Singing the Ice Dam Blues
A nature center in Wisconsin needs a solution for an uninsulated cathedral ceiling that is plagued by ice dams
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 2x6 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 1x6 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 (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.). 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 insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. .
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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of at least 25, enough to make a “huge” difference.
When it comes time to re-roof, Ott could add another 6 in. of EPS on top of the roof deck and his ice dam problem would be gone forever.
Putting foam insulation on top of the roof deck now would seem a good option now, says Robert Hronek, because it would leave the existing tongue-and-groove ceiling visible from the inside. But Ott says a new roof was installed last summer, keeping that option out of the mix.
The roof doesn’t have to dry to the inside
GBAGreenBuildingAdvisor.com Senior Editor Martin Holladay disagrees with Dorsett, finding nothing wrong with Ott’s plan to use the vapor-impermeable polyiso insulation on the underside of the existing ceiling.
“It isn't necessary for the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. to dry inward,” Holladay says. “It's fairly common to build a roof that has roof sheathing that can't dry to the interior (for example, when closed-cell spray foam is installed on the underside of the roof sheathing). As long as the roof sheathing is dry on the day when you install your rigid foam, everything should be fine.”
It’s just not a point on which Holladay and Dorsett are going to agree.
“It absolutely is necessary for the roof to have a drying path,” Dorsett replies. “Even 6 inches of closed-cell foam is semi-permeable at about 0.2 perm, whereas foil facers are not. That 0.2 perm isn't a high-rate drying path; it's about half that of a kraft facer, but 200 times that of six foil facers. (Three layers of foil-faced polyiso.) That's the difference between a tight Class-II vapor retarder and a true vapor barrier.”
Says Martin, “If you think that a layer with a permeance of 0.2 perm will make the difference between success and failure — due to drying through the 0.2-perm layer — let's just say I'm a skeptic. The reason that these roofs stay dry is not because they dry inward through 02.-perm foam; it's because they don't get wet.”
How to attach the foam and seal the edges
No matter what type of rigid foam insulation Ott uses on the ceilings, there’s still the matter of attaching the panels securely and making sure the perimeter is sealed to prevent air leaks.
Dorsett recommends construction adhesive to attach the sheets of insulation, tacked in place temporarily with nails if necessary. Butt joints should be sealed with housewrap tape, and Ott should leave a 1/4-inch gap around the edges, to be filled later with Dow’s Froth-Pak foam.
When Ott has installed the last of the rigid foam, he should finish the assembly with 1x4 or 2x4 furring attached with pancake-head screws that penetrate the ceiling by at least 1 inch, Dorsett says.
Holladay suggests cap nails to hold the insulation in place. Each layer will need longer cap nails or screws, and taping at least two layers of foam “will provide you with a belt-and-suspenders air seal.”
Once the furring has been attached, Ott can wrap up the job with the new layer of tongue-and-groove boards.
Attaching lights and a ceiling fan
“Reduce recessed can lighting like the plague,” Dorsett says, “since that would produce hot spots in the roof deck to start the whole ice-damming process.”
Instead, he adds, use surface-mounted fluorescent or LED fixtures and mount any fans on a furring strip, adding a second timber screw at that location for added security.
Ott could use surface-mounted metal boxes and conduit or, alternately, he could create a service chase for wiring by using 2x4 furring between the ceiling boards and the rigid foam.
Our expert’s opinion
Here's how GBA technical director Peter Yost sees it:
This is an interesting disagreement regarding the drying potential of this cathedral roof with interior rigid insulation. What difference does the slightly vapor permeable 5.5 inches of EPS make in comparison to 5.5 inches of foil-faced polyiso rigid insulation? It’s one of those rare questions: how fast will the roof assembly dry, in terms of its initial moisture load (moisture of construction), or if the roof leaks?
This sort of question is exactly what computer simulations with WUFI software were designed for (for more on this important modeling program, here's a blog I wrote on the subject). So I did a series of WUFI runs:
- Existing roof assembly with and without a roof leak
- Retrofit roof with interior 5.5 inches of FF PI rigid insulation, with and without a roof leak
- Retrofit roof with interior 5.5 inches of EPS rigid insulation, with and without a roof leak
I used spruce from the WUFI material database for the roof/ceiling deck, sticking with the default initial moisture content of about 3.5 pounds of water per cubic foot (that translates to an initial percent moisture content of about 14%).
Existing roof assembly with and without a roof leak
No surprise — the existing roof is “dry” and stays dry through the entire WUFI cycle of three years.
Without the moisture source, the moisture content of the spruce is about 1 pound per cubic foot (4% moisture content by weight) after three years and with the moisture source the moisture content ranges, seasonally, between a high of about 5 pounds per cubic foot and a low of about 1 pound per cubic foot. The existing cathedral roof handles a leak.
Retrofit roof with interior 5.5 inches of foil-faced polyiso, with and without a roof leak
Without any roof leak, the initial moisture load very, very, slowly dries to a moisture content of about 2.8 pounds of water per cubic foot of spruce after three years. No problem.
With the 1% roof leak, the spruce wood deck is almost completely saturated with a moisture content of about 48 pounds per cubic foot. There is essentially no drying potential if there is a leak, given the very low vapor permeability of both the asphalt roofing shingle claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. to the exterior and the foil-faced polyiso to the interior. Big problem.
Retrofit roof with interior 5.5 inches of EPS rigid insulation, with and without a roof leak
The initial moisture load in the spruce dries quite slowly down to a moisture content of about 1 pound per cubic foot. No problem.
But with the roof leak, there is really little difference between the two insulations; after 3 years, the EPS interior rigid insulation results in the spruce wood deck near saturation as well (45 pounds of water per cubic foot). That 0.5 perms for the 5.5 inches of EPS is overwhelmed by the moisture coming from the leak. Simply put, neither insulation at 5.5 inches provides sufficient drying potential to this roof assembly, should there be a roof leak.
My advice? It's a damn shame about that brand new asphalt shingle roof, because the right thing to do is to insulate on the exterior of this cathedral roof. With either the EPS or the foil-faced polyiso on the interior, the only way it can work is to guarantee that the roof will never leak.
And in terms of ice dams: meticulous air sealing, particularly at the eaves and any roof penetrations, is a must. This will be challenging in a major way at that doubled roof truss smack in the middle of the ceiling... yet another reason to go topside.
- Walt Ott
Mon, 01/27/2014 - 14:20
Mon, 01/27/2014 - 16:16
Mon, 01/27/2014 - 17:09
Mon, 01/27/2014 - 17:47
Mon, 01/27/2014 - 18:43
Mon, 01/27/2014 - 23:10
Tue, 01/28/2014 - 10:53
Thu, 01/30/2014 - 14:09
Thu, 01/30/2014 - 19:00