After years of confusion, it’s now clear that mold only grows inside buildings where excess moisture has accumulated. And more recently, the worldwide interest in reducing energy consumption has put a new emphasis on diagnosing the location and extent of heat leakage into and out of building enclosures.

Thermal infrared cameras have become standard tools for the building inspections focused on these issues.

Figure 1 shows a simple example of a hotel stairwell with an odor problem. The infrared camera shows a temperature pattern which suggests that cool water is escaping the fan-coil unit’s drain pan and wetting the carpet. Such wet materials grow the mold and bacteria which produce odors.

Figure 2 shows an example of outward air leakage from the top of a tall building due to the stack effect during cold weather. Warm air escaping through open joints near the top of the building creates a slight suction at the base of the building. Cold air leaks in at the base of the building to replace the outward-flowing air at the top, resulting in wasted energy and cold drafts.

To understand the use and the limitations of infrared imaging for moisture detection, it helps to understand infrared energy and the cameras that can see it.

Thermal Cameras

All surfaces above absolute zero emit electromagnetic waves in the infrared region. These wavelengths are almost all much longer than visible light, although at the edge of human perception, infrared laser pointers emitting light at about 0.75 microns are a familiar technology.

Slightly beyond that wavelength are the near-infrared waves which can be seen with “night-shot” image enhancement in conventional video cameras. Then, ten times longer than those wavelengths and beyond the range of conventional optics, are the middle infrared wavelengths between 7 and 14 microns. This range is called “thermal infrared” because when surfaces are between -40ˇ and +400ˇF they emit quite a bit of radiation in this range. The strength of their emission can be sensed and measured with special optics, and used to show surface temperature patterns, as in Figures 1 and 2.

Infrared technology was developed originally for military purposes. It’s the basis of heat-seeking missile guidance systems. But the cost of cameras has come down as economies of scale have reduced manufacturing costs. For example, the cost of a camera with a low-res imaging array of 160 x 120 pixels has dropped from over $20,000 10 years ago to less than $8,000 today.

And the cost of a high-res camera with a sensor of 320 x 240 pixels has come down from $40,000 10 years ago to between $10,000 and $20,000 today, depending on which features they include. These more economical prices have made thermal cameras cost-effective for moisture inspections as well as more complex energy investigations.

Inspection Examples Figure 3 shows the aftermath of a pipe-break in an office. After the obvious water was removed with wet-vacuums, the building was surveyed in preparation for drying. These images show the classic result of water ponding on the floor. It wicks up into the wall board, pulled by capillary suction. The thermal camera shows the moisture pattern as a band of cool surface near the base of the wall. That’s because the evaporating moisture takes just a bit of heat from the wall itself.

Modern infrared cameras can resolve a temperature difference of less than 0.2ˇF between adjacent pixels, so the fact that the moist area may be only 1.5ˇF cooler than the rest of the wall is no obstacle to forming the image.

It’s interesting to note the insulating effect of the base molding shown in Figure 3. Of course, the wallboard behind that base molding is still wet. But that fact is not perceptible in the infrared image. This illustrates the important point that these cameras do not “see inside walls.” Nor do the camera images “penetrate” into the wall.

Although, it shows very useful images, an infrared camera can only see the patterns created by the emission of infrared energy from surfaces. Meters must be used to confirm the presence of excess moisture. Then the investigator’s skill must provide the analysis which leads to locating the source of the moisture suggested by that infrared image.

Figure 4 shows an example of an investigation of water penetration through an exterior wall, leading to odor problems indoors. The image was taken during the winter, on the north side of the building. In that situation, moist insulation appears warmer than the rest of the wall because damp insulation allows more heat to flow from indoors to the cold outdoor surface. The thermal pattern is consistent with rain penetration through joints between the window frames and the exterior finish.

Further note that the thermal pattern in Figure 4 is consistent with the behavior of liquid water. It penetrates a wall and flows downward until it meets an impermeable layer at the floor slab. Then it spreads out horizontally. Such patterns are not typical of other thermal anomalies, such as missing insulation or air infiltration.

Consider also that in figure 4, the moist areas appear warm rather than cool, as in the earlier figures. But note that in all cases the moist areas appear different than the surrounding dry areas. These patterns help an investigator locate potential problems in seconds, rather than the hours required to generate a fine grid work of exploratory moisture content readings.

Thermal images on the exterior, such as that in Figure 4, are always specific to rapidly-changing thermal circumstances. For example, if the camera had been looking at the eastern face of the building facing the rising sun, the moist areas would have appeared darker (cooler) than the rest of the surface. At that time on the eastern face of the building, the sun would be warming the surface quickly, except where moisture had soaked the insulation, where the surface would lag the solar temperature rise because of the greater mass of the wet insulation compared to dry insulation.

Such sun-driven temperature differences are fleeting. In less than an hour any temperature differences between wet and dry insulation would have “flattened-out,” eliminating this obvious moisture related pattern. Such issues highlight the fact that outdoor investigations are much more challenging than interior investigations. Outdoors, thermal influences are very complex, and they change constantly through the day and night. Rain, clouds, and sunlight with it’s shadows from trees and other buildings all generate thermally-confusing patterns.

Tips for building inspections using thermal cameras: The use of infrared cameras is expanding rapidly, but it’s useful to keep several points in mind: 1. Thermal cameras only show surface temperature patterns, and those patterns include both emitted and reflected energy. Glass surfaces, in particular are problematic, because they reflect and show thermal patterns from nearby, not just the temperature pattern of the glass itself.

2. Indoor moisture nearly always appears cooler (darker) than surrounding dry areas. Outdoors, moist areas may be either cooler or warmer than surrounding dry material— but the moist areas will most probably be different in temperature, which helps the inspector identify suspect areas.

3. Be wary of infrared images that have been colorized. Colors can exaggerate or obscure the true thermal differences. In some cases such false coloring can be helpful, as in the case of printed reports, in which grayscale seldom reproduces well. But during the actual inspection, grayscale will probably be less confusing.

4. Be wary of images unsupported by a description of all thermal circumstances, including air temperature, current weather, wind speed and for moisture investigations; moisture meter readings.

5. Inspections of exterior walls require more time and skill, because the thermal environment is more complex, and because there are more layers of material. If you are an inspector, recognize that your costs will increase accordingly. If you are a buyer of inspection services, expect to pay more for exterior investigations.

6. Also expect to pay much more for a meaningful report—one which documents all the thermal circumstances rather than just showing interesting pictures. The time of an inspection may be a matter of moments, but the time required to construct a meaningful report of that same inspection may require many hours or even days.

These cautions are especially useful to keep in mind for the complex circumstances surrounding forensic investigations of exterior walls. On the other hand, infrared moisture detection is quite simple for the more common problem of locating excess moisture indoors, or for quickly surveying the exterior of a building to locate air leakage points and thermal bridges.

The images communicate very quickly and effectively to the people who have to deal with building problems, and they provide a useful record of the presence or absence of such problems. ❑

Lew Harriman is director of Research & Consulting at Mason-Grant, an international provider of building investigation services with headquarters in Portsmouth, NH (www.MasonGrant.com), and Bret Monroe is chief technical officer of Monroe Infrared Technology, a nationwide provider of thermal cameras and related training and inspection consulting services, headquartered in Kennebunk, ME. (www.MonroeInfrared.com).