An Introduction to Thermal Imaging
An infrared camera can help diagnose a variety of home-performance problems, including air leaks, missing insulation, or thermal bridges
Energy auditors and insulation contractors have been using infrared cameras to diagnose home-performance problems for over 30 years. Without opening up your walls or ceilings for inspection, a trained specialist can use one of these cameras to locate insulation voids, air leaks, moisture intrusion, thermal bypasses, and thermal bridges. It’s even possible to use an infrared camera to locate leaks in hydronic tubing embedded in a slab.
These tools are known by a variety of names, including infrared (IR) cameras, thermographic scanners, and thermal imaging devices. An image produced by such a camera is called a thermogram, and a trained user of the device is called a thermographer.
Although many people assume that infrared cameras measure surface temperatures, that’s not really how the tools work. An IR cameraA camera that provides an image showing radiation in the infrared range of the electromagnetic spectrum. Since the amount of infrared radiation emitted from a surface varies with temperature, a thermal imaging camera is a useful tool for detecting hot or cold areas on walls, ceilings, roofs, and duct systems. When used to scan a building envelope, a thermal imaging camera can detect missing insulation or locations with high levels of infiltration. Thermal imaging cameras can provide useful information when the difference in temperature (delta T) between the indoors and the outdoors is as low as 18F°; however, the higher the delta T, the easier it is to see building defects. actually measures the intensity of infrared radiation (radiant energy) being emitted by the surface it is aimed at.
The history of infrared cameras
Academic researchers began using infrared cameras to diagnose thermal envelope defects in the late 1970s, using cameras that cost more than $25,000 each. The cameras were cumbersome devices that were cooled by liquid nitrogen.
As new models of IR cameras were developed, prices began to drop. Gautam Dutt, one of the original “house doctors” at Princeton University, recalls, “For an infrared camera, we used a Magnavox unit marketed by Aga, the Aga 110. It cost about $13,000. It was expensive equipment, but it was cost-effective. The first thermal bypasses that I discovered took me weeks to discover, crawling through attics. Another one of our teams, working with infrared, could find thermal bypasses right away by using an infrared camera in combination with a blower door. That was the innovation we developed at Princeton — the combination of a blower door and infrared.”
Prices for infrared cameras continue to drop. It’s now possible to buy a useful IR camera for $1,500, and a high quality one for $5,000. Lower prices mean that more contractors can afford the devices; however, since an infrared camera in untrained hands isn’t very useful, anyone starting out in the field should budget some money and time for training.
John Snell — admittedly a biased source, since he owns a company that provides thermography training — explained in a recent JLC article that “a two-day training session in IR-imaging basics is worth about a year of flopping around on your own.”
Wait for appropriate weather
This article is intended as an introduction to the topic of thermal imaging. However, I am not a trained thermographer, so readers should not depend on this article for technical advice concerning the use of an infrared camera.
If the outdoor temperature is 70°F, you usually won’t get any useful information with your IR camera. That’s because the indoor temperature and the outdoor temperature are about the same; there usually won’t be any heat flowing through a building’s walls or ceilings under those conditions.
To conduct a thermographic inspection of a building, you need a minimum delta T (temperature difference) of 18F° between the interior and the exterior for several hours before the inspection begins. If the interior is at 70°F, you usually need an outdoor temperature that’s below 52°F or above 88°F.
If the weather isn’t cooperating, you’ll have to be patient and wait for the weather to change — or you’ll have to adjust the thermostat so that the heating system or cooling system makes the interior of the house unusually hot or cold. To be sure of an adequate delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures., most thermographic inspections are scheduled for early in the morning.
Early morning inspections are best for another reason: sunlight tends to warm exterior walls and roofs, complicating thermographic inspections. Ideally, you don’t want any sun shining on the building for at least three hours before the inspection begins. If the house has brick or stone veneer, the sun-free interval should be at least eight hours long. According to Snell, “Inside and out, in both summer and winter, too much sun can wreak havoc on otherwise acceptable conditions.”
