I will start with the problem statement: For our unvented roof (vaulted ceilings with skylights) we have no ridge vents and the rafter bays are closed off, all on purpose – but now we are unsure if it is safe to seal off the interior roof sheathing with closed cell spray foam (CCSF) because the roofing underlayment that we ignorantly allowed has such a low permeance value. I feel that we are now in an unsolvable situation? Is Martin Holladay or Joe Lstiburek around to help with this??
My husband and I are rebuilding a 3000 sq ft home that has vaulted ceilings (2 x 10 rafters) with 7 skylights and we’d like to use LED canned lighting as well. We are in the Pacific NW in the zone 4-5 region would like to make a well-sealed, energy efficient home. After researching the topic of vented vs. unvented roofs, it appeared that many FineHomebuilding articles and GreenBuildingAdvisor articles stated that sealing and insulating with CCSF directly against the interior side of the roof sheathing would be the best thing to do for vaulted ceilings with skylights.
We chose to do the unvented roof with spray foam (at least 3in CCSF + 6in FG batts). We haven’t yet insulated but we purchased a fairly expensive roof (stone-coated steel on 1in battens) because of the possibility of fire in our area. The roofing company knew that we were doing an unvented assembly (obviously, no ridge vents), but put Owens Corning deck defense down on the top of the roof sheathing before nailing down the battens. It’s a semi-impermeable vapor retarder with perms=0.23. We didn’t know that this might matter because the exterior roof underlayment had not been mentioned in any of the articles we had read. All the articles were about sealing off all the small air leaks (for which CCSF is great) and keeping the warm, conditioned air away from the cold roof sheathing to prohibit condensation/moisture from forming. We put the roof on last year.
Now we are getting ready to insulate walls and ceilings with spray foam. We were told by a premium homebuilder (who came to bid on the staircase) that the CCSF directly applied to the interior side of the roof sheathing would cause us moisture problems. After reading some more recently found articles about underlayments and some Building Science articles by Joe Lstiburek, I see that the deck defense underlayment may be a problem, since we now have no outward drying path to the exterior, and the CCSF on the interior underside of the roof sheathing will certainly be a vapor barrier and not allow drying to the interior. The steel roofing material is 1″ up off of the roof sheathing so there is air flow between the exterior roof underlayment and the steel roof shingles, but if moisture gets under the deck defense underlayment (is this a concern? moisture is sneaky stuff….) it won’t have a way to dry out.
We do not know what to do now to keep our unvented assembly – We must insulate in November but we don’t want our roof to rot over time. Any help out there???
Thanks for any help/ideas,
Linda E.
Replies
I think you have been given incorrect info.
Numerous sources, including FHB, warn against using OPEN cell foam on an unvented roof as the moisture will collect in the foam and possbily rot the sheathing. This can't happen with closed cell, which is impervious to moisture.
I would use unfaced batts to ensure that nothing is trapped in the fiberglass.
I did a lot of research on this subject over the last year before spray foaming my unvented roof this last summer. I doubt your metal roof is airtight, so moisture ought to be able to escape. I think.
Offered by an alledgedly humble DIYer who reads a whole lot.
Good luck.
Great question and it certainly highlights how small changes impact other trades. I too have done a lot of reading on this and have not come upond the issue before. I'd be interested to know what the experts say on this one...
Go with it...
Linda, first, this is not my area of expertise.
But a few years ago I did the same dance with the tech departments of a couple of "roofing systems" manuacturers as well as my town's building department. That was regarding the perm rating of various shingle underlayments when installed over SIPs.
The general consensus was: Pretty much any non-vented roof that has battens that raise the roofing material off of the deck of the roof will be fine with at least a semi-permeable membrane. Your roof, metal on battens, fit's into that group.
The greater concern was for a material like asphalt sihingles installed right on top of an underlayment like Deck Defense. It wasn't an indictment of the Deck Defense. It was that the heavy duty laminated shingles themselves can have a low perm rating. Even when a highly permeable underlayment is used under premium laminated shingles, the shingles themselves can restrict vapor transmission and can prevent drying.
For the most part, they all agreed that even with the added concern of the preceding paragraph, all would still be well.
We even gave opposing tech departments the opportunity to take a shot at their competitor's underlayment, and they didn't. They were all pretty much in agreement with one another.
You raised an excellent question, and based on what I was told a few years ago, Deck Defense is fine on a hot roof, especially on a metal roof on battens.
closed cell sprayfoam with semi-impermeable roof underlayment
Thanks to Mongo and others who have replied,
I have re-read older articles like "BSD-149: Unvented Roof Assemblies for All Climates" from the Building Science Corporation, and newer articles like "How to build an Insulated Cathedral Ceiling" revised in 2016 in GreenBuildingAdvisor, and have found that the general roofing assembly we intended to use is exactly what is described in these "unvented" articles. So it's reassuring that Mongo has at least found that some contractors and his building dept. think that the air space provided by the battens under the metal shingles and the semi-impermeability of the deck defense can still allow roof sheathing to dry out if necessary.
Its little paragraphs like the following that still jump out at me and cause me discomfort: in a FHB article "Synthetic Roofing Underlayment" (also by Martin Holladay of GBA), in the sidebar of the "permeance" table of different underlayments, (attached) the comment is made that synthetic underlayments don't allow upward drying and should only be used over vented ceilings that allow downward drying. Ouch! Also in another Building Science Digest, "BSD-106 Understanding Vapor Barriers," the recommendation section has the following:
• Avoidance of using vapor barriers where vapor retarders will provide satisfactory performance. Avoidance of using vapor retarders where vapor permeable materials will provide satisfactory performance. Thereby encouraging drying mechanisms over wetting prevention mechanisms.
• Avoidance of the installation of vapor barriers on both sides of assemblies – i.e. “double vapor barriers” in order to facilitate assembly drying in at least one direction.
Especially the second point - I know the deck defense is semi-impermeable, a vapor retarder, not a barrier, but at 0.23perms it still isn't going to allow drying at a very useful rate....
So..... thanks for the encouragement - I'm still feeling a little dicey about the situation, but maybe a little fortified. I think I will formally correspond this question to Joseph Lstiburek (by USPS mail!) at Building Science Corp. and see if he will respond. If he does, I will most certainly post his wisdom here, but I'm sure it will be at least a couple of weeks out.
Many thanks,
Linda
Response from Joseph Lstiburek of Building Science Corporation
So... I wrote a letter to Joseph Lstiburek and asked him my question. His response is pretty much in line with Mongo's above. Here it is:
from: Joseph Lstiburek
Your roof will work. The roof is designed to dry inward as such the permeance of the roof underlay is not an issue. Note that most unvented roofs with spray foam have asphalt shingles which are vapor barriers. The key is to use closed cell 2 lb/ft3 high density spray foam. Additionally the IRC and the IBC both explicitly allow this assembly in your climate zone.
Dec 13 (1 day ago)
to me
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Merry Christmas to me! Problem solved, case closed. I am ready to spray foam! Thanks to those who looked at my issue.
Merry Christmas everyone ! :)