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Community and Q&A

Roofing and insulation on old house

dipolojarvi | Posted in Green Building Techniques on

I have an 1850s farmhouse I’m renovating. To insure good insulation, I had the attic rafters spray foamed with closed cell foam about four inches deep.

I’m trying to plan for an eventual re-roofing, and I wanted to know if there are any best practices I should keep in mind at that stage for eleminating any issues that might arise from the spray foaming. The old roof has a few layers of shingles on it.

Is there a recommended roofing type for this situation? Any ventilation issues I can helpfully prevent moving forward?



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  1. GBA Editor
    Martin Holladay | | #1

    You haven't told us your location or climate zone. But no matter where you live, you don't have enough insulation in your roof assembly.

    Four inches of closed-cell spray foam has an R-value of R-24 to R-26. That is less than the minimum R-value required by building codes. In Climate Zone 4 and colder climates, you need a minimum of R-49 in your roof (according to the 2012 IRC). In Climate Zones 2 and 3, you can get away with R-38. But you don't even have enough insulation for those warmer climate zones.

    If you are planning to install new roofing, you'll want to remove all of the existing asphalt shingles. At that point you could add one or more layers of rigid foam above your roof sheathing to improve the R-value of your roof assembly. (I don't usually recommend sandwiching roof sheathing between layers of foam insulation, but you have kind of painted yourself into a corner here.)

    Unless you live in a climate zone with ice dams, you don't want any ventilation channels in this type of roof. If you do decide to add ventilation channels, the ventilation belongs on the exterior side of the uppermost layer of insulation.

  2. dipolojarvi | | #2

    Hi Martin,

    Thanks. Here's more detail.

    I live in Maine, climate zone 5.

    I had an energy analysis done (blower door) and followed the advice of the agent in putting in the spray foam between the rafters. Did I make a mistake in doing so? When I redo the roof, should I remove the spray foam and start again?

    All your advice is much apprciated.


  3. dipolojarvi | | #3

    Here's a bit more relevant detail.

    The closed cell spray foam is sprayed between the rafters, leaving the edge of the rafter exposed. The inside of the walls of the attic is similarly sprayed. The "sill" area in the attic is completely sprayed to keep chimney effect air movement through the walls and up into the attic to a minimum.

    If I made a mistake in the first place by spraying in the foam, any suggestions for solving the problem would be most appraciated. The attic is actually a nice room that would make a fine bedroom. Perhaps I should be putting more insulation on the inside instead of between the sheathing and the new shingles?

    Is a metal roof a better choice?

    Thanks again,


  4. GBA Editor
    Martin Holladay | | #4

    Q. "I had an energy analysis done (blower door) and followed the advice of the agent in putting in the spray foam between the rafters. Did I make a mistake in doing so?"

    A. Not really a mistake. The spray foam definitely added useful R-value to your roof. It just wasn't enough to meet minimum code requirements. (In fact, it was only about half the R-value you need to meet minimum code requirements.) This is a persistent problem with spray foam contractors; for some background on the issue, see It’s OK to Skimp On Insulation, Icynene Says.

    Q. "When I redo the roof, should I remove the spray foam and start again?"

    A. No. But I would definitely add R-25 of rigid foam above your roof sheathing when you install new roofing. For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.

  5. dipolojarvi | | #5

    Hi Martin,

    What a relief! Is there a concern with rot of the sheathing if I put the new rigid foam over it?

    And is it possible to do this in stages? When I roofed my barn, they only did one side the first year to save on the budget.

    Thanks again, and thanks for this forum. You're providing a really wonderful resource.


  6. GBA Editor
    Martin Holladay | | #6

    Q. "Is there a concern with rot of the sheathing if I put the new rigid foam over it?"

    A. There is always a concern, but you need to look forward, not backward. Sandwiching roof sheathing between two relatively impermeable foam layers is not ideal. In your case, however, it's probably the best option. (Unless you want to add more insulation on the interior side of your roof assembly -- which is certainly possible. The disadvantage of this approach is that you lose ceiling height.)

