sun has been and how that might have affected thermal signatures on the inside wall surface. Thanks to the effects of the sun, you will also encounter issues related to differences in the thermal capacitance of building materials (wood, brick, stucco, concrete) that can create confusing patterns at any time during the day or night.
A blower door is the thermographer’s best friend.
The Swing Seasons: Spring and Fall (and Sometimes Summer, Too)
During the so-called swing seasons of spring and fall (and sometimes summer, depending on where you live), it is common for building thermographers to start off their inspection day in winter conditions, go through spring and fall by midmorning, and finish up in summer mode by afternoon. This is especially true during the summertime in more northern climates, where outside temperatures can be around 45°F (7°C) at 7 am, warm up to 70°F (21°C) by 11 am, and hit 85°F (29°C) by mid-afternoon. If the interior temperature stays somewhat consistent at 70°F throughout the day, the direction of heat flow (and the pattern you are looking for) will certainly vary depending on what time you show up! This reinforces the importance of documenting the environmental conditions you are encountering at the time of inspection. Air temperature, sky conditions, and wind speed are just some of the important data points to record for your report, and they will certainly help with your analysis.
The sun’s position on the building can also create a situation where the east- and south-facing walls are showing summerlike patterns while the west- and north-facing walls are still in winter mode. At this point, it can be difficult to determine whether all the wall cavities in a building are empty, full, or some combination of the two.
The worst cases are cloudy days when the inside-to-outside temperature difference is less than a few degrees and there is no solar loading to assist with thermal transfer through the wall. Inspections on days like these are among the most challenging that you will ever encounter and sometimes, quite simply, these inspections do not work.
As an example, I was out one day last summer in northern Vermont conducting field training with a group of thermographers from a state weatherization agency. We pulled up to a house that had all of the windows open on a 68°F, cloudy day. Good luck! It took a few minutes to convince the somewhat reluctant homeowner why, in August, he needed to close his windows and turn on the furnace. Both ASTM C 1060 and RESNET recommend that there be a minimum temperature difference of 18°F (10°C) between the interior and exterior wall surfaces for at least four hours prior to testing. Even though these conditions were far from ideal, this was the only chance that we had to work together, due to scheduling. We eventually did spot some anomalies at this location, but it was certainly not my favorite situation. Had this been an actual IR inspection for a client, the best option would probably have been to reschedule, unless we could artificially alter the inspection conditions as we did (something the RESNET guideline also suggests).
During cold-weather inspections, thermographers always need to be aware of solar loading and its effects. Thermally charged exterior walls can reverse thermal patterns seen on the interior of a building. In many cases this pattern reversal can be misinterpreted as an uninsulated wall, when in fact solar loading has simply reversed the direction of heat flow, causing the studs to warm up and appear hotter than the cavity—a classic pattern of an uninsulated wall in cold-weather conditions.
It is common to encounter a situation where the inside ambient temperature may be warmer than the outside, but because the sun is heating the exterior wall surface, it is now hotter than the interior, sending heat flow in the opposite direction to what the thermographer is expecting.
These issues with solar loading are something to bear in mind when conducting warm-weather inspections, but in some cases solar loading can actually help thermographers to identify problem spots in a building.
The IR shot in Photo 4 of an east-facing wall was taken at midmorning on a mild summer day and shows apparent missing insulation in the exterior wall cavity (the lighter, brighter area above the window). The inside temperature was slightly warmer than the outside temperature, but because the sun had been charging the wall for well over an hour, heat transfer was moving from outside to inside. Where there was less insulation, there was more energy passing through the wall, revealing an empty cavity adjacent to ones that were apparently full.
Infrared patterns associated with the thermal capacitance of building materials can create confusing IR images during both warm- and cold-weather inspections. Both ASTM C 1060 and the RESNET guideline state that the effects of solar loading can last for up to three hours on light frame buildings and as much as eight hours on masonry structures. That is the amount of time recommended for direct sun to be off a surface before you conduct a thermal inspection. A tall order any time of the year, this can be especially troublesome during warm weather.
In the example in Photo 6 captured on the outside of a building during warm-weather conditions, the framing members appear to be hotter than the cavities (a classic pattern of an uninsulated wall in the cooling season). The relatively higher thermal capacitance of the wood studs, however, could help to explain why we are seeing this particular thermal pattern. It turns out the wall is in fact insulated.
