Image Credit: Walt Ott
More Q&A Spotlight
Winter is the season of icicles and ice dams, at least on houses whose roofs are poorly insulated and air-sealed.
Or, in the case of a nature center in Wisconsin, not insulated at all. Walt Ott is inquiring about the best way to fix a building with a completely uninsulated cathedral ceiling.
Writing in a Q&A post at Green Building Advisor, Ott outlines his strategy: on the underside of the existing 2×6 tongue-and-groove ceiling, add two layers of 2-inch-thick polyisocyanurate insulation, two layers of 3/4-inch-thick polyiso insulation, and then a layer of 1×6 tongue-and-groove boards as the finish ceiling.
There are details yet to be worked out, but is this plan fundamentally sound? That’s the question for this Q&A Spotlight.
And if this topic is ringing a bell as we approach the depths of winter, a second Q&A post deals with similar issues.
Nope, this just won’t fly
Ott’s approach has a major problem, writes Dana Dorsett. “You can’t use stacked-up polyiso against the roof deck,” he says, “since it would block the drying of the roof deck toward the interior.”
He can, however, use unfaced expanded polystyrene board insulation (EPS). Type II EPS, 5 1/2 inches thick, would act as a Class II vapor retarder with a permeance rating of about 0.54 perm, Dorsett writes. That’s about the same as the kraft paper used to face batt insulation.
And that, he adds, “would be sufficient for protecting the roof deck from excessive wintertime moisture accumulation, yet provide a path for seasonal drying during warm weather.”
Type II EPS would give Ott a roof R-value of at least 25, enough to make a “huge” difference.
When it comes time to re-roof, Ott could…
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9 Comments
Rigid foam should be
Rigid foam should be installed above and be vented above that. Any other use of rigid foam in my area has been replaced finding water damage always.
The most successful local contractor using rigid foam to the exterior around here always vents above his rigid installs. Always.
Let me repeat, always vent rigid foam installs in zone 6a.
aj
Same for any "hot" roof?
Doesn't Peter's WUFI study then suggest that for the same climate any "hot roof" done with a similar thickness of spray foam under the deck and unvented above the deck isn't a good idea, because the roof would be leak intolerant?
Yep- pretty much (response to Dick Russell)
The roof leaks would be a problem, and in this particular instance roof leaks are LIKELY, since there isn't sufficient R value in either the foil-faced iso or EPS case discussed to fully mitigate ice dam formation.
Though either would be a huge improvement in the severity & frequency of the ice damming, it would still happen multiple times during the lifecycle of the shingles. There's no question that it would be FAR better to put the foam on the exterior, independent of R value.
But note Lstiburek et al have stated:
"Code-compliant roofing system using ccSPF on plywood sheathing with cellulose insulation on
the interior has the capability according to WUFI to safely dry a leak up to 0.6% of the rainfall in
Minneapolis. "
See the discussion on p.9 (pdf pagination) of this document:
http://www.buildingscience.com/documents/bareports/ba-1312-application-of-spray-foam-insulation-under-plywood-and-osb-roof-sheathing
The vapor permeance of R25 ccSPF is a bit LOWER than the 5.5" of EPS under discussion here, which makes me wonder just how much leak Peter Yost was modeling compared to the volumes Grin, Smegal, and Lstiburek are considering?
Clearly nothing can withstand a fire-hose type of a leak, but there are many orders of magnitude of what might be considered a realsitic "leak".
What about a 12" EPS SIP?
With a cathedral ceiling design and if a 12" SIP with Type 1 EPS were used, one wouldn't encounter these problems that are typically found with a cathedral ceiling, correct? Of course assuming the SIPs are properly air sealed at the attachment points.
Vent above rigid foam assemblies in residential non flat builds
Spray Foam glues to substrate so leaks are the main source of failure.
Experience here in 6a is that rigid foam get bypassed. Then interior moisture is a disaster. It has happened here to many who think otherwise. WUFI does NOT model the bypasses that are impossible to build out of seamed sheet goods whether installed inside or to the exterior.
Science is needed which is WUFI. User field feedback of actual performance is the other half of truly understanding an assembly and how well it will perform.
Spray foam though I do not advocate it's use certainly can perform in an unvented assembly if installed correctly. Rigid foam is best installed with vent space above it. Why? Because we have seen too many failures. Too many. Why do school flat roofs build with out a vent space? Price, flat roof issues and they actually do have lots of water issues. Their frames are galvanized steel and survive quite awhile with leaks. They also do leak and do have to repair leaks.
aj
Might end up with a flat ceiling
If it were up to me I think you might have to end up losing the pretty wood planks and end up drywalling the bottom of the trusses and have a flat ceiling like other buildings do. I notice this seems to be an office so its not like "guests" are going to notice it. Then fill with about 12 inches (maybe more) of cellulose or fiberglass in this new "attic". It looks like the architect was trying to get a nice "natural" look but maybe did not know the building would be in such a severe climate.
expanded polystyrene
Wisconsin has the same big box store Iowa has, which sells expanded polystyrene for about half the price per stated R value as polyisocyanurate. Glad I did not take the advice last year to go with polyisocyanurate and good to see some evolution.
Insulation on top, new roof notwithstanding
It's too bad the roof is new--and too bad it's asphalt shingles, since a metal roof could be reused. But I wonder if it wouldn't be cheaper and easier to install the foamboard from above ( I would go with EPS) and then put on a new roof on than to install the insulation from below, fit it around the trusses (and insulate between the two chords of the double truss), put up a new wood ceiling, remove and reinstall the ceiling lights and fan, etc....
Other options
I think Steve is right in comment 8, that in spite of recent shingling, putting the insulation outside makes more sense, and may be cheaper. If that option continues to be off limits, what about using a rigid mineral wool insulation inside? Or framing a cavity for dense-pack cellulose? I'm curious about the expert view of these options.
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