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Q&A Spotlight

Insulating a Flat Roof

With spray foam and exterior rigid foam ruled out, what options are left?

This three-season room has a low-slope roof framed with 2x6 rafters. What's the best way to insulate this roof?

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?

This is the ceiling of the three-season room that Tom Jordan wants to insulate. Suggestions include closed-cell spray foam and a layer of exterior rigid foam. [Image credit: Tom Jordan]
“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.


  1. Premier_Building_Sup | | #1

    None of you three options from reputable companies meet unvented attic codes. unventeted attics must meet code R-806.4 ( I am not at my office at the moment so the code reference number might not be correct). Even if a company is reputable doesnt mean they understand all the codes. There are a ton of great companies that specializes in adding attic insulation to existing attics but have no clue on how a unvented attic should be insulated. It is possible but not easy to meet climate zone 5 energy code requirements with 6 inches of unvented attic space. Every home is different and if i had more information maybe i can help you with a solution that we can prove meets the current energy code to the local buildiing inspector. My first question is what city and state is the home located in? 2nd question is do you have a full set of plans for the home?

  2. sethw | | #2

    Since the job is in NJ, you should talk to your architect about what is the applicable Code. This sounds like it would fall under NJ's Rehabilitation Code as an Alteration. If so, and the framing is existing, you may only need to fill the existing framing cavities with insulation, rather than meet a prescriptive R-value. That may deal with the R-value question.

    In terms of the vapor trap question, I'd suggest either a "flash & batt" approach wherein you spray an R-20 layer of spray foam against the underside of the existing sheathing and then install a vapor open insulation on the interior side of that spray foam (mineral wool would be my choice but fiber glass batt would be fine) to fill the cavities.

    As far as contractors knowing what R-value should go where and anything at all about vapor diffusion, I work with a lot of contractors and very few know what to do in specific terms. Are they good at building stuff? Yes. Are they good at building science? No. But then again, I'm no genius either! ;)

    Hope that helps.

  3. User avater
    Elden Lindamood | | #3

    I frequently get frustrated with the number of people who what to turn unconditioned rooms into year-round living space, but refuse to accept the inherent problems with it. If Jordan is dead set against closed cell foam, he should simply accept that the cost of bringing the space into the conditioned envelope includes a new roof, regardless of how old the existing roof is.
    His aversion to spray foam simply reinforces the advantage, if not necessity, of re-roofing with rigid insulation above the deck. Then he can put whatever he wants in the cavity.
    Roll-roofing has a pretty short expected lifespan anyway, and his concern about leaks is valid, especially with this type of roofing. Put on a 45 or 60 mil EPDM roof and forget about it.
    If it costs too much to do it right, then maybe it should not be done.

  4. User avater
    Nils Bird | | #4

    I've been toying with this idea for flat roof insulation which will be a bit more expensive but avoids foam of any kind. On top of your roll roofing add several layers of comfortboard, (rockwool: resistance to compression 943 psf), OSB and then some more roofing. If any water does make it through the two layers of roofing it will be visible from the inside and dry to the inside. Should there be water that condenses inside the rockwool it shouldn't reduce the insulation value of the rockwool. Also any moisture in the rockwool could drain harmlessly off your roll roofing.

    1. User avater GBA Editor
      Martin Holladay | | #5

      For more information on that approach, see this article: "Building a Foam-Free House."

      In that article, I wrote:

      "Creating a foam-free unvented cathedral ceiling is challenging. The only approach that I can think of is to install a thick layer of semi-rigid mineral wool insulation above the roof sheathing, followed by another layer of roof sheathing. The mineral wool insulation would need to be thick enough to keep the lower layer of roof sheathing above the dew point during the winter.

      "While it’s fairly common to install semi-rigid mineral wool insulation above roof sheathing, the method is usually restricted to low-slope (flat) roofs. Using this method for sloped roofs would be considered experimental, so builders should consult an engineer before proceeding with this technique."

  5. Thompson Johnson Woodworks | | #6

    This comment is directed at the interior picture of the roof framing. I am not a structural engineer, just a 20 year carpenter. None of the rafters are continuous! Before you proceed with insulation work, that roof system should be evaluated. Even though it is NJ with very low snow loads; it appears, via this one photo, to need structural attention.

    Good luck with your insulation quest! You have a very common situation in climate zones 5+. There is not a magic solution, only the right one, which is expensive. Fur down the rafters for more R-value (after evaluation), air seal/insulate, and control the vapor diffusion. Or insulate partially on the exterior side of the roof sheathing, with an air control layer, insulate in the rafters bays, control the vapor diffusion, and re-work the fascia/soffit/etc. Not cheap, but needs to be done right either way.

    Also, it looks likely there is a dirt crawlspace below the three season room. The water vapor loads from the ground will definitely need to addressed down there as well. (I'm assuming the floor system or skirting walls are currently insulated or will be as part of this project.)

    1. User avater
      Elden Lindamood | | #7

      I thought about the structural implications as well. My thought was that it is unconditioned space now, so cold, and snow likely doesn't melt off quickly. Insulating a roof will certainly increase the snow load if you are increasing the insulation in a heated space. But going from an unheated/uninsulated space to a heated/insulated space might not increase the snow load much. You are increasing the dead load of materials though. I agree that it is worth having a qualified structural engineer look at it to be certain.

  6. ryan_kohl | | #8

    Caveat first: Based on the framing alone, I'm not sure I'd even think about adding any weight at all to this roof, and certainly not taped gyp board. I'm not sure how much snow NJ gets, but here in MN I wouldn't even think of finishing a room with that existing framing. Were it 2x8 framing and actually bearing on the wall... ok maybe. THAT SAID...

    There are multiple ways to skin a cat in a case like this, with or without re-roofing (and honestly, a new single ply membrane roof would cost maybe $400-500 to install yourself, and its easy). If the guy is worried about spray foam IN the house, then put it ON the roof... spray polyurethane roofing is still a thing, its (arguably) the best insulation there is, and you'd install it right over the existing roll roofing. Add some wood blocking at the roof edge (for fascia) to match insulation thickness and coat the spray foam, and done. Feel weird about spray foam roofing? I would too, its pretty wonky and hard to repair.

    Soooo, then install the same perimeter wood blocking right over the roll roofing to match insulation thickness, 5" or so of polyiso. Screw it down and glue down a 45 mil EPDM membrane. Probably $1000 all put together for an R-49 roof in a 2x6 frame roof.

    Now, if you were REALLY resistant to adding a new roof, AND the framing were sufficient, you could do an IRMA roof (inverted roof membrane assembly), where the roof membrane is covered by moisture resistant insulation (ie extruded polystyrene) and weighed down by intermittent ballast (ie something heavy and durable like pavers). You'd have to add blocking at the parapets and roof it in to hold the insulation in place, and the drainage is usually tricky without using roof or scupper. I'm planning a second story addition and residing/re-insulating of the house that is hopefully going to include a roof-top patio, and i'd do it in inverted; framing, roof membrane, insulation, filter fabric, treated blocking and wood deck. Will start posting my story when the time comes...

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