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Community and Q&A

Add rigid foam during re-roof

Robbcz | Posted in Energy Efficiency and Durability on

Our home on a barrier island 30 miles south of Wilmington, NC (close to NC/SC border) needs to have its shingle roof replaced. We’re planning on going to a standing seam metal roof.

If we know we want to eventually condition the attic, should we add rigid foam on top of our sheathing during the re-roof? Or spray foam the attic later?

If rigid foam makes sense now, what is the recommended type and thickness for this application?

Other house details
Climate zone: 3 Coastal
Built in 1947 (survived Hurricane Hazel!)
Solid board roof sheathing. Not sure of lumber size, but it’s not spaced/skipped (per my contractor)
Current attic is vented (soffit and ridge) and is unconditioned
Blown cellulose in attic
Heat pump outside, air handler and ducts in attic
Tongue and groove ceilings throughout house (not well sealed from conditioned space to attic)

Please let me know what other info is needed for recommendations.


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  1. GBA Editor
    Martin Holladay | | #1

    If you plan to transform your vented unconditioned attic into an unvented conditioned attic, using exterior rigid foam is preferable to using spray foam between the rafters, because exterior rigid foam address thermal bridging through the rafters.

    You are in climate zone 3.

    If you plan to install rigid foam above your roof sheathing, you have two choices:

    Option 1: You can install all of the insulation above the roof sheathing (in which case the rigid foam has to meet minimum code requirements for ceiling R-value); or

    Option 2: You can install some of the insulation above the roof sheathing, and the rest of the insulation underneath the roof sheathing (and in direct contact with the roof sheathing).

    If you choose Option 1, you’ll need R-38 of rigid foam (about 10 or 11 inches of EPS, 8 inches of XPS, or 6.5 inches of polyiso).

    If you choose Option 2, the code dictates the minimum thickness of your rigid foam layer. In Climate Zone 3, you’ll need at least R-5 of rigid foam (about 1.5 inch of EPS, 1 inch of XPS, or 1 inch of polyiso). The rest of your insulation (enough to reach R-38) will need to be installed later between your rafters.

    All of this (and more) is explained in this article: How to Install Rigid Foam On Top of Roof Sheathing.

  2. wattsup | | #2


    Martin is right about the two options, but there is a third option.

    If the rafters are deep enough, you could add a 1"-2" trim where the rafters and the roof sheathing meet, then put 2" of rigid insulation onto the trims creating air channels going from the soffits to the ridge vent, then spray foam the remaining depth of the rafters. You could also extend the depth of the rafters to reach the required R value or leave the rafters as they are and then add on layer of insulation over the rafters to create a better thermal break.

    Some questions;

    1) What is the pitch of the roof(s)?
    2) What is the depth of the rafters?
    3) How much head room is there in the attic?
    4) What is the access limitation to the attic?

    Usually when you look to insulate the roof over, under or a mix of both, you will also need to insulate the rake walls from inside the attic.

  3. Robbcz | | #3

    Martin, Leon,
    Thanks very much for your feedback and my apologies for my delayed response. I haven’t been able to get back to the house since my original post so I can’t answer all of Leon’s questions:

    1) What is the pitch of the roof(s)?
    Walkable, I’d say 5/12
    2) What is the depth of the rafters?
    Not sure
    3) How much head room is there in the attic?
    It’s been about 2 yrs since I was last in it, but I think it’s only about 7ft at peak.
    4) What is the access limitation to the attic?
    Very limited… ~2’x2’ scuttle in closet

    Our GC’s roofer is proposing this 2” thick polyiso/nail base (R-9) under the new roof:

    Our GC recommends using spray foam inside the attic at a later date when we’re ready to condition the attic. There seems to be concern with putting too much rigid foam on the roof due the house being in a high-wind zone (house is second-row oceanfront). Anyone have experience with rigid foam on the roof of coastal houses?

  4. GBA Editor
    Martin Holladay | | #4

    It should be possible to install thicker nailbase (rigid foam plus OSB) on your roof if you want, even in a high-wind area. Talk to an engineer about fastening requirements.

    In any case, R-9 nailbase is enough in your climate zone, even if you decide later to install fluffy insulation between your rafters.

  5. Expert Member
    Dana Dorsett | | #5

    With only R9 nailbase it makes it difficult to hit the code-min R38 total unless you have 2x8 rafters or deeper. A lot of houses in 1947 had 2x4 or 2x6 rafters. With a 2x6 you could get up to about R23-R24-ish using rock wool batts (provided it fits the spaceing) or dense-packed fiberglass, but that would still leave you a bit shy of R38, but you'd be most of the way there. With 2x4 rafters the most you'd get is ~R15 in the rafter bays, in which case you'd want to put at least R18-R20 above the roof deck.

    With the thermally broken rafters and ~ R35-ish insulation you can get pretty close to code compliance on a U-factor basis. To meet IRC code on that basis you'd need a U-factor of 0.030 or less, after accounting for the thermal bridging, the thermal performance of the roof deck, the roofing materials themselves (which not much for a metal roof) and any interior sheathing layers such as wallboard.

    For code compliance you're not allowed to add the R-value of any attic floor insulation to the roof deck insulation when taking the sealed insulated roof deck approach, but in your climate it does no harm to leave it in place, with or without a full R38 at the roof deck, and will in fact do some good.

  6. Coopsdaddy | | #6

    Question for you guys about the venting option mentioned buy the op,you wouldn't need soffit to ridge vent if ridged foam was on the top and spray foam under right ?

  7. dburgoyne | | #7

    I'm in zone 3 inland and installed per Martin's Option 2, with rigid insulation above as well as batt insulation below. I know that I overinsulated for my zone, but wanted high efficiency. I installed 2 layers of 1 1/2" (3 inches total) polyiso lapping joints, and installed R-38 high density fiberglass insulation below in my rafter bays. I applied Grace ice and water shield over the entire top of my roof insulation, and then an added a radiant roof barrier (Sharkskin) on top of that. My roofing type is concrete tile, so I ran two layers of roof battens, and attached them with 6" long screws into the roof deck. This gave me a secure attachment to hold the roof insulation down tight to the roof, and the double battens created a ventilated space above the radiant membrane to vent below the roof tile from the eave to the ridge.

    Added benefit of the double batten system is that any moisture and debris that gets through my roof tile will easily drain and wash out at the eave reducing buildup on roofing membranes. I reroofed my entire house with this method 8 years ago and the added radiant barrier and ventilation greatly reduced my cooling bills.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Question for you guys about the venting option mentioned by the O.P.: You wouldn't need soffit-to-ridge venting if rigid foam was on the top and spray foam under, right?"

    A. It's possible to create an unvented roof assembly if you want. Usually, I would advise anyone who wants to build an unvented roof assembly to choose to put the foam insulation either above the roof sheathing (the preferred approach) or under the roof sheathing (using spray foam -- a distinctly second-rate approach compared to rigid foam above the roof sheathing) -- but not to put foam insulation in both locations.

    For more information on this issues, see How to Build an Insulated Cathedral Ceiling.

    1. quantumgirl | | #9

      I know this is a really old post, but Martin, does you’re statement not to use foam on both sides of the roof sheathing only apply to closed cell foam?
      My unvented cathedral roof (2x6 rafters) is insulated with 5.5” of OPEN cell spray foam which only gives me an R value of about 20. Required is 38 in my climate zone 2. We’re about to reroof the house (asphalt shingles). Can I put rigid foam on top of the osb sheathing to create a higher R value or would that create any kind of issue with moisture and vapor?

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