Arkansas HVACR NewsMagazine March 2026

HVACR NewsMagazine March 2026

Tech News

Dew Point and Ventilation When we talk about moisture problems in attics, we’re referring to when surfaces sweat, which can then lead to other nasty consequences like fungal and bacterial growth. Sweating only happens when wet air makes contact with a surface below the dew point . The dew point is when the air is at 100% relative humidity (saturation) and cannot carry any additional water vapor with it. (Before anybody comes at me, I know air doesn’t actually “carry” or “hold” water vapor; it’s just a useful way to think about saturation. In reality, water vapor molecules exist in the air thanks to vapor pressure AND water’s partial pressure. As the air temperature increases, the vapor pressure increases. When there is more water vapor in the air, the partial pressure increases. The partial pressure cannot exceed the vapor pressure in our normal everyday conditions, so we get saturation when they’re equal. The ratio of partial pressure to vapor pressure IS relative humidity; a 1/1 ratio = 100%.) As a result, excess water vapor must come out as liquid water. In Florida, dew points above 70°F are very common in the summer months. The supply air in the ductwork is usually around 55°F, and of course, while we have duct insulation that makes the duct surface warmer than the air contents inside the duct, the duct surfaces are still significantly cooler than the unconditioned outdoor air that’s coming in. Therefore, when we bring in lots of unconditioned attic with no means of dehumidifying it or a clear path out of the attic, that moisture-laden air will come into contact with cold duct surfaces (and the air handler, if it’s installed in the attic). When those surfaces are below the dew point, the water vapor in the air will be pulled out of the

air and condense on those surfaces. They’ll sweat . Now, we can mitigate that a bit without conditioning the attic when we consider where we place those attic vents. Since we were young, we’ve been taught that “heat rises.” While that’s a simplistic way of describing heat movement, it’s largely true; warmer air is less dense, and less-dense air rises while denser air sinks — that’s thermal buoyancy and a key driver of heat transfer via convection . Wet air is the same. Despite how oppressive and “heavy” humid air might feel on our bodies, water vapor is actually lighter than air in terms of atomic mass. Most of the air around us is either nitrogen or oxygen, which are molecules N 2 and O 2 , respectively. One nitrogen atom has an atomic mass of ~14 atomic mass units (amu); multiply that by two, and the molecular mass of one molecule of N 2 is 28 amu . Oxygen has an atomic mass of 16 amu; multiply that by two to get a molecular mass of 32 amu . Water vapor consists of two hydrogen atoms and one oxygen atom (H 2 O), so since hydrogen’s How Moisture and Heat Behave

atomic mass is 1 amu, 16 + 1 + 1 = 18 amu .

Since water vapor is less dense than the other molecules that make up air, it will rise. However, convection is NOT the main way water vapor can make its way to the top of an attic.