Basement – Condensation on just 1 duct in unfinished basement


We live in the Northeast (PA) and have a new construction home that we have lived in for a year. The home is small (1300 sq ft, open concept first floor) with an unfinished basement. The ceiling of the basement was insulated with this super thick pink insulation w/paper backing (paper is marked R30), paper side up. We have a heat pump and central ac and keep the thermostat at 68 in winter and 71 in the summer. I keep a dehumidifier running in all seasons except winter. The basement is dry. All rooms on the first and second floor seem to heat and cool evenly.

At the end of July I noticed we kept having wet spots all over the floor along the front wall of our basement. I kept an eye on them for about two weeks, until I notice wetness on a piece of furniture that we have stored down there. I reached up and touched the insulation and water poured down through it. I pulled the entire batting of insulation down and see the duct was wet and dripping all the way across. This duct is the supply that goes up into our second floor (through outer walls) in the front 2 bedrooms (floor registers).

We pulled the insulation down that covered every other duct in the basement and all of the others were completely dry. I had the original HVAC installer come and he says his work is perfect and we MUST have a water leak (we have pex and our lines aren't anywhere near this area). I changed the dehumidifier setting to 40%, bumped the temp up to 72, and changed the filter (which was reasonably dirty but not the worst I have seen). I have kept an eye on it for several weeks now and the condensation is all but gone except for a few drips only on the elbows that bend up and go up through the wall (both sides, although one side is worse than the other).

Any theories, ideas, and suggestions/things to look for are appreciated so we know where to start. Checking the vapor barrier in our living room was the main idea that I was given but i'd like to avoid cutting holes in the walls if I could… We were also told to wrap the ducts but i'm not sure that's all it needs. In the next year or two we do want to finish our basement.

Best Answer

If the ducts do not have sealed air spaces at top and bottom when the ducts get cold the air around it becomes cold. Cold air is denser and heavier so it drops, pulling in fresh humid air from the attic or other airspace. As the air cools with the moisture condensing on the duct and it starts all over again.

Sealing the areas the ductwork goes up the walls at both ends is important and may be the root cause .

Your HVAC guy is a hack no one is perfect but they all say that. You just need to isolate any air flow and this is a common issue with multi story homes. The insulation is usually installed after the ductwork and sheetrock so they may not have sealed the chase but your hvac guy should have known this and told you to make sure the verticals shafts were sealed.