Upper vapor barrier with vented attic
I’m having a hard time getting over my concerns about condensation forming inside the roof and wall assembly. Over the last couple of winters I have seen pictures on Facebook build groups showing water damage on new house ceilings and walls because of condensation when warm moist air inside the house comes into contact with cold surfaces in the wall cavity or the ceiling cavity.
I want to make sure we build in a way that prevents this issue.
This will be a full house with a walkout basement in NW Kansas (climate zone 5).
We will have ICF on the basement 2×6 walls with ZIP upstairs. Attic will be vented.
We are shooting for less than 1 ACH50.
I am hoping to resolve the condensation potential in the walls by having around R12 exterior insulation with Rockwool Comfortboard 80.
For the ceiling though, I’m really struggling to visualize that. I’ve had several posts on here already asking about it. I think I’m beginning to understand the air barrier but what about the vapor barrier? Painted drywall isn’t a super great vapor barrier but maybe if your air movement is minimal the vapor transfer alone won’t be enough to cause issues?
I think we’ll likely end up with around R65 blown in on the roof.
Any thoughts would be much appreciated.
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Replies
dpilot83,
"Painted drywall isn’t a super great vapor barrier but maybe if your air movement is minimal the vapor transfer alone won’t be enough to cause issues?"
Yes, I would say that''s fair. Where you are a well vented attic insulated on the floor, with a good air-barrier on the ceiling below, simply doesn't experience moisture problems.
With that assembly you can get a bit of incidental mold on the underside of the sheathing in marine zone 4, and possibly excess attic humidity in the warm, moist parts of zones 1, 2 and 3, but in climate zone 5 it's a great balance between practicality and expense.
That’s comforting to hear. Thanks.