I was recently doing an infrared flat roof survey on an EPDM membrane covered roof and captured an interesting natural process that I’d like to share. The scan started about an hour after sunset on a 70 degree day. By the end it was down to about 55 degrees with condensation forming on the surface of the roof. I started seeing some very unique thermal patterns and was able to capture some good images. I’ll post the images below then explain what they show.
Warm areas at penetrations – looks familiar.
An open and shut case of moisture below the surface – right?
Wait a second! Why is moisture BELOW the surface affected by a cable ABOVE the surface?
Let’s take a closer look.
Upon closer examination we can see that the thermal exception resembles ice crystals. We can also see that the thermal imager is showing temperatures in the 20s. Of course the condensation would freeze at temperatures in the 20s, but it was never below 55 degrees that night. To understand what happened, it’s necessary to think thermally.
As stated above, the roof surface was covered with an EPDM membrane. One of the biggest challenges faced when performing an infrared roof survey on an EPDM membrane is that it is very reflective of thermal radiation. I often see reflections of parapet walls, RTU’s, or even the people I’m working with.
This time I was seeing a reflection of the sky, and the sky is cold! What’s interesting is that my thermal camera isn’t the only thing “fooled” by the reflected temperature. As condensation formed in a very thin film, it was flash frozen by the reflected temperatures. Since the actual roof temperature (not reflected temp) was above freezing, the condensation almost immediately thawed.
Case closed? Not quite. None of this explains why the frozen water appears to be warmer than the roof surface. For this we have to take into account the reflective nature of the EPDM and the thermal density properties of water. In addition to water having greater thermal density than the atmospheric conditions reflected from above, it’s also worth noting that ice on earth is warmer than the space above.