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Community and Q&A

Venting a Roof With Exterior Rigid Foam

cconti | Posted in Green Building Techniques on

Hi all,

I’m building a garage in central Maine (Climate Zone 6) with an apartment above the garage (I’m going to insulate the entire building so the garage can be kept at least decently warm in the winter), and I’m trying to build it with energy efficiency in mind while still not breaking the bank.

Following the concept of “the perfect wall” as explained in Joseph Lstiburek’s BSC article here, I’m planning to use 2″ of rigid foam (R10) on the exterior of the entire structure (outside the sheathing, inside the siding & roofing), with fiberglass insulation in the stud and rafter cavities.

My impression was that one of the purposes of the insulation outside the sheathing was to prevent moisture from condensing onto the sheathing, leading to rot.

When I went to my trusty local lumberyard with my plans, my guy there suggested venting the attic space with soffit and ridge vents and rafter baffles (diagram image included). That sounded wrong to me, because it would let moist air in underneath the foam insulation… which would seem to defeat the entire purpose of having the insulation there in the first place, both from a moisture and from a thermal perspective.

Instead, I had thought I would leave the attic space unvented and airsealed, with an air barrier between the finished, conditioned living space and the unfinished, unconditioned attic space (that air barrier is gypsum drywall, taped, mudded, and painted w/ latex paint). (my idea is included in the diagram attached)

I was searching through other previous questions here on GBA for the answer to this question, and I found another thread in which Martin referred to this article, all about insulating sloped ceilings:

In that article it explicitly says that vented attics are incompatible with my insulation plan of foam outside and fiberglass inside. So it looks like my lumberyard guy was wrong.

Additionally, that article also says that in my climate zone (6), the minimum exterior foam to keep sheathing above the dew point is R25…. the two inches of foam I was planning to use is only R10.

What should I do? How should I build this?

Thank you in advance for all your help. I am not a professional builder, just an amateur who knows just enough to be dangerous…

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  1. GBA Editor
    Martin Holladay | | #1

    Two inches of rigid foam is too little to protect your roof sheathing in your climate zone. If you want to put rigid foam on the exterior of your roof sheathing, you will indeed need at least R-25 of rigid foam above your roof sheathing, and R-11.25 of rigid foam on the exterior side of your wall sheathing (assuming that you are using 2x6 studs).

    If you want to install vent channels on the exterior side of your rigid foam, you can. But you should never install vent channels between the rigid foam layer and the layer of air-permeable insulation on the interior side of the roof sheathing.

    For more information on this type of roof, see How to Install Rigid Foam On Top of Roof Sheathing.

    For more information on walls, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. cconti | | #2


    Thank you for your reply. Given that it looks like I would need 5 inches of rigid foam on the exterior of the roof, do you think it would be better to simply forego the rigid foam on the roof altogether, and insulate appropriately using fiberglass only, using a vented attic space?

    Thank you for your help!

  3. Expert Member
    Dana Dorsett | | #3

    It's a lot cheaper to just build 18" taller (or use 18'' energy-heel trusses) to accommodate R50-R60 cellulose and have a fully vented roof than it is to insulate at the roof deck in a zone 6 climate. In zone 6 at least 50% of the total roof R has to be above the roof deck for dew point control.

    Rigid EPS or polyiso runs about 10 cents per R per square foot, so that R25 is going to run you about $2.50 per square foot, plus whatever you pay for the fiberglass between the rafters. Cellulose about 3 cents per R-foot, so even if you went to R60 the ENTIRE cellulose job would be less than just the foam for the insulated roof deck.

    Foam-overs make a lot of sense for retrofits in existing houses with inadequate space for R50 attic-floor insulation. But this is new construction- plan ahead and you'll come out ahead.

  4. cconti | | #4

    Hi Dana
    Thank you for your help. My problem is that I don't think I can use a truss (unless it's a very bizarrely shaped truss that I'm not familiar with!) because my living space is essentially a very large "room-in-attic" that I've accomplished using short exterior walls with a rafter-framed roof above... here's the front elevation.

    I'd be very curious to hear your thoughts or advice!
    Thank you!

