Moisture on closed cell flash & batt – Do I need vapor barrier?
I think the key to the question is, am I being fooled by unusual conditions?
I’m in the middle of renovating the upstairs of a 50’s Cape Cod in Zone 4. Insulation is flash & batt over the roof and walls as shown in the attachment (with the exception that I think instead of the batts with listed R-value like I asked for, I got 6″ batts on the roof and 3″ in the walls. Also no dry wall is up yet.). The area is unconditioned except for whatever heat migrates from downstairs (this keeps it at~60 deg F if the downstairs door is open). I will have a mini-split installed soon.
The moisture I found was only on the spray foam on roof deck–not in the walls. A cheap meter I have says it is 60 deg. with an RH of 69% upstairs. It’s actually warmer than that outside right now, but was recently much colder. The same meter brought downstairs has not fully acclimated yet, but says 68 deg, RH=59%. Outdoors it is 67 deg, 72% RH.
What I like to think is that this is just an unusual case where we had a good snow followed by warm temps. So all the snow melted causing high humidity and outdoor temps are high enough that the furnace is not doing much–but the temp hasn’t been high enough long enough for the surface of the spray foam to get above dew point. So the moisture observed is a rare occurrence (esp. once the upstairs is heated) that poses no problems. Is that wishful thinking, or do I need a vapor barrier….or dehumidifier?
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