Massachusetts Code for Vapor Retarders
I’m retrofitting a 1900 balloon frame farmhouse in northeast Massachusetts (Zone 5): leaving exterior alone (cement shingle over wood shingle over board sheathing), filling real 2×4 stud bays with recycled polyiso insulation (cut and cobble), running continuous sheets of 3/4 inch polyiso across studs (taped and acoustical caulk as air barrier), then building inside that 2×3 walls with batt insulation as service cavity. Inspector wants to know about vapor barrier and I’m worried he’s going to require polyethylene sheets. My understanding of Mass. Code is Class I or II vapor retarders must be installed on the interior side of frame walls in Zones 5-8. However, it also seems like Class III vapor retarder is allowable if using spray foam. Is it reasonable to argue that the cut and cobble 4 inch of polyiso (spray foamed around edges) in the bays plus continuous layer of polyiso is equivalent to spray foam and therefore qualifies for using Class III, i.e., sheetrock and latex paint? If not, does kraft faced batt insulation in the new walls qualify as Class II? There’s also some references to paint with a perm rating between 0.1 and 1.0 qualifying for Class II.
p.s. I’ve already lost the fireblocking battle; despite having stud bays completely filled with solid rigid foam through which I don’t believe any flame or gas could travel, the inspector is making me fireblock both top and bottom of walls.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part