    The fact of the matter is, if you have closed-cell spray foam on the underside of your roof sheathing, and vapor-impermeable roofing (for example, asphalt shingles) above your roof sheathing, your roof sheathing isn't going to dry out in either direction, no matter what you do.

    If you add rigid foam above your existing roof sheathing, make sure that you do it on a dry, sunny day. You don't want to add rigid foam when the sheathing is damp.

    Q. "Is it possible to do this in stages? When I roofed my barn, they only did one side the first year to save on the budget."

    A. Yes, you can do one side of the house at a time.

  7. Expert Member
    Dana Dorsett | | #7

    In a zone 4 location with ~R24 of closed cell foam on the underside of the roof deck you can install up to R36 in fiber insulation on the interior side of the closed cell foam without risk of wintertime moisture accumulation at the foam/fiber interface. You only need another R25 to hit code minimum. If you have as much as 5.5-6" of depth left to the rafters you can get there with rock wool or dense packed fiberglass, usually for less money than rigid foam above the roof deck.

    Even if you don't have the full depth for another R25, filling whatever depth remains between the rafters with fiber insulation is still the right thing to do from a fire hazard point of view (an empty channel behind gypsum board is a fire spread path), and reduces the thermal bridging of the rafters themselves. It also and reduces the thickness & expense of the exterior foam layers required to get to code minimum performance (or higher).

    How much rafter depth do you have left?

  8. dipolojarvi | | #8


    Thanks for your suggestions.

    I'm in zone 5.

    The rafters have been pretty well filled with spray foam. Couldn't I put more foam over the rafters themselves or build them out a bit?


  9. GBA Editor
    Martin Holladay | | #9

    Q. "Couldn't I put more foam over the rafters themselves or build them out a bit?"

    A. Yes. As long as you don't mind losing ceiling height, you can add more insulation on the interior side of your roof assembly.

  10. Dana1 | | #10

    Sorry about the typo- the parameters for how much fiber insulation is still moisture safe were indeed for zone 5, not zone 4.

    If you only have an inch or so rafter depth, it sounds like the rafters are probably square-ish beams on some non-standard spacing?

    Unless the spacing is not 24" or 16" on center (or even if they are) rather than adding on to the existing rafter depths, install some 2x6 perpendicular to the rafters on a 16" or 24" on center spacing. You can then fill up the 6-7" depth completely with open cell foam, damp sprayed cellulose, or damp sprayed fiberglass (JM Spider) and and meet code minimum R, and still have plenty of dew point margin at the interior face of the 4" closed cell. The new framing would also be on a spacing consistent for mounting standard sized sheets of gypsum board.

    You could also blow dry fiber at higher density in netting, but that's usually more expensive than damp-sprayed. Cheaper still (but possibly lower performance) would be to fill the shallow depth left between the existing rafters with split batts, then install R23 rock wool or R20-R21 fiberglass batts between the 2x6s. The key to a batt approach is to leave no voids. Air gaps between the 2x6 batts and the closed cell foam become a thermal by pass convection channel, which can reduce performance dramatically.

    Adding 4 more inches of closed cell foam could get you there too, but it would have a much heavier environmental footprint than 6" of open cell foam, due to the much higher polymer content and the extremely higher impact of the HFC245fa blowing agent compared to the water that is used for blowing open cell foam. Any fiber insulation would be even lower impact than open cell foam.

  11. whstrain | | #11

    In zone 4, I have a 1940s Cape Cod with 2-3" fiberglass batt under the decking that needs a new roof. I'm trying to keep the foam inches outside the sheathing as small as possible to change the look of the structure as little as possible and so am considering foam under and over the sheathing.

    Martin, would your disadvisement for foam on both sides of the sheathing apply is the spray foam between the rafters is open cell?

  12. GBA Editor
    Martin Holladay | | #12

    The best approach is to follow the advice in this article: How to Install Rigid Foam On Top of Roof Sheathing. In Climate Zone 4, you want to install rigid foam insulation with a minimum R-value of R-10 above the roof sheathing. You can do that with 2 inches of polyiso -- so the rigid foam won't be ridiculously thick.

