R-24 spray foam enough?
Hi there,
I’m in Zone 6 up here in Nova Scotia Canada. Got a old 150 year post and beam house that has some serious rot in the roof sheathing. The roof was insulated with fiberglass and no vapour barrier or proper ventilation, so it was mouldering in there awhile. Going to be laminating the rafters, ripping off the sheathing and putting on new sheathing and a metal roof. Thinking of doing an unvented roof assembly as there are dormers that would be heard to vent properly and I’m not convinced the baffles would adequately vent the sheathing. There is no attic, the upstairs right below the roof is our second floor and will be a living space, and so we would have to do a drop ceiling and make a mini attic to properly ventilate. Also losing 1-2 inches for vent space below the sheathing would cut into our living space. So i’m leaning away from ventilation but I am open to be convinced otherwise. To do an unvented with rigid foam on the outside of sheathing and batts on inside I would need an R25 on the outside. This is 5 inches of XPS rigid foam and seems a little over the top. My roofer wasnt gung ho on this idea. But is it the best route? Other option is spray foam. The code requires R 31 in sloped roofs here in Nova Scotia, that would be more than 5inches of spray foam, which I cannot afford. However the spray foam contractor says that 4 inches is fine. Is he telling the truth? If the code require R 31 why would R 24 of spray foam be ok? Also, according to spray foam contractors around here, 2 inches of spray foam is enough to seal the roof in as a proper vapour and air barrier. Is this true? If so I’m leaning towards 2 inches of spray foam to seal roof and then adding roxul batts below to reach my R31. Any insight into the potential consequences of just an R 24 in spray foam or choosing vented vs non vented, rigid foam exterior vs spray foam interior would be much appreciated! Thanks in advance.
Marshall
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Replies
Marshall,
Q. "To do an unvented with rigid foam on the outside of sheathing and batts on inside I would need R-25 on the outside. This is 5 inches of XPS rigid foam and seems a little over the top. My roofer wasn't gung-ho on this idea. But is it the best route?"
A. Absolutely.
Q. "Another option is spray foam. The code requires R-31 in sloped roofs here in Nova Scotia. That would be more than 5 inches of spray foam, which I cannot afford. However the spray foam contractor says that 4 inches is fine. Is he telling the truth?"
A. No. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.
Q. "If the code requires R-31, why would R-24 of spray foam be OK?"
A. It's not.
Q. "According to spray foam contractors around here, 2 inches of spray foam is enough to seal the roof in as a proper vapour and air barrier. Is this true?"
A. Not really. In Climate Zone 6, you have to make sure that at least 51% of the R-value of the roof assembly comes from either rigid foam on the exterior side of the roof sheathing, or closed-cell spray foam in the interior side of the roof sheathing. Here is a link to an article that explains more: Combining Exterior Rigid Foam With Fluffy Insulation. (Even though that article focuses on exterior rigid foam, the ratio rules also apply to interior closed-cell spray foam.)
Q. "I'm leaning towards 2 inches of spray foam to seal roof and then adding Roxul batts below to reach my R-31."
A. That won't work. If you want only R-31 insulation -- that's not much -- you still need a minimum of R-16 insulation in the form of closed-cell spray foam below the roof sheathing (that would be at least 3 inches of closed-cell spray foam).
Thank you for the clarity in your answers Martin. If 5 inches of xps rigid foam on the outside is best how do I ensure that the heat from the metal roof does not melt or effect the rigid foam ? Folks here at the local building centre say that the metal would melt the foam. Is this true? Can I strap the outside of the rigid foam to allow an air gap between the foam and the roof? Also if I use Nova seal above the roof deck as an underlayment , will this effect the drying ability of the roof deck? Thank you in advance.
Marshall,
Q. "Folks here at the local building centre say that the metal would melt the foam. Is this true?"
A. No.
Q. "Can I strap the outside of the rigid foam to allow an air gap between the foam and the roof?"
A. Yes. This approach is mentioned in my article, How to Build an Insulated Cathedral Ceiling, in which I wrote: "You can create 1 1/2-inch-high ventilation channels above the roof sheathing with 2x4s installed on the flat, with the 2x4s located above the rafters, 16 inches or 24 inches on center. Although this approach is less fussy than installing vent baffles underneath the sheathing, it usually costs more, because most types of roofing require a second layer of plywood or OSB on top of the vent channels. In some cases, these ventilation channels are installed above a layer or two of rigid foam. It's also possible to purchase nailbase (a type of SIP with OSB on one side instead of two) that includes integrated ventilation channels between the OSB and the rigid foam; one brand of these panels is Cool-Vent from Hunter Panels."
