Humidity in unvented attic (but we have closed-cell foam insulation)?
We have been living in our air-tight house (.44ACH50) for a couple months now. We live in climate zone 5.
We have an unvented conditioned attic space that we use for storage. Our ERV and the indoor component of our Fujitsu unit live up there, too. We have closed-cell foam sprayed under the roof sheathing, and then there is blown insulation under that (for a total of R-93)
We have noticed that the attic’s RH tends to stay 5 to 10 percent higher than the rest of the house. This wasn’t that big of a concern in the summer. The attic stayed between around 50 to 55 RH and the rest of the house was usually around 45 to 50 RH (we tried to keep it toward the low end).
However, it’s starting to get cold at night, and I have read that best practice for airtight houses is to keep indoor RH 40% when it’s 40 degrees and 30% when it’s 30 degrees.
The attic was at 51% RH, and the outdoor temps have been 60s in the day, 50s to 40s at night. We attempted to get the attic humidity down for the past two days. Using a combination of dehumidifiers and the “dry” mode on the Fujitsu, we got the main part of the house down to 35 RH, but the attic would not drop below 46. (We kept the attic stairs open, hoping for air exchange.) Then, overnight, we decided to turn on the fan option on the Fujitsu, thinking that would circulate the air in the house and even things out. It only budged about 1%.
We have a supply vent in the attic but not a return.
From what I can tell reading on GBA, elevated humidity in the attic is usually due to closed-cell spray foam, not open cell. So what’s going on here?
And should we be concerned? It is possible this elevated moisture is due to construction moisture and will eventually resolve itself? Is it possible the blown insulation is doing something? Do I need to be concerned if we can never get the attic below 45% RH, even in the dead of winter? Should we install a return grille in the attic (apparently this can be a code violation)?
I am just a homeowner. Not a builder. We are trying to be hypervigilant about potential moisture problems.
Thanks in advance!
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