    I don't advise going forward with less than R-10 of exterior rigid foam (assuming you are adding any exterior rigid foam at all).

    If there are valid reasons why you don't want to install R-10 of exterior rigid foam, the alternative approach is to install at least R-10 of closed-cell spray foam on the underside of the roof sheathing (without installing any rigid foam anywhere). That isn't as good as the rigid foam approach (because it doesn't address thermal bridging through the rafters), but it will work.

  13. whstrain | | #13

    Ilmari, you might consider some concepts from this article and apply to your situation. Particularly the section just below photgraph 9. Basicly, make sure you have a tiny air gap between the sheathing and the waterproof membrane on top of the sheathing.

    Martin, Your article directly calls out the things I have been considering. I was going to attempt to upgrade the roof to R49 from the 2012 IECC for zone 4. I am also planning to cover the sheathing with a stick-down waterproof membrane before the polyiso foam layer to double as an air barrier.I understand that 1/3 of the insulation should be on the outside of the sheathing. The idealized calculations for R38 and R49 lead me to understand that the minimum amount of foam I'm going to have outside is 2" for R38 and 3" for R49 (R38= R26.22 (69%) inside+ R11.78 (31%) outside & R49= R33.81 (69%) inside+ R15.19 (31%) outside. Now considering my 5.5" rater depth I need to get R6.15 /in to get to R49. Which means I either need to spray permeable foam and don't fill the whole depth, or blow in something else like fiberglass and maybe take the top side to 4". The current rafter bays are either finished or backed by chicken wire so I'll be doing it from the top side after the tear-off either way.

    Edit: I realized I had not included a question: Is the permeability of open cell foam enough to allay concerns about using foam on both sides of the sheathing?

  14. GBA Editor
    Martin Holladay | | #14

    Q. "Is the permeability of open-cell foam enough to allay concerns about using foam on both sides of the sheathing?"

    A. Yes. But cellulose is usually cheaper than open-cell spray foam.

  15. Expert Member
    Dana Dorsett | | #15

    Dense packed cellulose blown in netting on sloped roofs often turns out to be more expensive than open cell foam. Damp-sprayed mid-density is usually cheaper, so it depends- get quotes.

  16. dipolojarvi | | #16

    Two questions:

    If putting rigid foam on the exterior, should I have any concerns about the underlying sheething, and would venting prevent them?

    If trying to increase the interior R-value, could I use roxul instead of rigid foam and plaster over it with lath?



  17. GBA Editor
    Martin Holladay | | #17

    If you want to add ventilation channels on the exterior side of your roof sheathing (as part of a re-roofing project), you can. The usual way to do this is to install 2x4s on the flat, either 16 inches on center or 24 inches on center, from soffit to ridge, thereby creating 1.5-inch-deep ventilation channels.

    If you decide to do this, you won't be installing any rigid foam on the exterior side of your roof sheathing -- because these ventilation channels are designed to keep your sheathing dry.

    If you decide to install rigid foam above your roof sheathing, then your roof sheathing will be encapsulated with foam on both sides, so it isn't going to dry in either direction. As I wrote in previous comments, that isn't ideal, but it's acceptable. (You may still want to install ventilation channels above your rigid foam, in order to have a "cold roof" with the smallest possible chance of ice damming -- but in that case the ventilation channels won't be helping your sheathing to stay dry.)

    Adding additional interior insulation is also an option, as Dana and I have both written in previous comments. You can install Roxul mineral wool on the interior side of the existing spray foam if you want.

  18. dipolojarvi | | #18

    Hi, Guys, I find myself in the odd situation of having asked great questions a few years ago that I forgot to refer to when I finally did the roof job.. I'm going through all my old questions based on a problem that has now occurred. I've had a new roof put on over the old one. They laid down new OSB and synthetic underlayments. I've got a strange off gassing smell now in the attics and am wondering about how best to vent from the interior. Thanks.

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