I’m trying to zero in on a basement humidity problem I discovered this weekend. I recently built a house in North Central Massachusetts, Zone 5 and am I seeing signs of excessive humidity in my unfinished basement with no obvious sources of moisture. While poking around I noticed moisture on the inside of the poly that the insulators had installed on the “warm side” of the fiberglass batt insulation (The rest of the house is dense pack cellulose with now vapor retarder). At first I though I might have had a leak but then ruled that out as the moisture was consistent along the entire 30′ walk out section. I then notice a couple small puddles had not dried up by the filter on the well line I had changed a week ago. The well line and water softener tank have been sweating aggressively as well.
The foundation has a proper asphalt damproofing , there is an interior and exterior footing drain, a 6mm poly vapor barrier with taped seams, the basement has 30′ of exposed grade so ground water is not and issue and the gutter down spouts are tied into drains. The rim joist has 2″ of closed cell spray foam and all framing joints have received spray foam caulking. Upstairs the house has has been averaging RH of 45-50% which is controlled by the A/C and HRV. The house is considered fairly tight with less than 2 ACH.
Do you think I have a ventilation issue or moisture issue?