I am thinking of an unvented cathedral ceiling
user-6153094
| Posted in Energy Efficiency and Durability on
Hi. I am thinking of an unvented cathedral ceiling. Living in Hazelton, BC (Zone 4A, I believe), and would like advice on my proposed insulation plan. Staring from the underside: vapour-retarding paint; sealed gypsum board; 3/4″ strapping; 2″ or 3″ Roxul board.
I would like to achieve at least an R50.
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Replies
Mike,
I think that some of your question failed to post.
Two or three inches of Roxul has and R-value of about R-8 to R-12, so you'll need something else as well.
At 4998 heating degree days (base 18C) and a 12 month heating season, Hazleton BC is on the zone 6 / zone 7 boundary (5000 HDD-C) not zone 4:
https://www.greenbuildingadvisor.com/sites/default/files/Climate%20Zone%20map%20including%20Canada.jpg
http://legacyweb.theweathernetwork.com/statistics/degreedays/cl107g879/cabc1667
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/59387/Do-You-Know-Your-Building-Science-Climate-Zone
Mike,
Here is a link to an article that describes all the options for insulating unvented cathedral ceilings: How to Build an Insulated Cathedral Ceiling.
Briefly, this type of roof assembly requires either:
(a) a sufficiently thick layer of rigid foam (in your climate zone, one with a minimum R-value of R-25 or R-30) above the roof sheathing;
(b) a sufficiently thick layer of spray foam insulation (again, in your climate zone, one with a minimum R-value of R-25 or R-30) below the roof sheathing.
If you choose one of these approaches, you still need to include additional insulation to reach a total R-value of R-49.
Not sure if this is what I do to respond, but thanks for your prompt answers! Yeah - a lot missed out, don't know why, had em there, but here goes the parts missing: (paint; gypsum board, 1x4 strapping; 2-3"Roxul board); Roxul board attached to 2x10 rafters (thermal bridging reducer, breathable to inside); 5.5" Roxul mineral fibre batts; 4+ inches closed cell spray foam to underside of 3/8"plywood sheathing on top of rafters, 2x3 strapping on top of 3/8"sheathing, on topm of rafters facia to ridge(to help cool next layers and help stop ice damming); 1/2" plywood sheathing; asphalt shingles.
(Got that zoning answer - we're in a peculiar spot here - Skeena River Valley - but I'll check that out. Thanks.)
mike
Mike,
I usually calculate closed-cell spray foam at R-6 per inch (even though spray-foam installers often tout higher R-values). Four inches of closed-cell spray foam gives you about R-24 -- or R-26 on a good day.
7.5 inches of mineral wool is about R-30, so your total R-value is R-54 to R-56. That means that the foam layer is contributing about 44% or 46% of the R-value of the total roof assembly.
8.5 inches of mineral wool is about R-34, putting your total R-value at R-58 to R-60. In that case, the foam layer is contributing about 41% or 43% of the R-value of the total roof assembly.
In Climate Zone 6, you want 51% of the total R-value of your roof assembly to come from the foam layer.
In Climate Zone 7, you want 61% of the total R-value of your roof assembly to come from the foam layer.
(The percentage targets that I just listed are explained in this article: Combining Exterior Rigid Foam With Fluffy Insulation.)
The bottom line: your foam layer is too thin to keep your roof assembly safe.
Mike - I believe that Martin has misread your stack-up, He thinks that the foam is outside the sheathing, keeping it warm. But I think that you're using the spray foam under the sheathing, where it acts to air seal and insulate. The foam there is not very useful. You already have a vented space formed by your 2 plywood layers.You can use that first layer,3/8, as an air control layer, and use just mineral wool in the cavity, 3.5" plus 5.5".With the 2-3" of roxul board attached under the rafters you approach the R-50 you're looking for. The foam sprayed to the sheathing won't bring very much increase in R value over just mineral wool, given all the rafters which water down the R value. Also, you may want to consider using 2X4 on edge as the "strapping", run horizontally and filled with 3.5" mineral wool. The mineral wool batts are about 1/3 the price of the boards for the same r-value.Not as perfect as the thermal break from boards, but pretty good for a lot less $.
Kevin and Mike,
I didn't misread the stack-up described by Mike. Once you introduce closed-cell spray foam in the mix, the foam has to meet the minimum R-value requirements I discussed. If you ignore these minimum R-value requirements for the spray foam layer, you can get condensation on the interior surface of the (relatively cool) cured spray foam during the winter.
