Advice on yet another vented cathedral ceiling using the “peanut brittle” method
I am building my home and have a 24′ x 32′ section with 13′ walls then a cathedral ceiling at a 7:12 pitch with 2×10’s 16″ o.c. The roof is steel with an unvented ridge cap, DuPont synthetic roof underlayment and 19/32″ OSB on top of the 2×10 rafters.
I will be dropping the ceiling 20″ from the bottom of the ridge beam to create a mini attic for several reasons: ease electrical and ceiling fan installs, provide venting into “mini-attic” from each rafter bay into the space and out a gable vent on each end, and ultimately lower my heated space slightly closer to the living area.
I have read for a year on all the different ways to insulate cathedrals and believe I have come up with my plan – I am in central Montana and consider the zone 5.5 with my elevation of 6000′. Would appreciate comments on this plan and a few suggestions on the short flat roof insulation (about 10′). If I had deep pockets I would use CC foam and be done but. . . . . if I staple in 1″ wood spacers on the inside of each rafter bay and install two layers of 2″ Rmax Thermasheath-3 (4″=R-26.2 when combined), can foam in the rigid foam, and then spray 4″ of CC foam to completely seal off?
The rigid will provide some cost savings and the closed-cell foam should ensure a water/air barrier and bury the condensation point inside the foam. The transition near the peak is troubling me more, I intend to use 2x6x10’s parallel with the floor and nail to each rafter so they too will be 16″ o.c.
Advice on both the sloped plan and flat plan would be much appreciated.
For the walls (2×6) I intend to fill with 3″ of Rmax Thermasheath and then 2″ of closed cell and would like to hear suggestions on this? I am building it day by day and have more time than money. I have struggled with the walls and it seems like overkill but I want to mostly fill the wall cavity without going with the flash and fill method, our local SF guy who was quoted in GBA said they spray 3″ of closed cell to capture the condensation point and call it good, I would be concerned with the remaining open void in my walls, should I be?
My double top plates and floor plates were all put together with two strips of liquid nails floor adhesive between them – I am the builder and can attest to it- have the liquid nail covered Carharrts to prove it! Building code/inspectors non-existent, no mortgage building it with free and clear as I go. Thanks in advance. CJ
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You don't need anywhere near 4" of cc foam for an air seal, a flash-inch will do. It's environmental footprint is also comparatively much worse than polyiso (like Rmax Thermasheath) or EPS (like R-Tech.) The vapor retardency of closed cell polyurethane is also moot when you have air-impermeable foil-faced goods in the stackup. If you used Lapolla's HFO1234- blown 2lb foam the enviromental hit is about the same as 2lb polyiso, but almost all other closed cell foam in the US is blown with HFC245fa, which is a very powerful greenhouse gas. At 4" it would have to be installed in 2 lifts for both fire safety and quality reasons.
Polyiso like Rmax on the COLD side of the assembly will under-perform it's labeled-R, but can even exceed it's labeled R when the average temperature through the foam is in the 50sF. It sounds like you have 8" of space to work with for the insulation layers, and it's vented. Make the outer 3" of the foam stackup EPS, since it GAINS performance as it's temperature falls, then 4" of R-Max (which will now be warm enough to stay in it's performance zone) then an inch of open cell foam (easier to trim flush to the rafters) or an inch of closed cell foam as the interior side sealing layer. From a labeled-R point of view that 3" x R4= R12, plus 4" x R6= R24, plus R3.5-R6.5 for R40+ center-cavity, which isn't terrible for a zone 5 cathedral ceiling. (That meets code under IRC 2009.)
At 3" some closed cell foam is R20 and technically code-min, but at 3" it means the 20-25% framing fraction has ~R3-R3.5 thermal bridging from the warm side to the cold side, instead of R5.5-R6+ for a full-depth fill. The 3" closed cell approach will underperform a code min R20 fiberglass/cellulose wall. In a zone 5 location as long as the siding is back-ventilated (like vinyl siding) you don't need anything more than standard latex paint on the interior for a vapor retarder, and the assembly can dry in both direction. If you installed 3" of closed cell foam the foam's vapor retardency is about 0.2-0.4 perms, so all drying would have to be to the exterior. This is a less resilient assembly.
Skip the cc foam in the walls- at 3" you're talking an under-performing less resilient assembly at an insulation cost of $3 or more per square foot. If you want to use foam, 5.5" of open cell foam is at least as air tight will have lower thermal bridging for higher performance, and will cost no more than $2.50 per square foot. If you're still concerned about interior side moisture drives, install a layer of Certainteed MemBrain (less than 15 cents per square foot) between the foam and be done with it. The MemBrain becomes vapor open if the humidity in the open cell foam ever rises, which it might if you don't have sufficient drying capacity toward the exterior. In the dry midwinter air at 6000' in Montana the MemBrain would run about 0.5 perms, unless you're intentionally keeping the humidity above 40% RH in winter with humidifiers.
Thanks Dana for the answer, the siding will be milled ship lap from the beetle killed lodgepole on the property and spaced off the tyvek a 1/4". My biggest concerns were reading about the initial can foam seal breaking open between the board foam and the rafters. The CC at 2" to 3" would negate it if it were to happen with time. The walls with OC and a vapor barrier will be much less labor intensive as well - vapor push to exterior is my concern there as well as a good air seal - I have timber frame and exposed beams throughout the wall and ceilings. Any thoughts on the mini attic ceiling? Same as the sloped? Not enough space to blow in cellulose and keep a good air passage for rafter venting. Use the "Pan cake" method of stack rigid foam and seal with an 1' - 11/2" of CC to seal in? The stack would surpass the height of the 2X6, wondering if I can slip in between the 2X6 and lay a layer of rigid on top of the 2X6 breaking the bridge and reach in and spray CC at the ceiling/wall transition then cont. stack between 2X6 and finish it with a flash of CC?
Adam,
Q. "Any thoughts on the mini attic ceiling? Same as the sloped? Not enough space to blow in cellulose and keep a good air passage for rafter venting."
A. You need a vented ridge, not gable vents. If you do that, you'll be able to fill up the attic with cellulose. Of course, you'll need sturdy ventilation baffles to keep the ventilation channels open.