GBA reader Erik is building a new house in southwestern Washington state, and he’s thought through most of the details with care. But as the time nears to install the standing-seam metal roof, Erik realizes he may have overlooked something important in the construction details.
As he envisions it, the unvented roof would be insulated with the “flash and batt” approach — a 3-inch layer of closed-cell spray foam applied to the underside of the roof sheathing followed by insulation batts to fill the remainder of the 2×12 rafter bays. Above the 5/8-inch roof deck, Erik sees synthetic underlayment and then the metal roof.
“Now, as we get close to putting the roof on, with quotes from roofers in hand and a few selected based on their experience with standing-seam, I realize that one thing I did not fully understand is ensuring the sheeting can dry in at least one direction,” Erik writes in a Q&A post.
Erik sees four options for venting the roof:
1. Adding a layer of rigid foam above the sheathing, which would require a second layer of plywood over the foam to provide a solid substrate for the roofing.
2. Adding a vent channel under the metal roofing and using a vapor permeable underlayment, which also would require a second layer of sheathing.
3. Use of a mesh layer such as Cedar Breather between the underlayment and the roofing.
4. Use of a vapor permeable underlayment or 30-pound felt over the sheathing, followed by the roofing.
There are drawbacks with some of these options, including higher costs and, in some cases, awkward aesthetics. But Erik would probably choose the second option as the least objectionable. Or should he be worrying about this at all?
That’s the topic for this Q&A Spotlight.
Just make sure the roof deck is dry
GBA Editor Martin Holladay suggests Erik’s original roof plan may not be problematic after all — providing the roof sheathing is dry when the spray foam is installed. In fact, most flash-and-batt roofs do not have a built-in path for the roof sheathing to dry, which is also the case when the roof is insulated completely with closed-cell foam.
“Here’s the idea,” Holladay says. “You make sure that the roof sheathing is dry on the day that the spray foam contractor installs the spray foam, and then you don’t worry.
“As long as the roof sheathing starts out dry, it shouldn’t take on any new moisture until you get a roof leak,” Holladay continues “If you’ve got a roof leak, you’ve got wet sheathing — but that’s the case with any roof. In that case, you repair the roof leak or install new roofing (and you make sheathing repairs if necessary).”
If guaranteeing a drying mechanism is important, he adds, Erik has two options. The first is to choose a different kind of roofing that’s vapor permeable, such as concrete tiles. The second is to install a ventilation channel above the sheathing along with a second layer of plywood, an option Erik has already covered.
“Most people wouldn’t do this,” Holladay says, “but it’s your house and you get to spend your money how you want.”
Another possible approach, suggested by John Clark, is to install the metal roofing over 2×4 purlins, skipping the second layer of plywood altogether. Some but not all manufacturers of metal roofing would allow this, Holladay replies, and Erik would need to find out whether solid decking is required under the particular kind of roofing he’s planning on using if he’s interested in pursuing this option.
Metal roofing will let moisture escape
Jon R sees some merit in using , which is sold as a “ventilated underlayment.”
“With perhaps .7 perms and good air sealing below it, the sheathing sees some but not much wetting,” Jon writes. “In my opinion, a [Cedar Breather] vent channel plus breathable underlayment would work very well to remove this amount of moisture.”
Erik worries that metal roofing is an impermeable, moisture-trapping material. Jon, however, points out that while metal can’t breathe, the seams between sections of roofing can.
“So you don’t have to move moisture very far,” he says. “A guess is that even a crinkle wrap breathable roof underlayment would suffice — but [I have] no idea if one exists.”
Dana Dorsett also sees some drying potential with that type of roofing, particularly when it’s attached to purlins that have been installed over the structural roof deck. “With proper underlayments there is still some outward drying,” Dorsett says, “but in often-rainy, sometimes windy Skamania County [where Erik is building his house] purlins would be a safer bet.”
Michael Maines isn’t quite as optimistic about the ability of a metal roof to release moisture.
“I don’t think much drying is happening through the seams on a metal roof,” Maines says. “Maybe on a conventional screw-down rib/panel, agricultural-style roofing, but certainly not on double-lock standing seam, and unlikely on the snap-lock standing seam [Erik] noted. (Jon R., I’m really not out to get you —this is a topic I discuss often with clients and builders.)
“One note to add to Martin’s advice,” he says, “if you install metal roofing with any air space below, be sure to install a waterproof roofing underlayment, because you have created a condensation machine. Many roofing underlayments are water-resistant but not waterproof.”
The Cedar Breather option
Erik sees the use of Cedar Breather over the sheathing as a promising option. But, he wonders, would that detail create the “condensation machine” that Maines has warned him about.
“So, if I went the Cedar Breather route would I want to use non-breathable underlayment or breathable?” he asks. “And would I be inviting more problems by putting all that condensation on the underlayment instead of the roof surface?”
Maines doubts many installers would look forward to installing metal roofing over the Cedar Breather, a 1/4-inch-thick mesh designed by its manufacturer, Benjamin Obdyke, for use under shingles or shakes.