  5. Expert Member
    Dana Dorsett | | #5

    The cheapest way to do a foam-over approach is using reclaimed roofing foam from commercial re-roofing/demolition, which in my neighborhood is almost as cheap as batts, typically 1/3 the cost of virgin stock roofing foam. ($15-20 for a 4' x 8' sheet of 3" polyiso instead of $55-60.) With 2x6 rafters and R23 rock wool and six inches- two layers of 3" or three layers of 2" roofing polyiso (R30-ish, after cold climate derating) you'd be there in an assembly no thicker than a typical 2x12 / R38 type cathedral ceiling and would have plenty of dew point margin.

    My go-to local sources are Nationwide Foam in Framingham MA, and Green Insulation Group in Worcester. Both almost always have some supply of 2" & 3" roofing iso, but sometimes you have to wait. It's easiest to deal them if you have your own truck, call in to see what they have in stock, condition & price, have them hold the amount you need, and go get it. Nationwide will ship (for a price) if the quantities are big enough, but I'm not sure if that makes sense depending on how much you need.

    You might be able to find a source of used foam more local to you, eg: (some of these folks are asking sky high prices for used-goods, but you can always feel free to dicker.)

  6. Expert Member
    Michael Maines | | #6

    Chris, you can frame that roof with trusses. The bottom part of the truss will be "parallel chord" and the upper portion will be more of a conventional truss. Vent and insulate it like your lumberyard suggested in your first picture, minus the foam.

    Ideally, the insulation baffles should be air-sealed, especially if you go forward with fiberglass insulation. I would use cellulose, especially as you are also in Maine and there are plenty of qualified installers.

  7. GBA Editor
    Martin Holladay | | #7

    There are lots of ways to go forward with this plan. You can frame the roof with rafters or with trusses.

    The insulated sloping roof assemblies can be vented or unvented.

    The insulation system can include rigid foam or just cellulose. (If you just use cellulose, you will need a vented assembly.)

    All of these options are explained in my article, How to Build an Insulated Cathedral Ceiling.

  8. Irishjake | | #8


    I'd echo Dana's comments about Insulation Depot and reclaimed foam insulation. Dave Volpe @ Insulation Depot can get you almost any kind of rigid foam insulation, with notice. If you buy in volume, you can save thousands and thousand of dollars. My home has R-70 roof, R-50 walls, R 38 below grade walls, and R-22 below slab (HERS index of 2 with my PV). I probably saved $40,000 buying reclaimed insulation. I was able to insulate my entire house and detached barn/garage (with apartment above) for less than $12,500.

    FYI - You'll be hard pressed to find an engineer to sign off on more than 4" of exterior continuous rigid foam on the roof in ME, NH, or VT, without putting some kind of "over-roof" in to carry the loads. I'm in central NH and have an 85lb snow load, which makes it very hard to find a screw to attach all that insulation which will pass - if you are building to code that is......

    Yes BSC has written articles about roofs that have 6"+ foam on them, and yes they had 8+ ft of snow last year. To paraphrase a response I got, when I asked about such a roof via email - "....they are also engineers and architects and were comfortable with the design.".

    Good Luck!

  9. cconti | | #9

    Hi Brad,

    Thank you, that's really helpful to know about getting reclaimed foam at lower prices (it astonishes me how much a 4x8x2 sheet of foam costs new, even when buying in bulk!).

    At this point given that we'd need at least five inches of foam to make our dew point rating, it probably doesn't make sense to go the unvented, foam-on-outside approach with this roof/ceiling. Instead, now I'm thinking about going a more traditional, vented, insulation-on-the-inside approach and using dense-pack cellulose (at R-4 per inch, the dense-pack cellulose ought to get us to R-42 or so with vented 2x12 rafters. And while R-42 isn't great, it satisfies my code officer and seems like the best we're going to be able to do without drastically increasing the cost of the building.

    That said though, if anyone has any ideas for how we can get to higher than R-42 without significant added expense, I would be very, very eager to hear them.

  10. GBA Editor
    Martin Holladay | | #10

    If you are planning a vented roof assembly with cellulose insulation between the rafters, you can improve the R-value of the assembly (and reduce thermal bridging through the rafters) by installing a continuous layer of rigid foam on the interior side of the rafters.

  11. cconti | | #11


    That is an excellent idea, thank you!

