Building a Foolproof Low-Slope Roof
A homeowner in the Pacific Northwest wants to detail the roof correctly, but it may already be too late
Carolyn Wood is building a house 80 miles north of Vancouver, British Columbia, and if nothing else she'd like to get all the details in the roof assembly right. The question is whether the house is too far along to let her reach that goal.
The roof, with a 2-in-12 pitch, is framed with I-joists, strapped with 2x4s, and sheathed with 1/2-in. plywood. Above 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. , the roofers plan to install NovaSeal roofing underlayment and standing-seam metal roofing.
Below the sheathing are two layers of Roxul mineral wood insulation, providing a total of R-36. Wood plans to finish the ceiling with 1x6 tongue-and-groove boards. Against her builder's advice, there will be no polyethylene vapor barrier in the ceilings or in the walls, but Wood would like to know whether, as she has recently heard, there should be a layer of drywall between the T&G ceiling boards and the insulation.
As currently built, the roof assembly has 1-inch-high ventilation gap between the top of the insulation and the underside of the roof sheathing, Wood writes in Q&A post at GreenBuildingAdvisor, but the question is whether 1 inch will be adequate. That's the topic for this Q&A Spotlight.
Yes, the ceiling needs an air barrier
There's no question that an air barrier should be installed between the ceiling boards and the mineral wool insulation, write both GBA senior editor Martin Holladay and Dana Dorsett.
"Unless measures were taken to fully vent the 1 1/2-inch air space between the 2x4 purlins to the exterior in a manner compliant with [the International Residential Code], using poly would be a BAD idea," Dorsett writes. "But you can use a 'smart' vapor retarder such as Certainteed MemBrain or Intello Plus, detailed as an air barrier."
Either 1/2-inch plywood or oriented strand board (OSB), with all seams seamed with acoustic sealant, or a layer of gypsum drywall would work, Dorsett says. "If you use the gypsum option, painting it with 'vapor barrier latex' (about 0.5 perms) would be the right thing to do," Dorsett adds. "OSB and plywood are already somewhat vapor-retardant, and would be sufficiently vapor retardant to work in that climate as long as there is at least some ventilation going on under the roof deck."
Holladay suggests the gypsum drywall route, and directs Wood to an article he wrote on insulating low-slope roofs last year. In that article, building scientist Joe Lstiburek recommends an air space of at least 6 inches between the top of the insulation and the roof sheathing for low-slope roofs.
And there's the rub. As it stands, there's only a space of about 1 inch.
The advice so far has been conflicting
Although Wood hadn't overlooked the twin issues of ventilation and insulation, guidance was anything but clear-cut.
"We had considered the insulation and roofing construction before and throughout the construction process," she writes. "Unfortunately, it now turns out our builder and others in the industry where we live were not well informed about building science.
"We have tried to educate ourselves on the topic, but a lot of it is confusing. We are also finding conflicting numbers about the space required between the insulation and sheathing in a vented ceiling. We had understood we had sufficient venting given we are venting on all four sides of each roof."
After reading Holladay's article, Wood is leaning toward the use of gypsum drywall as an air barrier, and adding vented cupolas on each roof to prevent moisture buildup.
Wood notes that on the island where she lives, there are no building codes and no required inspections except for electrical work.
Malcolm Taylor suggests that even in the absence of a locally approved building code, Wood's house might be subject to the provincial building code, which mandates a 2 1/2-inch air space between the insulation and the roof sheathing.
But the larger issue, Sal Lombardo writes, is that local tradespeople may not be receptive to doing things in a way that doesn't conform to what they know.
"I find its not uncommon to garner information from good building science sources, such as this site or Building Science Corporation, and when you refer to this information or relay the same in conversation to the contractor, craftsman, or mechanic in the field, they have little to any idea what your talking about," Lombardo says.
"Much of this 'theory' seems to be lost on the day-to-day contractor doing the work in the field. In my area, if you stray from the 'accepted' standards, you're suddenly in uncharted waters, getting looks of confusion and skepticism."
Consider an unvented roof assembly
The 1 inch of space between the top of the insulation and the roof sheathing is far less than the 6-inch minimum that Lstiburek recommends for a nearly flat roof. What now?
Holladay says that removing a layer of insulation in order to create a deep enough ventilation space (an option Wood is now considering) really isn't an option because the roof still needs to meet minimum R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. requirements.
"It sounds like you are far advanced in your construction, and you may have painted yourself into a corner," Holladay says. "If that's the case, it's an object lesson to others who may be reading this thread: It's always a good idea to nail down your insulation details before construction begins. If you hope to figure these details out at the last minute, it may be too late."
Under the circumstances, he suggests Wood consider an unvented roof assembly instead, following the guidelines provided in his article. (Unvented roof assemblies require the installation of a layer of rigid foam above the roof sheathing, or a layer of spray foam insulation applied to the underside of the roof sheathing.)
But as the conversation unfolds, Wood as another idea: "To increase the airflow, our latest solution is to attach 2x2s to the bottom of each rafter and drop the insulation down by the extra 2 inches," Wood says. "All our ceilings are 9 feet or more. We should then have about 4 inches of space between the top of the insulation and the bottom of the roof sheathing. We will then install the plywood, and make sure air sealing is well done."
Our expert's opinion
Here's GBA technical director Peter Yost's take on Wood's situation:
Other than the general belt-and-suspenders approach to promote building assembly drying, the main reason for venting in a cold climate is to assist in preventing ice dams. I am assuming that ice dams are not a concern for this climate/site and for this 2:12 roof pitch with standing seam metal claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. and a membrane to its interior.
The main reason for going without venting is complex roof geometry. I don’t know anything about the roof geometry/complexity in this case. But we all agree that vented or unvented, a continuous air barrier for the roof assembly trumps everything. And Martin was right on in strongly encouraging proof of air tightness with a blower door test; even more important when trades are not used to anal air sealing.
This is a special case because our options are narrowed by the stage of construction. I am going with Martin on this one: spend your time and money on getting a continuous air barrier and make this an unvented roof assembly rather than spending any more time and materials on creating a vented roof assembly.
Martin is recommending interior spray foam installed on the underside of the roof sheathing; I am a bit reluctant to make specific recommendations on this assembly without seeing details like photos and drawings. But the spray foam gets you the air barrier you need and more R-per-inch of depth than other cavity fill insulations. One particular recommendation on spray foam installation is to use certified companies AND installers (either per ABAA or SPFA certifications). Spray foam does not install itself, contrary to some popular beliefs.
Maintain drying potential to the interior (no interior vapor retarder), use proper spot exhaust and manage interior relative humidity.
- Photo courtesy Carolyn Wood
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