At first blush, inspecting buildings with thermography might appear to be a rather straightforward endeavor. While this is partially true, there are some points to consider. Heat transfer in buildings can be dynamic and understanding the thermal conditions and how they change during the inspection process is essential to interpreting the data collected. Additionally, it is important to understand that the thermal patterns present in a building may not always present themselves as might be expected .  Here is an example of a precast panel on a hospital, one would expect nice even signatures across the whole panel but as you can see there are many areas that appear not to be insulated. This type of inspection is an induction inspection which is different from an air leakage survey where you might use a blower door to pressurize or depressurize the building.

The general premise of IR inspections of buildings is that abnormal heat loss or heat gain can be detected, and a determination of the root cause is made based on the detected thermal pattern. Detecting abnormal heat loss or gain often is the simpler part of the overall equation, particularly with advances in imager technology. Connecting the dots to arrive at the root cause is where mistakes can be made. Assumptions cannot be relied upon to provide the answers we seek, so the next step in the interpretation of images is further investigation of the building system where the abnormal thermal patterns have been detected. This may be done by a contractor or the thermographer or both.

The keys to interpreting a thermal signature in a building can be summed up as follows:

• Second Law of Thermodynamics
• Anomaly size, intensity, and location
• Building construction emphasizing:
  • Capacitance
  • Type of insulationSiding, brick, stone, stucco
• Inspection conditions emphasizing:
  • Sun
  • Shadows
  • Wind
  • Ambient
    • Air temperature
    • Moisture
    • Relative humidity
    • Dew point

In cases where the shape is regular, for example in a rectangle, the anomaly might be associated with framing and could be missing or defective insulation. An irregular pattern that crosses the apparent boundaries of framing, however, might not be missing insulation, so more investigation is warranted. Locations that are between floors, under windows, or other fixtures are often spots that are hard to insulate and they may be voids as well.

Intensity can also be a good indicator of the root cause. An area of a large thermal difference could indicate trapped moisture. Wet construction materials may be detected or hidden as a result of variations in conductivity and capacitance.  If the moisture is on surfaces evaporative cooling may be the dominant thermal pattern.  Lower intensity patterns can be the result of conduction losses due to other factors, such as air movement in and through the insulation, or internal heat or cooling sources in direct contact with wall systems. HVAC ducts routed behind insulation, appliances that generate heat energy directly contacting or in close proximity to the exterior wall, and spots where there is a greater occurrence of thermal bridging can all cause irregular patterns of differing thermal intensity.

The downside here is that the same key thermal signatures that can be helpful may also mislead. So again, the assumption is the enemy of due diligence, and it is imperative that deeper investigation of detected anomalies be undertaken to ensure proper image interpretation.