Q. "If I use Nova seal above the roof deck as an underlayment, will this effect the drying ability of the roof deck?"
A. NovaSeal is vapor-impermeable. (For more information, see Synthetic Roofing Underlayments.) If you want a roofing underlayment that is vapor-permeable, use #15 asphalt felt or one of the more expensive vapor-permeable synthetic underlayments.
That said, roof sheathing can't dry to the exterior if it is covered with rigid foam or metal roofing. It can only dry to the exterior if there is a ventilation channel above or below the roof sheathing, with no intervening layers that are vapor-impermeable.
Thank you Martin!
Perhaps I should clarify my options here, as I now see them:
Option 1
Rip off rotten deck and install 5/8 inch plywood sheathing
3 inches of closed cell spray foam below roof deck: R18
Fur out rafters in order to accomodate 3 1/2 inch roxul batt below this: R 14
No vapour barrier below this
Metal roofing attached directly to sheathing
Total R 32 - meets Nova scotia code, dries to the interior, solid seal, more than 51% of insulation against roof deck as spray foam
With this assembly can I put nova seal above the roof deck? Or would it have to be vapour permeable? Nova scotia's code is R 32 but we are Zone 6 and according to the U.S. code we really need a total of R 49 with R 25 in spray foam. Should I follow U.S. code or Nova scotia code? 3 inches of spray foam is enough to follow the 51% rule but not the R 25 rule.
Option 2
Rip off rotten roof deck and install 5/8 inch plywood
Install asphalt felt as air barrier
Install 4 inches of XPS: R20
Cover with 5/8 inch plywood
Install Nova seal
Install metal roof directly on top of that
Install 31/2 inch roxul below sheathing for R=13
No vapour barrier below this
Total R=34 meets Nova scotia code but does not meet U.S code, dries to the interior, more than 51% of R value is exterior rigid foam, does not meet R25 on exterior rule
Do I need ventilation above the foam board by installing 2x4s on the flat as you suggested? Ice damming is likely in our climate, but with the nova seal it should not leak,no?
Option 3
Rip off rotten roof deck and install 5/8 inch plywood
Install asphalt felt as air barrier
Install 5 inches of XPS: R25
Cover with 5/8 inch plywood
Install Nova seal
Install metal roof directly on top of that
Install 51/2 inch roxul below sheathing for R=22
No vapour barrier below this
Total R=47 meets Nova scotia code and almost meets U.S.code, dries to the interior, more than 51% of R value is exterior rigid foam, also abides by the R=25 rule
Option 4
Rip off rotten deck and install 5/8 inch plywood sheathing
4 1/2 inches of closed cell spray foam below roof deck: R27
Fur out rafters in order to accomodate 3 1/2 inch roxul batt below this: R 14
No vapour barrier below this
Metal roofing attached directly to sheathing
Total R 42 - meets Nova scotia code and is closer to U.S. code, dries to the interior, solid seal, more than 51% of insulation against roof deck as spray foam, meets the R=25 rule
Still unsure if I can use Nova seal on above assembly.
Which do you think is best? Am I getting closer? Thank you for sharing your time and knowledge.
Marshall,
Lots of questions -- I'm not sure I have the time now to wade through everything. A few quick points:
1. Asphalt felt is not an air barrier. To create an air barrier at the sheathing level, either (a) tape the sheathing seams with a high quality European tape, or (b) use taped Zip sheathing, or (c) install a peel-and-stick product like Grace Ice & Water Shield.
2. According to the NovaSeal installation instructions, the manufacturer only permits the installation of their product over ventilated roofs. You can't use it for an unvented assembly.
Thank you for those points Martin. I am very grateful for your help through this. If you do get a chance at a later date to wade through my ramblings I would very much appreciate it. The major two questions are: should I follow N.S. code R 32 or U.s code R 49 for my climate Zone 6? Is R 25 for exterior rigid foam or interior closed cell spray foam a hard and fast rule or can I get away with just ensuring that the spray foam or rigid foam represents over 51% of the R value? Thank you greatly.
Regarding R-25 vs 51%, the 51% is the more important number. The R-25 number is just 51% of the R-49 insulation required by IECC 2015.
