This is an easy assumption to make. When we consider moisture, we often think of evaporation. When moisture evaporates from a surface it cools that surface because it takes heat energy from the surface to change state. That’s why rubbing alcohol feels cool on your skin. It’s evaporating and removing the heat energy to do so. For that reason, caregivers will rub it on people for fever reduction, hence its name. Esoteric medical history is not why we’re here though. Moisture; not always cold.

Aside from evaporation, moisture behaves in different ways thermally depending on the situation. Water has a very high thermal capacitance, among the highest of all materials. Due to this fact we often utilize water in thermal applications, i.e. heat exchangers. So, when drywall is wet, it’s the water that’s creating the thermal pattern that we detect. Unfortunately, our infrared camera doesn’t know the signature is moisture. A moisture meter must be used to verify the signature is in fact moisture. Similarly, with roof insulation, subflooring, any building material. Water stores heat energy in a different amount than dry building materials and may move it at a different rate too due to a different thermal conductivity.

If I have moisture in an exterior wall that is exposed to sunlight during the day, the moisture is going to heat up during the daylight hours of the diurnal cycle. It won’t get as hot as the surrounding dry material, because it requires more heat energy to change temperature than the other materials do. Since it has such a high thermal capacitance though, it will stay hot longer once the sun is off the wall. So, depending on when you view that exterior wall, areas with trapped moisture might appear warmer than the surrounding areas or they might be cooler. Three possible reasons; difference in thermal conductivity, evaporation or as discussed here different thermal capacitance.

During a building inspection, it’s necessary to understand not just what’s going on thermally at the time of the inspection, but what might have been happening hours before. Where was the sun? Was there wind? The diurnal temperature range. Was there exterior moisture on a wall surface? All these conditions will have thermal effects that can last beyond when they are obvious to see.

So, the answer to the question posed at the beginning; when it’s wet it’s cold, right? Not always. Sometimes moisture is cool, sometimes it’s warm, and sometimes it might even be the same temperature as the surrounding materials because the entire system is in thermal transition. Keep your mind open and keep Thinking Thermally.