If only Kevin Hoene’s choices for a new roof boiled down to a choice between an EPDM membrane and metal, his life would probably seem a whole lot simpler.
But Hoene, building a new home in Illinois and on the boundary between Climate Zones 4 and 5, will soon be weighing the pros and cons not only of different roof coverings, but also of what type of insulation to use, whether it should go above or below the roof sheathing, and whether the roof should be vented or unvented. In other words, nothing seems off the table.
“Our house is being built near the boundary of Zone 4 and 5 in Illinois with a 1:12 pitch metal roof,” Hoene writes in a Q&A post at GreenBuildingAdvisor. “I’ve done a lot of research on low-slope roofs because our builder does not have a lot of experience with flat roofs.”
After reading a couple of articles on the topic by GBA Senior Editor Martin Holladay, Hoene is leaning toward an unvented roof assembly (see the sketch above), which would include cellulose insulation in the rafter bays and 4 inches of extruded polystyrene (XPS) rigid insulation above a layer of airtight sheathing. That would be followed by a second layer of OSB or plywood sheathing and, finally, the metal roofing.
“Does the diagram look like a good approach?” he asks. “Any details that I should pass along to our builder about the roof trusses?”
Those questions are the start of this Q&A Spotlight.
Choosing the type of roofing
A first step, Holladay suggests, would be finding out whether the type of metal roofing Hoene is considering can be installed on the roof he’s planning. The pitch is very low — rising only 1 inch for every 12 inches of run — and some types of metal roofing need more than that.
Hoene has discussed the possibility of using 60-mil EPDM instead, which apparently won’t present any aesthetic issues, but he wonders whether the membrane will perform as well as a standing-seam metal roof.
“On low-slope roofs in the snow zone, EPDM will work better than standing-seam metal, since it can’t leak even under high winds, whereas metal roofs will,” writes Dana Dorsett. “With limited slope to drain well, it can take forever to be rid of any leakage moisture. EPDM won’t last as long as metal, but there’s a reason it’s the most common roofing for very low slope roofs.”
Kevin Dickson also likes EPDM. It’s “great,” he says, and it’s available in white, which would reduce cooling loads in the house.
An alternative, Dickson adds, is spray polyurethane, which will last “indefinitely” if it’s well maintained, meaning that it should be recoated every 10 years or so, and inspected carefully every year.
In further conversations with a roofer familiar with low-slope assemblies, Hoene is told that a fully adhered 90-mil EPDM roof applied over 1/2-inch DensDeck Roof Boards is the best option.
“Is the 90-mil thickness overkill?” he asks. “Most of the applications I have read about use 60-mil or 75-mil, but we do want a roof built to last.”
What kind of insulation will work best?
Hoene’s original plan was to use 4 inches of XPS, which has a nominal R-rating of 5 per inch. Better take that with a grain of salt, Dorsett says.
“Four inches of XPS won’t perform at R-20 for the full 50+ year lifecycle of most metal roofing,” he writes. “As it loses its blowing agent over a handful of decades its performance starts out higher than R-20, but eventually drops to about R-17. The R-5/inch labeling is based on something like a 20-year average performance.
“R-17 might be sufficient for dew-point control, but it might not be,” he continues. “If instead you went with 5 inches of EPS [expanded polystyrene], its R-value will be pretty much the same on day 25,000 as on day 1, since its much lighter blowing agent dissipates quickly, and its labeled-R is its fully depleted R.”
There are two other choices — polyisocyanurate and spray polyurethane foam.
“Another roofer has proposed using 4.4 inches of polyiso insulation above the sheathing for an unvented assembly,” Hoene writes. “This builder said that XPS and EPS aren’t compatible with the EPDM adhesive. Quote from builder: ‘Like pouring gas in a styrofoam cup, it melts it away. They are typically used in ballasted or mechanically attached systems.’
“The issue I have with polyiso is that I have read that the effective R-value in cold weather is lower, and polyiso should only be used in warmer climates,” he adds.
One solution to polyiso’s less-than-optimum performance in cold weather, Holladay says, comes from building scientist John Straube, who once told him, “One option is to stick with polyiso and just make it thicker. If we do that, let’s call polyiso R-5 per inch.”
Either open- or closed-cell polyurethane foam could be sprayed on the underside of the roof sheathing, another possible route to take.
Where should the insulation go?
Hoene has sketched an unvented roof assembly, but his builder and roofer both suggest he consider a vented roof assembly (see Image #2 at the bottom of the page).
“They have not done an unvented roof with the layered foam above the sheathing and seem more comfortable doing the following approach and say it would save a lot of money in time and materials,” Hoene says.
The builder suggests blown-in cellulose 16 inches deep, leaving the area above the insulation open for ventilation. Hoene adds: “He recommended no venting in the center of the roof due to more chances for leaks.”
“It might be time to find a different roofer,” Holladay replies. “If you can locate a roofer familiar with commercial construction, you’ll be in better hands — because installing rigid foam above the roof sheathing is a standard method of insulating low-slope commercial roofs.
