Unvented Attic – All Closed-Cell Spray Foam or Foam and Batts
We are into the interior of our new home construction and discovered condensation on the closed-cell foam/fiberglass batt interface. The insulation is at the lid with the following roof detail (top to bottom): standing seam metal roof, Polystick MTS Plus underlayment, 1/2″ OSB sheathing, 2″ (R-13) of Gaco F1850 closed-cell spray foam insulation (vapor permeance <1 perm at minimum thickness of 0.44″), R-30 unfaced fiberglass insulation. The insulation company says that the condensation is due to the warm moist air from drying the drywall and paint. Due to the difficulty in drying the fiberglass insulation, the insulation company has agreed to remove the fiberglass batts, dry out the attic, and replace the insulation. We are located in climate zone 4C (25 miles east of Seattle in the Snoqualmie River Valley). We are conflicted between replacing the fiberglass batts or going with a full spray foam application (total of 7″ for ~R-45). What do you think? How should we treat this attic space going forward? Should it have air circulation through our ERV or connection with the underlying rooms?
Thanks!
Replies
If you are under construction it may not be an issue and your insulator is probably right. Drywall mud and paint can create a crazy amount of moisture that will never be duplicated. In the winter months condensation is even worse. We run fans in addition to keeping heat at lower temp (just keep it warm enough for drywall to cure). If you set temp too high it will encourage quicker release of moisture from drywall. Also, crank a window open for a few minutes in winter months to let some of humid air work it’s way out and dry air in. I’ve been doing flash and batt for years and have never had an issue. The foam does need to be the right thickness and you have to use a depth gauge while spraying to make sure you are on depth. Also, your home is going to be sealed tight, so an ERV (or similar) would be wise. Also, need to make sure bath vents and kitchen vents are sized properly for removing moist air. Your HVAC can advise the best on that. I run timer switches on bath fans, so they run after shower is done for 30 min b/c there’s still a lot of moisture in the air after the shower. If you’re having an issue w/ moisture well after install of drywall mud and paint, you could have a problem. Maybe bad foam mixture or something. But, if you’ve been experiencing the extreme winter temps and they are in process or recently completed the drywall and paint it’s just the residual moisture from that. The homes sealed tight, so needs to air out.
Your insulator is probably right, but 2 inches of foam seems marginal for zone 4. Check out this from the magazine.
https://www.finehomebuilding.com/2018/01/11/choosing-right-thickness-exterior-foam