Foam sandwich using 2-in open cell spray foam + rigid foam roof sheathing?
hertzr
| Posted in Green Building Techniques on
Hello,
I am in the planning phases of a complete re-do of my zone 4-A Cape’s second floor. My goal is an ACH of 1 with R30+ walls and a R60 roof. My approach is a hybrid wall/roof assembly using both exterior rigid foam + fluffy/air permeable insulation between studs (figure 4 and 7 of the BSC link below). I plan on using a fluid applied WRB on the exterior sheathing.
Because of my ACH goal, should I be looking at open cell spray foam in addition to the fluffy insulation between studs? Am thinking 2-in open cell because it’s more vapor open compared to closed cell (30 perms at 2-inch thick?). I know it’s borderline risky in my mixed climate zone. Plus I’d purposely be creating a foam sandwich on the roof sheathing.
If open cell is too risky, should I consider a smart vapor retarder, such as Siga’s Majrex, to encapsulate the underside of the air permeable insulation that’s between the roof rafters? I will likely have to run HVAC lines in the small attic space between the underside of the roof framing and my airtight drywall ceiling.
My plan is to contract out ~50-75% of the work using a detailed build-to spec; I want to add these details to the spec. Thanks in advance for your help!
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Replies
What is the purpose of open cell in the walls? If it's plank sheathing there may be an argument for open cell for the enhanced air tightness, but if plywood/OSB you'll get higher performance at a lower cost out of rock wool insulation.
At the roof the amount of exterior foam matters. If the exterior foam is more than 30% of the total R it's fine to use all-fiber or open cell foam in the rafters bays. If it's less than 30% of the total you'd need a minimum of 1" of closed cell foam on the underside of the roof deck (2" is better- at 2" most closed cell foam is barely into the Class-II vapor retardency range, and not a true vapor barrier), but the combined exterior + closed cell needs to be more than 30% of the total R to avoid moisture accumulation in the fiber insulation.
For a zone 4A climate 4" of 2lb polyiso roofing foam (R23) above the roof deck + R30 rock wool (fits in 2x8 rafters) would get you to R53 at center cavity, but would outperform R60 loose fill at the attic floor. If the rafters are 2 x10s the rafters could be filled with 1.8lb fiberglass (~R39) and it would hit ~R62, with a comfortable 37% R-ratio.
Even at the 37% ratio there needs to be at least a Class-III vapor retarder (such as standard latex primer on gypsum board) on the interior side, but a smart vapor retarder is even better. It's possible to use the Intello Plus vapor retarder in lieu of blowing mesh for installing 1.8lb fiberglass, (an idea promoted by 475 High Performance Building Supply, distributor of Intello products) which isn't a bad way to go.
Awesome, thank you for the feedback Dana! Like most Cape’s, my roof line is a bit complicated - 4/12 on the front with 1950 slanted Pine plank boards, and 1.5/12 on the back with OSB sheathing. All roofing framing members are 2x8s.
Am planning on 6” taped polyiso on the roof, which will be installed over a fluid applied WRB on the existing sheathing. I may put a layer of crinkly Tyvek over the top layer of polyiso for peace of mind. This is followed by 1-by strapping, another layer of OSB, felt, and shingles or aluminum/metal shake panels.
Under the sheathing, I plan on using rockwool - either R30 bats for the 2x8 rafters or 2-in open cell to help with air sealing the planks + R24 batts. If using only the R30 batts I could staple a smart vapor barrier to the framing to create a decent secondary air barrier. My primary air barrier is the gypsum panel ceiling.
The gypsum ceiling directly touches the underside of the front slanted framing. The slanted portion transitions to a flat ceiling ~2.5-ft from the ridge. Above the flat ceiling is a small angled attic space that’s 18-in at the ridge and shrinks to 0-in high at the soffit. I unfortunately will likely have to use this space to run HVAC/ERV lines.
I will use a similar approach for the walls (I have a mixture of plank wall sheathing and OSB), but use 4” polyiso and R15 rockwool. My siding will likely be bith brick veneer and fiber cement or metal shake.
Thank you again for your help and advice!!
>"Am planning on 6” taped polyiso on the roof, which will be installed over a fluid applied WRB on the existing sheathing. I may put a layer of crinkly Tyvek over the top layer of polyiso for peace of mind. This is followed by 1-by strapping, another layer of OSB, felt, and shingles or aluminum/metal shake panels."
There is no advantage to the crinkly Tyvek when it's next to an air gap between the OSB nailer and polyiso. Cheaper flat-stock is fine.
>"Under the sheathing, I plan on using rockwool - either R30 bats for the 2x8 rafters or 2-in open cell to help with air sealing the planks + R24 batts."
When using a fluid applied or fully adhered membrane ( Ice & Water Shield, Blueskin, etc) as the WRB over the plank sheathing it will be air tight, and there would be no advantage to the open cell foam- the WRB has already made it air tight. Open cell foam isn't very smooth, and would leave air pockets/voids between the foam & batts. A 2" shot of open cell foam would be an unnecessary expense that would be outperformed by a 2" thicker rock wool batt.
Great, thank you again again for the help Dana! The more I think about it the more I like the idea of R30 rockwool with a smart vapor retarder on the underside. This is something I could do, which would help save money.
One additional question - what are your thoughts on by-brand low temp performance of polyiso? I’ve found some data on Thermax but not on other manufacturers. For an exterior roof/wall application, is Thermax worth the ~2x cost delta compared to another brand (say Rmax)? Am also thinking about a foam sandwich to help increase assembly R values at low temps, where a 2-in or 3-in layer of EPS/XPS would reside over 4-in or 3-in layer of polyiso. I’m not in wild fire country, but fire resiliency is at least a consideration. Thermax’s performance becomes a mute point if I do the foam sandwich approach.
Thanks!