Attic Venting Condensation
I have read several articles on GBA and other trade and building sites and still have concerns how to solve my issue. I have the classic wisconsin 1-1/2 story house build in 1980 with vaulted ceiling (12/12 pitch) with knee walls. The rafters are 2×6 24″ on center with rafter collar ties appox. 10′ long creating an 8′ tall room with a 5′ slant walls and an accessible upper attic. The shingled roof was very recently replaced. All the old fiberglass insulation in the 2nd story slant wall, knee wall, and upper attic was removed with the exception of the cathedral living room where the insulation is not accessible. Below rafters and rafter collar is 2″ EPS white foam and then 5/8″ drywall. The drywall and foam is in good condition and I am not planning on changing or removing any of this existing finish. So my partially completed plan was to air seal all the ceilings, electrical, plumbing drywall seals, wall plates, etc in the ceiling plane and knee wall attic. I understand I cannot get to R50 in the 2×6 slant wall rafters and adding on top the roof sheathing isn’t an option on a new roof so the best I can do has to be good enough. In the 2×6″ cavity slant wall I installed 1/2″ XPS on top the existing 2″ EPS and then 3-1/2″ R15 rockwool leaving approx 1-1/”2 air space between the rockwool and roof sheathing. The soffit has continuous vents as does the attic ridge. The rafters in the knee wall will now become the thermal envelope and be as follows -roof sheathing-2″ airspace- 1/4″ dow XPS or 3/4″ EPS foam field built vent channel(sealed to joists) -rockwool R23 (2×6″ rafter extension to accommodate the insulation). – and lastly 1″ polyiso foam sealed and taped joints. The upper attic will be R23 rockwool between the collar ties and either r30 fiberglass rolls perpendicular or cellulose blown on top. I also insulated the rim joist in the knee wall with sealed 2″ XPS and R50 rockwool, painted and sealed the 3/4″ plywood floor in the knee wall. I understand this isn’t perfect but as good as possible with what I can DIY and not break the bank. Everything has been going good until this week. The rim joist and floor of the knee wall is completed and as I was installing the R25 in the knee wall rafters the weather turned bad. The last week the temp in the 20’s and 30’s with very high exterior humidity and will remain this way for the next week. The indoor temp is 64 with 73% high humidity. Condensation is already showing up on the 1/4″ XPS rafter air vent channel and the 3/4″ EPS rafter vent channel above the rockwool before I can get the 1″ ISO foamboard up. NOW WHAT??? do I have to remove the rockwool and dry it out or replace it? Should I poke holes in the site build sealed vent channels to allow the rockwool to dry into the vent channel space? Should I do something different in the knee wall rafters? Sorry for the long rant but The more info maybe the better understanding.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part