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Q&A Spotlight

Roof Assembly for a Getaway Cottage

An owner-builder looks for comments on his plan for insulating a low-slope roof

This proposed roof assembly for a small getaway cottage includes several layers of rigid foam, although one may be eliminated for reasons of cost. Concerns focus on the use of a peel-and-stick membrane in the assembly.
Image Credit: Scott Gibson

Plans are taking shape for Quinn Sievewright’s holdiay home: a small retreat with a shed roof that will be built in Climate Zone 4 near Vancouver, Canada. During the winter, the building won’t be occupied full-time, but enough so that Sievewright has included several layers or rigid foam insulation in the design for his low-pitch roof. (The drawing at right shows how he’s proposed to build it.)

Inside, the roof assembly will include exposed 3×8 Douglas fir beams topped by 2×6 tongue-and-groove boards. That’s followed by two or three layers of rigid foam insulation, plywood sheathing, and EPDM or another type of membrane roofing.

“The house designer (not necessary a qualified architect or experienced builder) says we need Ice and Water Shield for ice dam protection,” Sievewright says in a Q&A post at Green Building Advisor. “I fail to see in what layer of the build-up that would go in the ‘warm,’ unvented assembly we are proposing, and what it would achieve when one has a fully-adhered type roof like an EPDM membrane.”

Sievewright says that he recognizes that the air seal would typically be located on the bottom of the rafters, but that won’t be possible with the construction he’s now considering.

“The open-rafter architecture is beyond my control but is commonly done, even though it’s perhaps not the ideal method,” he writes. “As such I am trying to come up with the best solution.”

Has he? That’s the topic for this Q&A Spotlight.

The roof plan is problematic

Ice & Water Shield is a brand of peel-and-stick membrane made by GCP Applied Technologies. It’s commonly used in roof assemblies, particularly in snow country and areas where wind-driven rain is a problem. It’s usually applied directly to roof sheathing.

But Steve Knapp sees problems ahead for this particular assembly. “Typically you would have an airtight drywall layer at the ceiling, and your T&G would be installed over this,” he writes. “Are you planning to use Ice & Water on the interior as the air barrier?”

Further, there’s a question of whether the roof assembly as proposed would have enough insulation to meet code requirements. Sievewright says that his plan to install 1 inch of polyisocyanurate and another 6 inches of expanded polystyrene (EPS) should yield an R-value of about 37. But the R-value is probably going to be less than that, and if he eliminates the polyiso because of cost, the total R-value could be closer to 26 or 27, based on an R-value for EPS of about 4 per inch.

That’s roughly what Sievewright says is required for a flat roof in that area. Knapp, however, says that R-49 is recommended for a roof in Climate Zone 4. He suggests that Sievewright look for reclaimed rigid foam insulation, which costs about one-third as much as new.

Using Ice & Water Shield is fine

Unlike Knapp, GBA senior editor Martin Holladay sees no problems in using a layer of Ice & Water Shield as an air barrier above the tongue-and-groove ceiling. In fact, he says, that’s a “time-honored method” pioneered by builders using the PERSIST construction method in the late 1960s.

“It’s a standard approach, and you apparently understand the principles behind the need for an interior air barrier,” Holladay says. “The only worry that I’ve heard (especially among people who call themselves ‘chemically sensitive’) is the worry that some brands of peel-and-stick may have an odor. Whether this is a problem or not is a subject of debate.

“If you worry about odor, you can always install a layer of plywood or OSB above the tongue-and-groove ceiling as your air barrier,” he continues. “Of course, the seams would need to be taped. If you decide to use plywood or OSB as your air barrier, you could skip the peel-and-stick.”

Holladay adds that if Sievewright goes with EPDM roofing, there’s no reason to include a peel-and-stick membrane to protect against ice dams.

“If you are worried about ice dams, you need (a) an airtight assembly, and (b) enough R-value,” he says. “Your plan to install R-37 of rigid foam is OK (if legal), but R-49 would be better.”

Including a vapor barrier in the assembly

One problem Sievewright is running into, says Malcolm Taylor, is that British Columbia building codes are not always aligned with best practices from a building science perspective.

“You do need an interior vapor barrier,” he writes, “although that could be a number of materials and doesn’t have to be ‘fully adhered,’ although that will help during construction. Depending on the permeability of the foam you choose it could be the foam, it could be poly, or it could be an underlayment like Grace Tri-Flex. I’d be inclined to use the [Ice & Water Shield].”

