Polyurethane/Icy/Cells combination
I’ve been sifting thru the poly, icy, cell threads and I’ve not seen any posts that propose using a combination of these products in a heating climate, so I’d appreciate any input from you folks.
We’re doing a barn-to-house conversion in Michigan. Post-beam, balloon construction. Cathedral ceilings will be filled (2X6 rafter bays) with polyurethane up against the sheathing; then covered on the inside with 1″ extruded polystyrene (“pink”) sheet as a thermal break/vapor barrier, taped, drywalled.
QUESTION 1: Is the resulting R value enough in the roof? I believe PUF outgasses to R6.5/inch X 5″= 32.5 + 1′ of XPS @ R5/inch = 37.5Rs total). We’re going to be heating 3,500 sq ft, radiant hydronic panels and infloor fired with a condensing boiler.
The insulation contractor suggested that we spray 3″ of polyurethane (R- 19.5 total) into the 2X6 wall cavities against the exterior wall sheathing to seal everything and leave the remainder empty.
I was going to either leave the remainder of the bay empty as the contractor suggested or fill the void with cellulose for more Rs. In either case I’d install “pink” foam sheet on inside, tape, and drywall.
QUESTIONS 2,3,4: If PUF is a vapor barrier, isn’t it on the wrong side of the wall cavity in this climate? Besides that, wouldn’t I end up with double vapor barriers if I also install pink foam (also a vapor barrier) on the inside? Shouldn’t this foam be open cell Icynene rather than PUF? That way any moisture getting into the wall should migrate to the outside (OSB/plank sheathing, felt, board-on-board cedar)
I may be thinkin too hard, but I’d really like to do this right the first time since money is dear around here. Thank you all for any insight.
Replies
"The insulation contractor suggested that we spray 3" of polyurethane (R- 19.5 total) into the 2X6 wall cavities against the exterior wall sheathing to seal everything and leave the remainder empty.
I was going to either leave the remainder of the bay empty as the contractor suggested or fill the void with cellulose for more Rs. In either case I'd install "pink" foam sheet on inside, tape, and drywall."
Do a search on a "mooney wall".
You are doing a work putting up the foam for only a small increase in insulation value.
"QUESTIONS 2,3,4: If PUF is a vapor barrier, isn't it on the wrong side of the wall cavity in this climate? Besides that, wouldn't I end up with double vapor barriers if I also install pink foam (also a vapor barrier) on the inside? Shouldn't this foam be open cell Icynene rather than PUF? That way any moisture getting into the wall should migrate to the outside (OSB/plank sheathing, felt, board-on-board cedar)"
This is a case where the term "vapor barrier" is meaningless and vapor retarder is more meaningfull.
Vapor barrier implies a relatively thin material with minimal insulation value, but that restricts the flow of water vapor.
Both the PU and the XPS have significant insulation properties.
There won't be condensing surface. However if there is any leak in the walls and you get liquid water in that area it won't be able to dry out.
Seems like overkill to me to spray and then go over with foam board. Yeah, I know, thermal bridging. But what alot of extra work for modest gain.
Your heating plan seems great. I went condensing boiler in my house, what are you going with?
always shooting for r 44+ in ceilings in WI and r19 in walls is minimum accepted. in ceilings here i figure an inch of blown in takes 2 yrs to pay for. never used puf except for vapor barrier, or basements
There is no comparison between SPF (spray poly foam) and fiberglass of cellulose.
SPF blocks all air infiltration. The other traditional insulations do not. The "R" comparison is a labratory experimentally derived number with no wind or tolerances involved. Sprayed in foam fits perfect every time. Does FG?
I would follow the insulation contractors advice and save yourself a lot of time and money.
Stu