“The approach shown in your sketch is associated with failures, because there isn’t enough of a difference in elevation between the ‘inlet’ vents and the ‘outlet’ vents. I call this type of ventilation ‘faith-based venting.’ Air rarely follows the ‘smart arrows’ that some designers draw to indicate where the air is supposed to enter, and where it is supposed to leave.”
The same roofer who has suggested 90-mil EPDM also would prefer keeping all of the insulation below the sheathing. Hoene says he would insulate from below using blown-fiberglass with some sort of adhesive that allows it to bond with the decking, or to use spray foam.
Holladay is unconvinced. “I stand by my advice,” he says. “For an unvented flat roof, you want at least some — or ideally, all — of the insulation to be above the roof sheathing. That way, the roof sheathing stays warm and dry.”
The recessed can light conundrum
There is yet another wrinkle to this roofing saga: the use of recessed light fixtures in the ceiling. They would be installed in the drywall that serves as the air barrier. Hoene wonders if that’s going to be a problem.
Holladay says, “Absolutely, that’s a problem.”
He continues, “You want to minimize all penetrations and electrical boxes in an airtight ceiling, and all penetrations need to be very carefully air sealed. If you care about energy performance, you won’t have any recessed can lights in your ceiling.”
Dorsett adds that certain types of recessed lights come with gaskets to make them airtight, and they are rated for contact with insulation. But, he says, they should be inspected and installed carefully.
“There are also surface-mount LED fixtures that can be mounted on standard electric boxes that present a far smaller and far shallower penetration into the insulation layer, which may be a better alternative,” Dorsett says. “Any penetration of the ceiling gypsum needs to be detailed for air tightness.”
Our expert’s opinion
Here’s how GBA Technical Director Peter Yost sees Hoene’s situation:
Per usual on roofing and attic questions, I like to check in with Brian Knowles at RoofsPlus, a local high-performance roofing company. Here is a summary of my discussion with Brian:
Is a 1:12 standing seam roof OK? Yes, but use a full-coverage membrane such as Ice and Water Shield as the roofing underlayment. Also, use a hidden-clip system rather than exposed fasteners, and back up the standing seam lock seam with a high-performance sealant. Brian’s company uses Geocel Tripolymer.
Using an EPDM membrane would be OK as well. But, exposed roofing membranes typically don’t fail in the field of the installation, but at joints and laps. These need to be inspected annually. Brian’s company never uses 45-mil membranes but finds the 60-mil performs well. They have never used 90-mil EPDM.
For both types of cladding, the devil is in the details at the eaves: With a slope this low, making a watertight seal between either the membrane or the standing seam and the fascia is tricky, particularly if the site has wind exposure. Brian has found that even manufacturers’ recommended eave details are not robust and his company has developed some of their own details.
Here is Brian’s cut: “Fully adhered EPDM systems (where there is no curb or parapet termination) need to be stripped in to metal drip edge with cover tape. The published details will show the EPDM fastened under the drip edge metal. They show cover tape over the drip edge and onto the EPDM. This detail leaves the strip of cover tape vulnerable to damage from sliding ice and snow. Our manufacturer trainers have approved a slightly different detail that has a more successful field application in areas where ice and snow are a factor (see Image #3, below). Bear in mind that these edge details will require regular inspection over the years to maintain the bond at the edge. A parapet or curb detail is preferred when using EPDM.”
Color matters. In cold climates, white membranes and white metal roofs can be associated with significant night-sky radiation and much cooler attic spaces.
On the question of whether to vent the roof assembly: A dedicated space of 1 to 2 inches just underneath or above the structural roof sheathing will vent the assembly. The space created between the structural roof deck and the finished ceiling is an attic space. This space can be vented along any of the margins of the attic but typically with vents at the eaves or sometimes rooftop turbine or “whirlybird” vents. Brian and I agree that the attic (or in this case almost a plenum) space needs to be either completely inside or outside the conditioned space and it needs air flow in either case. But note: If the attic is outside, then it means venting it with outside air; if the attic is inside, then it means introducing inside air flow. (Read on for more details…)
In Brian’s experience, even though a 1:12 pitch gives you negligible stack effect air movement or venting, when the roof cladding — particularly a darker one — is exposed to solar gain, that attic space heats up quite a bit and air flow develops even without stack effect from eave to eave or eave to ridge. So, the “high-low” eave vents in the second hand-drawn image will work fine for venting this “outside” attic space. And of course in this case, the finished ceiling plane is where the continuous air and thermal boundaries will be.
On the other hand, you could insulate and air seal this roof at the roof line, in which case the attic space created between the roof and the ceiling plane becomes an inside space. It’s important to create air flow for this space, even if it is just passive vents that relieve this space.
But note, if you air seal and insulate the finished ceiling plane, then the roof assembly above can dry to the interior. On the other hand, if you insulate and air seal the roof line, and you don’t vent the underside or topside of the structural roof deck, then you’ll have little to no drying potential in either direction.