Although Jon R sees a potential problem with “sandwiching plywood between two impermeable layers,” Taylor does not. “There wouldn’t be any sandwiching,” he replies. “Both membranes would be on the exterior of the roof sheathing.”

Taylor adds that the building code recommends that the vapor barrier be “close enough to the interior to avoid condensation at design conditions.”

“In other words,” he says, “if you push it toward the outside, you need to be able to show it won’t be a problem. Your stack-up with the insulation above would be fine.”

Insulation and structural questions

On the question of choosing insulation, Dana Dorsett suggests that both the polyiso and EPS have about the same cost per R, so there’s no advantage to the layering Sievewright is suggesting. Using two layers of polysio, each 3 to 3 1/2 inches thick, is probably going to be cheaper than 1 inch of polyiso and two layers of 3-inch EPS, as Sievewright originally proposed, Dorsett says.

“If you used reclaimed roofing foam (both 3-inch and 3.5-inch are common standard thicknesses), it’s greener, too,” Dorsett adds. “Rigid foam of all types shows up regularly with reclaimers, surplus and salvage materials dealers, often at a small fraction of virgin stock goods at the local distributors, which can extend the budget quite a bit.”

Tim Rudolph adds two other considerations for the roof: structural stability in a seismic zone and fire resistance.

“A T&G ceiling may not provide an adequate diaphragm for seismic forces on the West coast,” he says. “This would lead to the solution of placing sheathing over the T&G for the roof diaphragm and thus the air barrier. You didn’t indicate the spacing of the 3×8 beams, but you may need to use staples on the roof diaphragm if there are not framing members at the correct spacing for the panel edges, or increase the 2×6 to 3×6 or 3×8 to be able to create an adequate roof diaphragm with staples.”

Good point, Taylor says, adding that in coastal seismic zones roofs must be sheathed with panel goods such as plywood or OSB and fastened according to an approved nail or screw schedule. “It’s unlikely that having the plywood separated from the structure by foam would meet this requirement.”

Rudolph raises one last point: fire resistance. If a fire-rated roof assembly is required, he says, a layer of DensDeck Roof Board from Georgia-Pacific could be added below the EPDM roofing. It could replace the top layer of plywood over the foam.

Adding 2x8s for roof ventilation

The conversation has prompted Sievewright to huddle with the designer, who now wants to add some 2x8s on the flat across the top of the insulation (and under the plywood) to introduce some ventilation to the roof.

“There would then be vents in the exterior soffit front and rear,” he says. “Is there any merit in that approach in such a low-pitch roof assembly?”

No, says Taylor, the plan doesn’t make sense.

Holladay adds this: “If you want a vented roof assembly, you should follow the recommendations in this article: Insulating Low-Slope Residential Roofs. (Here’s the short version: 2x8s on the flat won’t work.)

“The approach we have been talking about up until now is an unvented approach. It’s routine for commercial roofs. If your designer and contractor have no familiarity with commercial work, that might explain their confusion on this issue.”

Our expert’s opinion

GBA technical director Peter Yost added this:

To prevent ice dams, you need continuity in the thermal (insulation) layer as well as continuity in the air control layer. You need to maintain this continuity within the roof assembly and at the transition from the wall to the roof at the eave.

For the roof assembly, the key is to manage convection and conduction at penetrations, given that you have configured the roof with adequate insulation. On the wall-roof transition, the key is to have a robust and continuous overlap of the wall and roof air barriers.

There is nothing wrong with designating an exterior air control layer. But how will it be connected for air barrier continuity at the eave?

The biggest issue with this sort of tongue-and-groove roof deck is when — as is typical — this deck runs on the exposed roof beams from inside to outside the structure. Beams and a T&G roof deck that extend through the eaves and even the gables can be an air leakage and thermal bridge nightmare. It’s best to treat the eave and gable overhangs as completely separate add-on ladder overhangs instead of extending the roof assembly from inside to outside conditioned space.

In terms of the code or an inspector requiring an interior vapor retarder: 6 inches of any rigid foam insulation qualifies as an interior vapor retarder. Six inches of EPS has a vapor permeance of around 0.6 perms; six inches of XPS, around 0.16 perms. Either of these Class II vapor retarders provides at least some drying potential to the interior for the 1/2-inch plywood layer, while the Ice & Water Shield — a Class I vapor retarder — provides none. I think it is important to either tape the rigid insulation or install a non-Class I, taped, flexible sheet good to double up the air barrier and connect this layer to the wall air barrier.

