Review of insulation/air sealing retrofit plan…
For a cathedral ceiling, still a popular subject of concern I see. I have gotten advice before and used this site to increase my understanding of many building issues. The ceiling currently, from the inside, has V-groove pine boards, then a 12″ kraft faced fiberglass batt, no air sealing. There is a vent channel above the fiberglass of unknown thickness and quality (I ran into the builder and asked if it was 2″, and he replied “probably less”). The decking is 3/4″ OSB, asphalt shingles above (now at 25 years). We are in zone 6 and the roof and ceiling is steep at 9/12 (which may not be relevant). We do get ice dams.
Our plan is to take down the boards and remove the fiberglass batts, from the inside obviously. We can then inspect the decking integrity. If okay we would attach 2″ furring strips to the sides of the rafters (which are 24″ oc) and pushed up to the decking to allow for a secure vent channel (there are soffit and ridge vents). Then 2″ of rigid foam against the furring strips, sealed at edges with foam (we have a gun), caulking, tape or a combo of all 3. We would then return the fiberglass batts, slightly compressed as there is now about 9 or 10″ available for the 12″ batt (I know there is a small loss of R). This would bring us to the bottom of the rafters where we would put another 2″ of foam using 4×8 sheets with all seams well taped. This should make a decent air barrier and slow heat loss.
Any general advice is welcome. We know there are probably better options (i.e. exterior foam), but have decided against that for a # of reasons (expense and complicated application to name two). I have a couple uncertainties about the specific foam products to use, now that I have researched and found it not so easy to get precisely what I might want.
I thought EPS would be best for the upper 2″ so the high permeance would allow drying into the ventilation channel (but not easy to find unfaced EPS here, and I wonder just how much perm is needed). Next best would seem to be polyiso with the fiber facing in terms of permeance. It is against the cold vent channel so I had thought to avoid polyiso here due to R loss in the cold, but maybe not a big deal considering overall choices. I’d like some flexibility to try to find recylced stuff. Is 2″ of XPS too impermeable for this location and thus to be avoided?
And for the lower 2″ beneath the rafters; how permeable does that need to be? If not a concern at this location I’m thinking foil faced polyiso would work well and provide a couple extra R’s via reflective space (there will be furring strips securing the foam sheets so a 3/4″ space would exist). If foil is a bad idea, then fiber or paper faced polyiso okay?
Thanks in advance for input and for doing this site.
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If you're putting foam on the exterior of the fiber it either needs to be a sufficient R-ratio with the fiber for dew-point control at the foam/fiber boundary. In climate zone 6 that would be 50/50 for the foam-R/fiber-R.
If it's less foam-R than that ideally it needs to be at least semi-permeable (>1 perm, >2 perms would be better). Unfaced 2" EPS would be OK, but if you can't find unfaced EPS locally, use 1/2" unfaced XPS (2-2.5 perms). At 2" all XPS would be under 1 perm, as would most fiber faced polyiso.
If half-inch XPS is too flimsy you can support it in the center with a 1.5-2" wide strip of 2" foam glued to the OSB with foam-board adhesive as a spacer.
For the underside-of-rafter foam foil-facers would be fine- take care to air-seal it carefully. The seams can be taped with a quality thin aluminum tape (Nashua 324a or similar- something with a temperature rated adhesive should last forever in this application, but not a no-name tape of unknown adhesive quality.) The edges should be caulked or can-foam sealed to the framing.
I was unaware of the fiber to foam boundary issue, so thanks for that info. I will try to find 2" EPS. Are you familiar with neopor (Halo is a specific brand, from Canada)? I'm not sure I could get it (or how expensive), but it's EPS with graphite and a higher R value than typical. Not sure if the permeance is similar to other EPS though. Do you happen to know?
NeoPor runs about R4.5/inch, somewhat higher than EPS without the graphite. It runs about 1.5 perms @ 2".
Halo Exterra has similar R-value, but lower permeance, less than 0.9 perms @ 2".
http://www.amcfoam.com/PDFs/Halo%20MPDS-052814.pdf
The higher R/inch foam isn't really worth chasing when it is being undercut by thermally bridging rafters. A better value is to spend the foam budget on higher-R for the continuous foam that thermally breaks the rafters, not for your top-side air barrier for the fiber.
Howard,
Dana's advice is good, although a little conservative. Your planned 2 inch vent channels provide good insurance for keeping your roof sheathing dry. The wood rafters also provide some moisture buffering capability, as well as the ability to provide a drying path to the ventilation channels.
In short, this type of sandwich -- ventilation channels / rigid foam / fiberglass / continuous rigid foam -- is fairly low risk, even if your ventilation baffles are made of low-perm foam. It's a good assembly, especially if you pay attention to airtightness when installing the continuous layer of rigid foam facing the interior.
Thanks Martin and Dana. We'll do a careful job taping and sealing the polyiso, knowing this will do much to limit moist air entry. I may have located suitable 2" eps sold in Canadian big box stores not too far away (Canadian product). This is Plastifab Plastispan HD, stated to have R value of 4.04 per inch and 3.5 perms per inch. I don't yet know the cost though, which might change my mind. Big box stores more typically sell another of their products (Durofoam - a coated eps with low perm), though apparently not in the US in this area. The Plastispan can be special ordered to Canadian stores.