“It makes a good capillary break,” Maines says. “But it is flexible and will result in oil canning, the bane of metal roof installations. And it’s not thick enough for significant air flow. That’s good, because air flow (on cold nights following sunny days) is when the bottom of skip-sheathed metal roofs produce condensation. But it also means that you won’t get significant drying. At least that’s my semi-educated opinion on the matter. (I’ve only used Cedar Breather and similar products under cedar shingles; even clapboards and solid board sheathing is difficult to install well over it.)”
Malcolm Taylor has the same concerns about installing metal roofing over a flexible substrate like Cedar Breather: “All the flashing and penetrations in the roof rely on gasketed fasteners, which in turn rely on everything staying tight, with no flexibility between the panels and what is underneath,” he says.
In the end, Maines says, if Erik wants a more resilient assembly, he’ll need to vent the roof either above or below the sheathing. Furring the entire roof with 2x material and then installing a second layer of sheathing is an added cost, just as installing baffles below the sheathing before spraying the closed-cell foam would be. “But if you want a vented roof, those are your options,” he says, “unless something else changes.”
Jon R, however, believes the 1/4-inch vent channel created by the Cedar Breather would be enough.
“I suggest that compared to the amount of wetting from below in Erik’s roof, moisture movement via a 1/4-inch vent gap is very significant, not negligible, and more resilient than no gap,” he says. “Larger would be wasted expense. On a roof designed to unvented specs, 1/4 inch *is* code compliant.
“There is a question of outside airflow under metal roofing being a net plus or minus,” he continues, “but that’s true at 1/4 inch or 2 inches and arguably true with no intentional gap. A little bit of moist outside air gets under metal roofing even with no gap (and the moisture it sometimes deposits has a harder time getting out). I find it unlikely that for moisture in Erik’s design, a 0-inch vent is fine, 1+ inch is fine and somehow 1/4 inch is a negative.”
Don’t overthink the problem
After mulling the suggestions GBA readers have made, Erik is still left with a question or two. If he installs the roofing over the sheathing without an air gap, for example, should the underlayment be vapor permeable?
“You are overthinking this question,” Holladay replies. “There won’t be any outward drying if you install a standing-seam metal roof above your underlayment with no air gap, so the vapor-permeance of your roofing underalyment is irrelevant. Use whatever you want.”
Holladay also weighs in on the Cedar Breather option, telling Erik it’s not a standard approach for standing-seam roofing and should be cleared with the roofing manufacturer.
“If I had to speculate, I’d say that real ventilation drying through the Cedar Breather is unlikely, because (a) the gap is too small to allow for significant air flow, and (b) it’s not clear to me what type of soffit vent or ridge vent you’d use with this approach,” Holladay says. “The lack of air flow means that condensation in the springtime (when warm weather follows snowfall) is less likely than it would be with a metal roof that has a more generous ventilation channel.”
Our expert’s opinion
GBA Technical Director Peter Yost had these thoughts:
I believe it is just as important to have dedicated directional drying potential in roofs as it is in walls. (For more information on this point, see this article.) In fact, given the increased moisture and higher risk of leaks that roofs see, it’s probably more important in roofs than it is in walls.
Vapor permeance of standing-seam metal roofing: When this roof cladding is installed directly against the roofing underlayment, there is little to no air movement to create “in situ” vapor permeability to the assembly. The doubled-over laps and, in many cases, sealant used in the seams mean at least a Class II vapor retarder rating for the roof cladding if not Class I. It’s a different story if the metal roof cladding is in a different configuration — corrugated roofing, for example.
(Note on permeance versus permeability: Permeance is a metric of vapor movement of a given thickness; permeability is per unit, such as per inch.)
Vapor-permeable roofing underlayments: If you create a ventilation space between the standing-seam metal roof cladding — using technique and spacing acceptable to the roof cladding manufacturer — there are plenty of options for vapor-permeable roofing underlayment to provide exterior drying potential for the roof sheathing. They include , , and .
Spacer mesh under standing-seam roof cladding: I checked in with Brian Knowles at a local high-performance roofing company, Jancewicz & Son. “I wouldn’t do it,” he said. “We use the 1/2-inch foam backer rod and it creates a small air space between the vapor permeable roof paper and the steel. You need a flat surface to avoid oil canning on the finish panels.”
The use of backer rod run vertically in the middle of the metal panel — to prevent and create soffit-to-ridge air movement — appears to be pretty common in the roofing industry (I did a Google search on “using backer rod under standing-seam metal roof” and got plenty of roofing contractors commenting on the technique). But I could find no information on just what drying benefit there is to this practice. It certainly prevents oil canning but a 1/2-inch gap would provide pretty limited air flow potential between the cladding and the roofing underlayment.
The building code and unvented roof assemblies: In my opinion, the current code is making a big mistake by implicitly “equating” the thermal and moisture performance of exterior rigid insulation and closed cell spray foam up against the roof sheathing (Section R806.5 – Unvented attic and unvented enclosed rafter assemblies). With a vapor-impermeable roof cladding/roofing underlayment, the former can be configured for significant drying potential to the interior and the latter has no significant drying potential in either direction.
I am currently working on an article on the topic of moisture performance of unvented roof assemblies for The Journal of Light Construction. It should be out in the April or, more likely, the May issue. Stay tuned.