    The only question then, is how to hang the drywall board on the sloped portion of the ceiling, where the drywall will be in direct contact with the foam board… maybe just use extra long drywall screws?

    Also, what is the best way to attach the foam board to the underside of the rafters? Adhesive caulk? Screws with large washers?

    Many thanks!!!

  12. GBA Editor
    Martin Holladay | | #12

    Rigid foam is attached with cap nails. Cap nails are available at any lumberyard.

    The drywall is either (a) fastened with long screws through the foam, or (b) fastened to 1x3 or 1x4 furring strips, 16 inches on center. The furring strips are screwed to the rafters through the foam.

  13. cconti | | #13

    Got it, many thanks!

    Given all the help and advice I've received here, I think I'm going to go with roof assembly that looks like the following:
    2x12 rafters with Advantech sheathing, a moisture barrier, and screw-down metal panels. Vent baffles in the rafter cavities, a ridge vent, 2 inches of XPS on the inside (lower) side of the rafters, with blown-in dense-pack cellulose in the cavities. If my math is correct, that should give me an R-52 roof, which I'm happy with.

    My walls are going to be 2x6 studs, Advantech sheathing, moisture barrier, 2" XPS, vertical strapping/furring strips, and LP SmartSide lap siding, with either faced fiberglass batts or blown-in cellulose or fiberglass (haven't decided yet…) in the cavities.

  14. Expert Member
    Dana Dorsett | | #14

    Chris- Can I convince you to use polyisocyanurate instead of XPS on the interior side, at least for the vented roof?

    Foil faced polyiso is much easier to air seal than XPS since you can use 1.5" -2" purpose-made decent quality aluminium tape (eg Nashua 324a, sold at most box stores.)

    Foil faced polyiso is far more fire-resistant than XPS, with a higher kindling temp, and it chars in place rather than dripping burning liquids even while burning. (Fire-rated Dow Thermax polyiso doesn't even need an interior side thermal barrier to meet code in many locations.)

    Polyiso is usually substantially cheaper per R than XPS (about 9-10 cents per R per square foot vs. 13-14 cents/R-ft^2).

    Polyiso's pentane blowing agent is far more benign than the HFC134a used in all XPS made in the US. (Just the 100 year global warming potential aspect is telling: Pentane @ 7x CO2, HFC134a @ ~1400x CO2.)

    When installed on the interior side, polyiso's fully-depleted R-value is about R6/inch, whereas after 50 years XPS will have dropped to about R4/inch, after it's blowing agent has escaped, doing it's climate-damage.

    On the walls, EPS is both cheaper & greener. After the XPS loses it's blowing agent it's performance will be identical to EPS of equal thickness & density.

  15. cconti | | #15

    Hi Dana,

    Huh, interesting!
    The reason I had planned to use XPS over polyiso is that I had read (on this site and others) that polyiso's R-value performance drops in cold climates whereas that of XPS/EPS actually rises (where I'm building gets very cold in the winters!).

    You certainly do make a compelling case for polyiso…

    I guess my question then is, if all you said is true, why would anyone use XPS???

  16. Expert Member
    Dana Dorsett | | #16

    XPS (& EPS) will outperform polyiso if it's on the exterior, where it's a lot colder. When it's on the exterior the immediate hazard to building occupants from a flaming point of view is also somewhat lower.

    My presumption was that "... so the garage can be kept at least decently warm in the winter" the interior will be kept well-above freezing high 40s to low 50F, maybe higher. Or did you mean in the 20s rather than single digits F?

    The thermal conductivity of polyiso is about the same as XPS when the average temp through the foam is +10C or +50F at least as tested from one polyiso manufacturer:

    So when the garage interior is in the mid-50s or 60s polyiso sample in that test should be performing near it's absolute peak, and above it's tested-rated R-value, though it'll suck pond water when the interior of the garage is significantly below 0C / 32F.

    But performance varies by vendor, and the vendors don't publish nice charts of how performance varies with temp:

  17. cconti | | #17

    Ah ha, I understand.

    I think what will actually end up happening with the garage during the winters is that it will be heated while we're there (the property is a lake house), but when we're not there the plumbing will be drained and the heat turned off, so it will be allowed to drop well below freezing in between stays, then when we come back we'll heat the place back up and keep it above 50°F for the duration of our stay.

