Water vapor from the air condenses on air conditioning ducts in humid climates. It’s as normal as poorly insulated bonus rooms making occupants uncomfortable or cigarettes causing lung cancerâ€ . Condensation on ducts is most common in crawl spaces and basements, where the air is more likely to have a higher dew point.
But it also happens in conditioned space and occasionally even in attics. That’s why you have to be careful about burying ducts in attic insulation in humid climates, especially in coastal areas. The 2018 International Residential Code (IRC) and International Energy Conservation Code (IECC), however, have passed new requirements they say will prevent condensation on such buried ducts. Will they work?
The problem with buried ducts
Before getting to the new IECC requirements, let’s understand the issue a little better first. The potential for duct condensation is a humid climate, cooling season problem. My friends out west regularly bury their ducts in air-permeable insulation (mainly fiberglass and cellulose) and don’t worry about condensation. The air in the ducts may be as low as 55°F, but their summer dew points are usually below 50°F. There’s just not enough water vapor to condense.
In humid climates, that’s not the case. The photo above shows condensation on a duct in a crawl space in Hilton Head, South Carolina. Outdoor dew points can hit 80°F in the summer there and average somewhere in the 70s. With 55°F air moving through supply ducts, you’ve got to have well installed insulation on the duct with significant R-value to keep the surface temperature above the dew point.
When that supply duct is in a vented crawl space, it’s really hard to keep that surface temperature above the dew point because the temperature…