Condensation at the bottom of the main waste stack in basement
During our recent cold snap here in Chicago, I noticed condensation dripping from my main waste stack in the basement. It’s a single story home with a full basement, and with Passive House levels of air tightness and insulation. The waste stack goes through the Master Bath on the first floor, surrounded in the wall with Roxul batt for sound deadening. There’s 2′ of cellulose in the attic. Never any evidence of condensation/moisture on the bathroom ceiling or floor.
I’m pretty sure it’s condensation since it entirely goes away once temperatures move above 35 F.
At 20-30F there are small amounts of condensation sitting on top of the lip of the first elbow in the PVC directly below the 1st floor sub flooring. Not enough to drip down onto the basement floor, but it’s definitely there.
Only when temperatures were at 0F did I notice any condensation falling off the PVC onto the basement floor. And it was only when temperatures fell to -24F, without the wind chill, that more than a foot of the PVC pipe began to show condensation and drop water onto the basement floor.
We also had enough snowfall on the roof that it was slowly falling into the vent stack on the roof (confirmed by looking up on the roof), but the stack was never completely blocked (never any sewer gas smell or trouble flushing toilets).
The waste stack has a straight shot from the roof to the elbow just below the 1st floor sub flooring in the basement, so I’m pretty sure snow was falling in from the roof and landing at the elbow before sitting there and slowly melting.
When I googled “condensation on waste stack vent” there were any number of hits, typically involving people trying to hunt down the source of mysterious moisture in a basement, and always during the winter.
I’m just wondering if this is normal? Anyone else seen or heard of this?
Apart from when temperatures were at or below zero, there’s not much water involved. Even at -24F we’re talking about wetting the floor, not collecting it in buckets or even significant pooling.
Worth doing anything about it? As long as my subfloor stays dry, do I care if the basement floor gets a little bit of water during extreme cold snaps?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part