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I received an e-mail from a firefighter in Canada:
I have a question for you. Following the rekindle of a fire in an apartment building, a question arose over the use of the thermal imager (TI). The original fire was in a structure which was insulated with cellulose insulation. The department used the TI especially during overhaul and found no hot spots. A fire watch was established but during the night the fire rekindled. Assuming the fire was properly overhauled, would it be possible that the TI did not uncover the hot spot? We thought that because the fire happened during severe wintry conditions, the hot spot remained hidden from the camera due to an accumulation of ice over the area. We know that the TI does not see through water and the heat was not intense enough to melt the ice and thereby indicate to the camera that there was a possible ignition source.
The overall issue is how construction features and weather conditions can challenge your overhaul efforts.
Consider the Material
This situation out of Canada is one that could happen almost anywhere in North America. The first issue to consider is the role of insulation. Its job, in basic terms, is to prevent the transfer of heat, keeping it from escaping into an attic during winter, or prevent it from infiltrating the house in summer. Insulators do not discriminate in how they block heat, which means that a good insulator that keeps heat from entering the attic may also keep heat from reaching your thermal imager.
This does not mean, however, that the TI is useless in overhaul. I have found unknown hotspots in cellulose insulation, preventing a large fire in the attic. The key is going to be how big the heat source is, compared to how buried it is in the insulation. Most attics have 12 to 16 inches of cellulose; time may result in the material compacting to as little as four or six inches. In general, only the smallest heat sources will be hidden by thick insulation. However, you will still have to consider the normal fire movement and behavior.
If you can see that fire has extended to an area, or you believe the fire should have spread to an area, you should overhaul it as though the fire was there. Remember, you cannot rely solely on one skill, indicator or tool. If your senses tell you that a fire has spread, then act on that. If your senses tell you the fire has not spread, but the TI indicates that it has, consider making a small inspection hole. Err on the side of caution; an extra inspection hole is easier to explain than sending three companies back to the building for a rekindle.
Consider Weather And Water
Our friend in Canada also included in his e-mail that he was concerned that ice on the insulation may have played a role in missing the hot spot. Certainly, any amounts of water can hide heat signatures from your thermal imager. Ice, puddles and even very wet walls can play havoc with your efforts to detect hot spots. Normally, just being wet will not cause major problems. Sometimes, the cooling effect of water can actually make it easier to identify hot spots with the TI.
However, bodies of water, like puddles and coatings of ice, are a different issue. These will block infrared radiation, preventing your TI from seeing anything on the other side of the water or ice. In colder climates, the issue of ice or snow build-up on a surface is a real issue. If you cannot clear the frozen covering from the surface, you may have to scan the area from a different direction.
Also remember that the TI detects differences in temperatures. The effects of the sun, as well as the fire, may hide small temperature differences. Sometimes, a quick spray of cool water will help define a picture by hiding the masking heat and allowing the true hot spot to be identified. Sometimes, however, the hot spot will be hidden by an environmental issue no matter what actions you take. At times like these, you will have to resort to your traditional skills to detect and extinguish hot spots.