Ahh I see. Thank you for that clarification Brendan. So 51% is the most important thing, however the Nova scotia building code of a total R32 is not best practice.It is simply allowable in my province. The R 25 then is assuming that you follow best practice based on the IECC guidelines. Interesting.
Does this mean as long as the 51% rule is adhered to you should not have rot in the sheathing? Or is there a minimum total R value that makes this hold true?
Marshall,
Q. "Does this mean as long as the 51% rule is adhered to you should not have rot in the sheathing?"
A. Yes. But of course, if a homeowner installs additional fluffy insulation on the interior in 10 years, the assembly could be thrown into failure mode.
Q. "I there a minimum total R value that makes this hold true?"
A. No. A higher R-value lowers your energy bills. A higher R-value (in and of itself) won't make your sheathing safer from rot.
What makes your sheathing safe from rot is keeping the sheathing warm during the winter -- and that depends on following the ratio rules.
Yes, that is correct. Following the 51% rule will prevent interior moisture from condensing in the sheathing and causing problems.
If you are using exterior rigid foam, the 51% keeps the sheathing warm enough the condensation doesn't pose a problem.
If you use 51% interior closed-cell spray foam, then the interior foam forms a barrier that prevents interior moisture from reaching the sheathing. The 51% is important with the spray foam because it keeps the interior surface of the foam warm enough that you don't have condensation issues on that surface.
You can of course still have rot in the sheathing due to roof leaks and such! There are always ways for water to cause problems ;)
Marshall,
Q. "Should I follow N.S. code (R-32) or U.S code (R-49) for my climate Zone 6?"
A. That's up to you. The rate of heat flow through an R-49 roof will be only 65% of the rate of heat flow through an R-32 roof, so the R-49 roof will lower your energy bill.
This is generally worth reviewing:
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/17964/vented-vs-unvented-roofs
Also note that a second layer of plywood above foam will be cold.
Also note, 5" of XPS may have a labled value adding up to R25, but well within the lifecycle of a house it's performance will drop to ~R21 as it loses it's HFC blowing agents. Using 6" of EPS would be R25 now, and still R25 many decades later.
If there is any fiber insulation under the structural roof deck, the R-fraction has to be met where the fiber and low-permeance layers meet, which keeps the fiber insulation from accumulating condensation (or frost) over the winter. It only takes 2" of closed cell foam under the roof deck to provide a Class-II vapor retarder (it would meet Canadian code definition of a "vapour barrier" at that thickness), which is sufficient to protect the roof deck, but only R12-ish type performance. But with R12 of vapor retardent air impermeable foam under the roof deck it means you then only need another ~R13 above the roof deck to mitigate the wet fiber insulation problem. That's do-able with 3" of EPS with a vented nailer-deck above, or a 3.5" EPS nailbase panel (or a 3" polyisocyanurate nailbase panel). With that stackup sandwiching the roof deck 5.5" of rock wool (R23) snugged up to the 2" polyurethane foam layer brings it up to better than IRC code min performance. Even though it may not be quite add up to R49 at center-cavity, the R12+ thermal break of continuous foam over the rafters improves performance enough to more than make up the difference.
Sandwiching the roof deck is fine- the deck never is never expected to dry toward the exterior, since it really can't (much) with a #30 felt + asphalt shingle layup (that works out to about 0.1 perms, almost as tight as 4 mil polyethylene vapor barrier), and is often wet (or snow-bound) on the exterior surface. The key factors are to get the interior side permeance under 1 perm (2" of 2lb polyurethane is typically 0.4 to 0.7 perms). Even though the roof deck is colder in that stackup than if the R25 was on the exterior, the lower vapor permeance limits the seasonal uptake, yet it's also sufficiently vapor open to dry seasonally toward the interior in your climate zone.
The following set of WUFI simulations do not include exterior foam, but does demonstrate that the roof deck fairly moisture-safe with as little as 2" of ccSPF on the underside of the roof deck, with the rest of the R being fiber insulation on the interior. In these simulations the roof deck is even colder than it would be with R12+ above the roof deck, with correspondingly higher moisture content than with the sandwiched roof deck scenario. See the summary in Table 3, noting the "2" ccSPF + spray fiberglass" column, and the rows for Int. Falls & Minneapolis:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
If it works at R49 total for a north facing 3:12 roof with dark shingles in International Falls MN, it'll work even better in NS with R12 above the roof deck, and less than R40 below the roof deck.