Polyiso as Vapor Retarder in Cathedral Ceiling
Hi folks,
I am improving the insulation in a cathedral ceiling in Zone 5. Existing insulation was kraft faced ‘balsam wool’ batts about 1″ thick. Rafters are 2×6.
The roof is vented, so I’ve installed vent baffles, and then filled the remainder of the 2×6 cavity with Rockwool batts. My plan is to install a continuous 1″ layer of recycled foil faced polyiso across the studs as a thermal break, and then 1/2″ Gypsum board over that.
Am I correct that the taped polyiso is adequate as my vapor barrier?
FYI – I understand this improvement doesn’t get me to code minimum for my zone, but it’s all that’s in budget and a big upgrade over what existed previously. I should be going from whole wall R3 to about R23 per my Ekotrope calculations.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
If it's foil-faced polyiso, then yes, it's a good vapor barrier when taped. If there are any significant holes in it, stick some tape over those too. I like to use foil tape with foil faced polyiso, but flashing tape works well too. With the often not quite perfect edges on reclaimed polyiso, using a wider tape can make your taping job easier.
Bill