Depending on the amount of moisture that is present, wet building materials react slowly to temperature changes, and can appear relatively cooler in the morning hours before the sun has charged the material and warmer in the evening after the surface has been warmed by the sun and air temperature throughout the day.
This is also seen in concrete foundations. The uninsulated concrete wall appears to be warmer than the wood siding of the building in Photo 6—a pattern that you often encounter when it is warmer inside than it is outside. Yet in this example, the outside air temperature was about equal to the inside temperature. The foundation probably appeared warmer simply because of its higher thermal capacitance, not because of any additional heat transfer through the wall.
Consider the Exterior Surface
I mentioned earlier that inspections from the exterior are generally not practical on a sunny day in warm-weather conditions. At the very least, however, you should still conduct a visual inspection of the exterior to discern the position of the sun and note any variations on the exterior surface. Are there differences in siding color? Is a soffit overhang or an adjacent building casting a shadow on the wall? Are trees shading a section of the wall? All of these can directly affect interior thermal patterns.
The IR image in Photo 7 was taken last summer in northern Vermont from inside the home. The thermal pattern of apparent missing insulation between the roof joists is clearly seen at the top, with the warmer bays and relatively cooler framing members. The exterior wall, however, showed a somewhat more-confusing pattern that was initially difficult to interpret.
The top quarter of the wall appeared to be warmer than the bottom three-quarters but both were still cooler than the ceiling. Was there missing insulation here? Varying wall thicknesses? Different angle of the sun? Possibly, but there was no way to tell for certain… at least from the interior! Instead, the pattern turned out to be caused by two different colors of siding, as seen in the photo taken from the outside (see Photo 8). The darker painted surface absorbed more of the sun’s thermal energy, heating up the painted parts of the wall more than the unpainted areas.
Always Use a Blower Door
Regardless of the season, always use a blower door. Using a blower door enhances air leakage thermal patterns that are not often seen under natural-pressure conditions. It is also beneficial when the inside-to-outside temperature difference is marginal. The blower door helps drive warm or cool air movement patterns across the envelope, making swing season and summer inspections both possible and practical.
Depressurizing to about 20 Pa is usually sufficient. This provides enough of a draw to move air so that its effects can be seen thermally. It can take a few minutes (sometimes more) for the pattern to emerge, but as the blower door is running, watch for sources of warm air infiltration. This approach certainly works for air leakage, and it may also help to reveal insulation patterns that are not seen naturally.
During warm-weather inspections, air leakage sites can also be affected by both warm and cool air. You are likely to encounter both types of thermal pattern (hot and cold), depending on the source of the infiltration. Warmer outside air or exterior surface temperatures (such as sources with solar loading or coming from attic areas) are going to create patterns that appear warmer than the surrounding wall surface. Air infiltration from the north side of a house, or entering from the basement, will appear cooler. In many instances, you will see both patterns right next to each other in the field of view, indicating two potential air leakage pathways (see Photo 9).
Warm-Weather Inspections Can Be Effective
With the right training and a keen understanding of the conditions, you can use thermal imaging very successfully for warm-weather inspections. While having an IR camera with the right specifications is also important, there is no substitute for proper training and experience.
When you do conduct a warm-weather inspection, always plan on working either inside only, or at night and in the early morning on both sides of the wall, to get the best results. Keep in mind that whatever time you choose, if you are aware of the sun’s impact on the structure, you can use solar loading to your advantage to help bring out thermal patterns in a wall. Knowing whether you are in winter or summer mode is critical, so keep asking yourself: Which way is heat transfer occuring?
And remember: A blower door is the thermographer’s best friend. A blower door can drive air movement and create associated thermal patterns that might not be detected naturally.
So have at it! If you are a thermographer who lives in a hot climate, you probably already know how great a tool IR can be for warm-weather inspections. Probably you are already using it to define thermally deficient areas where improvements can be made to reduce cooling costs. For thermographers like myself from northern climates, the message here is that we can conduct successful scans throughout the year, including spring, summer, and fall. No longer do we need to be confined to the cold, winter months to do our infrared work in buildings.
Matt Schwoegler is an instructor and consultant with The Snell Group, where he has been working with thermal imaging since 2002.
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For more on IR in heating and cooling climates, and for information about training and equipment, go to www.thesnellgroup.com.
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