And as for roof venting: it’s really hard to get much driving force in a 1:12 pitch shed roof. Introducing a vent channel in this roof assembly is not worth the added expense, in my book.


  1. User avater
    Dana Dorsett | | #1

    Ice dams? In Vancouver B.C.? Why is that even a topic?
    Seriously folks, there's never enough persistent snow in zone 4C to be concerned about ice dam issues! The mean monthly temperature even for January in Vancouver is well above freezing a ~ +39F, (even the average daily LOW in January is above freezing), and fallen snow only rarely persists for more than 3-4 days. Snowfalls of a foot or more are once or twice per decade type events.

    A bigger issue for low slope roofs in that temperate rain forest type climate is keeping the roof deck dry enough to avoid heavy moss growth given the limited number of sunny hours and high number of active rain hours. Vancouver is overcast more than 50% of the time, and overcast to mostly cloudy ~70% of the time from early November through late March, getting very little solar assist for drying the roof, ventilated or not.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Dana Dorsett
    When I first read your comment, I thought the blunder might have been mine (due perhaps to my Vermont-centric approach to building).

    A quick check revealed the truth: it was Quinn Sievewright's designer, not anyone involved with GBA, who raised concerns about possible ice dams.

    I think you're right about snowfall in Vancouver.

    -- Martin Holladay

  3. Brendan Albano | | #3

    Since Dana brought it up, I was curious:

    Are there any problems caused by moss growth?

    It's like you're getting a green roof for free! ;)

    The moss is one of my favorite thing about the pacific northwest.

  4. Malcolm Taylor | | #4

    Vancouver and environs
    Yes you can often golf in January, but Vancouver is right up against the Coast Mountains. Within fifteen minutes of the municipal boundary there is a ski resort. North Vancouver borders an recreational area with the most active wilderness rescue team in Canada, where hikers die in avalanches on a regular basis. The problem is more likely that Quinn used the wrong climate zone to describe the location of his cottage that that he shouldn't worry about ice dams.

    However, his designer's comment probably has more to do with the building code than anything. It requires eave protection against ice dams everywhere without making exceptions for the mild coastal climate.

  5. Quinn Sievewright | | #5

    Add strapping?
    Hello again
    Thanks for featuring this question - hopefully it helps future readers.

    I've got an additional question if I may, more construction practicalities related but with a building science implication....

    The contractor would like to add some 1x4 strapping perpendicular to our underlying 4x8 beams (beams installed on 48" centers, strapping would perhaps be 24" centers) above the rigid foam. He wants to do this to ease the installation of the overlying ply so that only the strapping needs expensive long screws to penetrate the rigid foam and the ply can be installed with short screws penetrating just the strapping and countersunk flush (long screws seem to all have large heads which would show through on the overlying EPDM).

    We could fill the space between the strapping with additional insulation (approx 3/4" for 1" nominal strapping) or just leave it as an "air gap" linked to soffit vents, or not vented at all. I gather the 'venting' wouldn't be needed in this assembly to address moisture build up from internal sources, but wasn't sure if it might help in summer as the black EPDM will get quiet warm? Perhaps not though.

    As always, your advice greatly appreciated.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Quinn Sievewright
    The contractor's plan will work, as long as an engineer approves the type of fastener, and the length of the fastener, used to hold down the 1x4 strapping. Those fasteners are being asked to do a lot of work, so they had better be right for the assembly to work.

    The air gap is a benefit, not a problem. It can be vented or not, as you prefer.

  7. Quinn Sievewright | | #7

    Mold on strapping material
    Hello all,
    We are a few weeks in on this build and so far the crew have been great at embracing the Zip wall systems and thinking about air sealing and thermal bridging etc as we progress.

    The roofing materials have arrived and the 1x3 untreated strapping has arrived but is very moist and covered in black and white mold spots. Clearly the lift has been left out in the rain and been covered with a tarp or something to prevent drying. I've spread it out to dry in the sun but I'm concerned about introducing this mold into the roof assembly. Should I be concerned? Or is the material perfectly fine to use? Thanks!

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Quinn Sievewright
    Mold on lumber isn't that unusual. You have the right idea -- the lumber needs fresh air and sunshine, with breezes.

    As long as it isn't sopping wet when it is installed, everything will be fine. A few black stains won't cause any problems.

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