Roof Assembly for a Getaway Cottage
An owner-builder looks for comments on his plan for insulating a low-slope roof
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 3x8 Douglas fir beams topped by 2x6 tongue-and-groove boards. That's followed by two or three layers of rigid foam insulation, plywood 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. , 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 damA ridge of ice that forms along the lower edge of a roof, possibly leading to roof leaks. Ice dams are usually caused by heat leaking from the attic, which melts snow on the upper parts of the roof; the water then refreezes along the colder eaves working it's way back up the roof and under shingles. 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 barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.?"
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 (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.) should yield an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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 constructionA method of construction including rainscreen cladding and foam insulation installed on the exterior of the building’s frame. Developed by the National Research Council of Canada in the 1960s, PERSIST is an acronym for Pressure-Equalized Rain-Screen Insulated Structure Technique. (Some PERSIST builders prefer a different acronym: REFORM, for Rigid Exterior Foam Over Rubberized Membrane). Most PERSIST buildings have no insulation in the stud bays or rafter bays. Instead, 4 to 8 inches of rigid foam insulation is installed on the exterior of the wall and roof sheathing. The PERSIST system requires the installation of a rubberized asphalt membrane between the exterior foam and the wall and roof sheathing; this membrane acts as a water-resistant barrier, air barrier, and vapor barrier. 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 3x8 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 2x6 to 3x6 or 3x8 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 conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow. 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 spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. .
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 XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation., 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 n& 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.
- Scott Gibson
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