Tom Jordan is a painting contractor in northern New Jersey who’s at his wit’s end as he tries to figure out a remodeling job at his own home.
It seems simple enough. Jordan is turning a three-season room on his 1930s house into a fully conditioned space. The room measures 12 feet by 16 feet and has an 8-foot ceiling. The flat roof overhead is framed in full-depth 2x6s, finished with what Jordan believes is roll roofing (a type of asphalt roofing).
“My issue is how to correctly insulate the ceiling,” he writes in a Q&A post. “I’ve spent what seems like hundreds of hours online and had a few insulation contractors look at the room, and I can’t believe the inconsistencies that exist on this subject.”
In Climate Zone 5, the prescriptive table of the International Residential Code requires at least R-49 of ceiling insulation. With 2×6 rafters, that’s a tall order, and Jordan adds a couple of major caveats.
“The roof is only four years old, so removing it is not an option,” he says, ruling out a layer of exterior rigid foam. “The room does not have any venting, nor is it possible to vent … Before anyone suggests closed-cell spray foam, I’m ruling that out. I have no confidence in the ‘healthiness’ of the product. It might be a great product, but I don’t want it in my house.”
Those are the ground rules for this Q&A Spotlight.
You need exterior insulation
Convenient or not, Jordan will need exterior insulation over the roof sheathing in order to meet code minimums, says Ryan Lewis, advice that is seconded by GBA editor Martin Holladay. And it will be important to maintain the correct ratio of interior to exterior insulation.
“You need sufficient exterior insulation, typically rigid foam — but I guess you could use other stuff, too,” Lewis writes. “In Zone 4A you need R-15. If you are in Zone 5 you need at least R-20.”
In order to meet the R-49 requirement, Jordan will need the balance of the insulation — an R-value of 29 — on the inside.
“Ripping up the roof to add exterior insulation is just not an option,” Jordan replies.
Instead, he proposes making the rafters 7 inches deep and then adding two layers of R-15 mineral wool for a total of R-30. Drywall would be the air barrier, and the primer and paint would act as the vapor retarder.
The consensus for exterior insulation, despite his objections to it, is emblematic of Jordan’s frustration.
“This is unbelievable in that three different insulation contractors (all reputable) gave me three different opinions,” he says. One recommended R-19 fiberglass batt insulation with ventilation baffles. The second contractor favored two layers of rigid foam with two separate ventilation spaces. The third said he’d apply 2 inches of spray foam and “call it a day.”
Why spray foam makes sense
If there is no way to vent the area below the roof sheathing, as Jordan has indicated, leaving a gap between the insulation and the sheathing won’t help, and may be a problem, says Bill.
“If someone suggested a ventilation space in an unvented ceiling assembly, then they probably don’t understand the physics of how the assembly actually works,” he adds.
In an unvented “hot roof,” Bill says, the key is to keep moisture away from the sheathing, and the only reliable way of accomplishing that is with closed-cell spray foam. Bill has a shallow-pitch roof at his own house, and no way of venting it. He used 6 1/2 inches of closed-cell spray foam (R-38) for both air and vapor sealing, and he’s had no problems.
An experienced spray foam installer is essential. The contractor that Bill used told him that temperature and pressure of the two agents that mix in the spray gun are critical.
“He constantly adjusted these things throughout the job as he was applying the foam,” Bill said. “I had zero issues with improperly cured foam anywhere. I think you’ll find that if you use an experienced spray foam contractor you won’t have any problems.”
What happens if the roof leaks?
If Jordan were to go the spray-foam route, he wonders what would happen in the event of a roof leak. With a layer of impermeable foam coating the under side of the sheathing, where does the water go?
“You’re pretty much screwed if you get a roof leak,” Bill tells him. “The water won’t be able to get out, will pool up on top of the spray foam, and soak into the sheathing. You need to make sure your roof is properly flashed and sealed everywhere.”
Bill suggests waiting a month or two before applying the foam, just to make sure there are no roof leaks. Every time it rained, Bill says he took a strong light and checked the sheathing for evidence of a leak. When he was sure there were none, he felt safe in adding the spray foam.
The cut-and-cobble approach
There are no takers on Jordan’s idea of stuffing the rafter bays with mineral wool for an R-30 ceiling and calling it good enough. For one thing, that amount of insulation is below code.
What about putting two layers of 2-inch rigid foam between the rafters, and filling the rest of the cavity with R-15 of mineral wool? “I understand it does not meet code,” Jordan says, “but will it suffice? Do the 4 inches of rigid foam mimic what 4 inches of closed-cell spray foam would do?”
Cutting sheets of rigid foam to fit between rafters or studs is called the “cut-and-cobble” approach, Bill says, and the only situation where it’s safe is in a vented roof. “With your unvented roof, a cut-and-cobble install will never seal well enough,” he says, “so you’ll end up with moisture getting to the underside of the sheathing, which is bad.”
Further, Jordan’s view that spray foam is the “next asbestos” is unwarranted, Bill adds.
“I really think you should reconsider your stance on spray foam,” he says. “As long as it’s installed properly, the cured foam is a very stable material. Worst case, it would be something like the old lead paint — safe unless you disturb it. Even asbestos is considered safe if it’s encapsulated and not disturbed.”
Our expert’s opinion
GBA Technical Director Peter Yost had these thoughts:
Venting “flat” roofs: In order for venting to work, you need air flow — two holes and a driving force. It’s pretty easy to add two vents. The problem is the driving force. There simply is not enough air movement from wind, especially in low buildings, and there is really no stack effect.
Can you mechanically ventilate? I have never seen this done in a way that does not make flat roof assemblies wetter rather than drier. It’s most often because there is no continuous air control layer between the insulation below and the vent space but it is also because setting up the sensors for controlling the mechanical ventilation is challenging.
Solar reflectivity/absorptance of roof cladding: “Cool” roofs that reflect solar energy significantly reduce drying potential in most flat roof assemblies.
Evaluating spray foam: Given what we know now about global warming potential (GWP), someone asked me the other day, would I have used closed-cell foam on the exterior of my house as I did in 2002-2007?
In my exterior wall assembly, the closed-cell spray foam was my continuous water, air, and thermal barrier. That alone makes it pretty hard to give up on. And I know this Spotlight is about a flat roof, not a wall, but my point is that sometimes there is just so little else that can do the job.
Additionally, closed-cell spray foam is now available with HFO blowing agents with a global warming potential (GWP) of approximately 1. That is a huge environmental footprint change for closed-cell spray foam. Having said that, spray foam is job-site chemistry. There are lots of variables to manage to get the chemistry right (for more, see this article from BuildingGreen).
Finally, when applying spray foam inside a building, keeping the building unoccupied for 72 hours after application is a good idea.
Air sealing: It seems as though just about every other Q&A on GBA hammers on airtightness, and in flat, unvented roof assemblies, you need to be fanatical about this. The lack of inherent drying potential of flat roof assemblies with membrane claddings means you’ve got to get the air control layer right.
You might have a look at Joe Lstiburek’s article “Doubling Down: How Come Double Vapor Barriers Work?” While the assemblies he cites have worked, they are not the most robust and they are pretty dependent on solar heating. Double vapor barrier or not, get the interior air control layer right.