Very windy days aren’t good for thermography. Ideally, the wind speed should be 8 mph or less. The roof and walls should be dry.
For an inspection of an occupied house, it’s a good idea to tell the homeowners to move furniture and pictures away from exterior walls 12 hours or more before the scheduled inspection.
Start on the outside of the building
Most IR cameras need to be adjusted by the user each time images are made. Typical user-adjusted settings include:
- A choice between a color image or a grayscale image.
- Settings for the upper and lower temperature limits (the span setting and the thermal level setting — sometimes referred to as “gain” and “contrast”).
Assuming that the home’s walls have conventional wood framing without a layer of rigid foam insulation, a thermographer hopes for weather conditions that produce images showing details of the building’s framing. For example, if the camera is looking at the outside of a house during the winter, vertical stripes should be visible on the walls. These are the studs, which act as thermal bridges, leaking more heat than the insulation between the studs.
If the framing members aren’t visible, that usually means that either (a) the weather conditions aren’t appropriate for a thermographic inspection, or (b) the house is well built, with details designed to minimize thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. . Examples of such walls are SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. walls, ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure. walls, or walls that include exterior rigid foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .
If the wall framing is visible (or if the lack of visible framing is explained by the presence of rigid foam insulation), a thermographer inspects the outside of the building, looking for anomalies. An IR camera will easily reveal common thermal bridges like studs or poorly insulated rim joists.
The camera will also reveal insulation voids. In some cases, it’s possible to see stud bays that were left uninsulated. In other cases, blown-in insulation may have settled, leaving a void at the top of each stud bay. Stud bays with hot-air ductwork show up very clearly.
Get out your blower door and move inside
After inspecting the exterior of the building, a thermographer usually moves indoors. As with the outdoor inspection, the idea is to scan surfaces for anomalies. After an initial inspection, most thermographers set up a blower door and depressurize the building.
The blower door increases the flow rate of air leaking into the building through cracks in the walls, floors, and ceiling. Since the outdoor air is at a different temperature from the indoor air, the air cools (or warms) the surfaces it encounters in the area of the crack, creating visible plumes in thermal images.
According to “Guidelines for Thermographic Inspections of Buildings,” a standard produced by RESNET, “The thermal image for air leakage will appear as ‘fingers’ or ‘streaking’ showing as dark when cold air is observed and lighter colors when warm air is viewed. The thermal images will produce irregular shapes with uneven boundaries and large temperature variations. These air leakage sites are often at joints, junctions or penetrations in the enclosure. There is often a temperature gradient within a finger or streaking area. … Take care to discriminate between thermal bridging sites and thermal bypass or air leakage sites. Thermal bridging sites will not change size or shape during the inspection.”
An IR camera can be a useful tool for pinpointing leaks in a low-slope roof. Such an inspection is best performed at night after a sunny day. For a useful thermographic inspection, the roof must be dry.
When the sun shines on a low-slope roof, it heats up the roofing and the top of the insulation below. Once the sun sets, the insulation begins to cool; however, damp insulation cools at a much slower rate than dry insulation. That’s why leaky areas of the roof show up as warm spots when viewed at night.
Pitfalls for the unwary
The best thermographers are those who understand building construction, understand building science, and have been conducting thermographic inspections for many years.
Almost anyone can point an IR camera at a building and fiddle with the knobs until the colors are pretty, but it takes a trained and experienced expert to tell the difference between evidence of thermal bridging, air leakage, and moisture.
Last week’s blog: “Energy Predictions for 2012.”
- Tom Barbour - Thermal Image UK - http://www.thermalimageuk.com
- Infrared New England - http://www.infrarednewengland.com
- Chuck Evans - http://homecert.com
- Infrared New England - http://www.infrarednewengland.com
- David Valley - Massachusetts Infrared - http://www.massinfrared.com
- InfraMation newsletter - November 2006
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