The ratio rules for foam-to-fluffy apply to closed cell + fluffy, in the same way as they apply to rigid foam + fluffy. Although the article I linked to (Combining Exterior Rigid Foam With Fluffy Insulation) talked about rigid foam, the same ratios apply when the foam layer consists of closed-cell spray foam under the roof sheathing.
For zone 6 the ratio of R outside the first condensing surface to the total R needs to be 50% or greater. In zone 7 it has to be 60% or greater. The IRC prescriptives for zones 6 & 7 presume R49 for the total, which approximatly your R50 goal:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
Hazelton is on the line, so it'll be safe to split the difference even if it's just on the zone 7 side of the line. (Look up your base 18C annual HDD averages from a local weather station, if you think the site I linked to had it wrong).
If you're going with only 50% or a bit less it's prudent to put a smart vapor retarder on the conditioned space side of the fluff, to limit interior moisture drives without unduly impeding drying. In your stackup you could probably do just fine with air tight 2mil nylon on the underside of the rafters & 5.5" Roxul before installing the 1-2" of rigid Roxul board. It's not as moisture safe as installing another inch of closed cell foam, but it's not bad (and a heluva lot cheaper.
Rather than 1-2" of rigid Roxul, using 2" thick rafter edge strips of rigid polyiso and using batts designed for 2x8 framing with the 2-mil nylon tight to the insulation would be higher performance and cheaper than rigid rock wool.
I am very grateful for all you knowledgeable types out there. Martin and Co., what do you think of the following, then:
1/2" gyproc; 1x 3 strapping; smart vapour retarder; 2 inches rigid polysio rafter edge strips (as per DD) on 2 x 10 rafters; R22 and 2x R14 Roxul comfort batt (12.5 inches in all) stuffed into the 11.25 inch cavity; 3/8 inch plywood sheathing; full 2" air venting between 2 x 2 strapping on top of rafters; 1/2" plywood sheathing, membrane; asphalt shingles. Should I mention 8:12 slope?
What happened to the closed cell foam? At anything less than 50% of the total R as closed cell foam under the roof deck the interior side vapor is necessary, but not sufficient (especially in a location with 12 months of heating season.)
Even with the smart vapor retarder you'll need something like 2" of closed cell foam on the under side of the roof deck to fully protect the roof deck. The 3/8 " plywood runs about 1 perm when dry, and even if fully vented on the top side you'll want the interior side to be WELL under 1 perm year round.
The comparable stackup that will likely work is (separated by line breaks for clarity, and top to bottom I don't have to stand on my head :-) )
-asphalt shingles
-membrane
-1/2" plywood sheathing
-2 x 2 strapping / vent space
-3/8" plywood sheathing (<< is 3/8" sufficient from a structural perspective? This is the structural sheathing layer- it probably needs to be 1/2" or 5/8".)
-2" closed cell polyurethane (~R12 and 0.6 perms)
- ~R38-R40 rock wool batt (9.25")
-2 mil nylon vapour retarder
-1x3 strapping
-1/2" gyproc
The variable permeance 2 mil nylon will limit the amount of moisture accumulation in the rock wool, the 2" polyurethane puts a ~half-perm vapor retarder between the ~1-perm sheathing and outdoor air in the vent space, guaranteeing that the sheathing stays dry. The closed cell foam is only ~25% of the total R rather than 50-60% , but that should be enough to keep from soaking the rock wool or forming frost as long as the 2-mil nylon is air tight.
Thank you, Dana.
Thank you, Dana.
Hi Dana - I'm forgoing the pray foam approach (and R50!) - very costly - and going to the vented cathedral ceiling described by Martin. According to Martin, "thin plywood" can be used as baffling to preserve the 2" airspace between insulation and roof sheathing. Does this require a spray foam undercoating? I couldn't see that requirement in Martin's article: How to Build an Insulated Cathedral Ceiling, and couldn't see the difference between that and my 3/8" sheathing on top of the rafters, 2" airspace, then 1/2" sheathing.
Thanks for the responses!
A roof deck that vented from below with a soffit-to-ridge venting scheme does not need any special treatment beyond the venting.
With a vented roof deck you'll still need an interior side vapor barrier (it's called out in the NBC- anything under 1 perm would meet code), unlike most of the US. In local climate you could probably get away without it but it's marginal. Six mil polyethylene detailed as an air barrier is good enough, and would make the code inspector comfortable. With an 8:12 pitch and 2" vented air space the rationale for using a smart vapor retarder is nil.
Thank you again.