    (Or who knows... if the building insulation proves to be effective enough that it doesn't take too much energy to keep it above freezing in between stays, maybe we'll keep the heat on, at 45°F or so?)

    But either way, your point about the rigid foam in the roof being INSIDE of the cavity insulation, and therefore warmer, is very well taken.

    And as my friends all say, I am nothing if not cheap, so if polyiso has all those benefits you listed AND is cheaper than XPS, that makes me a happy guy (I've already started contacting various reclaimed/factory second foam distributors!).

  18. Expert Member
    Dana Dorsett | | #18

    From the foam reclaimers the prices will be all over the place- hard to say which will come in cheaper. Virgin stock, polyiso is cheaper (almost as a rule).

    IIRC Green Insulation Group usually has a stock of polyiso factory-seconds (usually dinged facers or corners) they get from one of the regional foam fabs.

  19. GBA Editor
    Martin Holladay | | #19

    Q. "Why would anyone use XPS?"

    A. Green builders -- or builders who seek out advice on green building web sites -- try their best never to use XPS.

  20. cconti | | #20

    I guess I had always thought of "green building" strictly in terms of building energy-efficient homes in order to minimize the environmental impact of the home.

    It never really dawned on me (although in hindsight it should have) that minimizing environmental impact doesn't just mean considering how much energy the home uses, but also means considering the selection of the building materials themselves.

    Lesson learned.

    That said though, XPS foam is very popular in the building industry, so it must have certain benefits over polyiso (at least, as far as the less-environmentally-aware crowd is concerned).

    I'd love to hear the thoughts of those more experienced and knowledgable than myself!

  21. cconti | | #21

    After doing a little Googling on the subject, maybe the most succinct answer I found to my question above was in this YouTube video:

    In that video, they basically lay out two benefits of XPS over polyiso: 1) moisture resistance (apparently polyiso is more vulnerable to moisture and the Dow guy says it's not suitable for environments where it might encounter moisture) and 2) toughness (the builder guy says he's had issues with polyiso panels being damaged on the job site, whereas XPS tougher).

    I'm not terribly concerned about the toughness of the material… I can be careful enough not to damage the polyiso.

    As far as the moisture issue, in my application I'm going to have foam in two places: on the outside of the wall sheathing, under the siding, with an air gap, and deep inside the roof assembly, inside the sheathing and inside the cellulose. I don't think moisture will be an issue deep inside the roof assembly (if it is I've done something really wrong!), but the wall assembly looks like it very likely would allow moisture to contact the foam (in fact I think the air gap is designed intentionally to do so!).

    So it sounds like polyiso could work very well in my roof assembly, but it might be a better plan to stick with XPS in the wall assembly.

    Martin, Dana, would you agree?

  22. GBA Editor
    Martin Holladay | | #22

    Here on GBA, we advise the use of either EPS or polyiso for walls and roofs that require rigid foam. We usually don't advise the use of XPS because the blowing agents used to manufacture the XPS injure our planet's atmosphere.

    Either EPS or polyiso will work just fine for your wall assembly or roof assembly.

  23. guillow | | #23

    Seeing if I can revive this question as it has some great information. I was curious as to why 2" on polyiso on exterior of sheathing while using a vented assembly beneath is a bad idea? The polyiso would be for thermal break for the rafters. Putting them on the outside would allow for better internal drying than putting the at the end of the rafters no?

    (edited for clarity)

  24. guillow | | #24

    To clarify, the context is zone 6.

  25. MichaelAndrew | | #25

    Martin I saw where you mentioned not to use foil faced foam board as a ventilation channel? I have a gambrel roof and in sections it's tight against the side roof. I was going to do a 1"-1.5" air channel to allow air to reach the ridge vent. I was going to use rigid foam along the inside of the roof rafters then put a piece of rigid foam on top of those spaning inside each rafter/wall cavity. then fill with rigid foam due to it's high R value per inch.. also was going to use the same rigid foam as a baffle vent in the upper attic section with the foil side facing roof deck with 1.5" gap. do you have a different recommendation? do you not believe in radiant barriers?

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