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Community and Q&A

Insulating underneath wood shingles underneath asphalt shingles

drewbug | Posted in GBA Pro Help on


I’ve done my research and read quite a few relevant posts and articles here, but I couldn’t find an exact match for my situation.

Legend has it that my home was built in 1876 in Philadelphia and moved here to Harrisburg via barge sometime after that. I’m in Climate Zone 5.

My wife and I are looking to build an insulated cathedral ceiling. Yes, I’ve read the wonderful GBA article 🙂

I’m inclined to first have 3 1/2 inches of closed-cell spray foam installed, then blow-in a layer of cellulose below that. The rafters are only at most 5 inches deep, so I understand that this would require attaching new deeper rafters.

However, this is all complicated by two things:

1. Looking up from the interior I see skip sheathing and (in the gaps) what I believe are wood shingles. Outside, I see asphalt shingles. I don’t know if there is anything between the asphalt shingles and the wood shingles, and I’m not sure how best to safely check.
2. We would like to switch to a metal roof at some point in the future, so I’d like to avoid doing anything that would make that harder on us.

I have a feeling rigid foam insulation might form part of the solution here.

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  1. GBA Editor
    Martin Holladay | | #1

    It's very common for older houses to have wood shingles on skip sheathing, with asphalt shingles installed over the wood shingles. In most cases, the roofer who installed the asphalt shingles probably installed a layer of plywood roof sheathing above the wood shingles. (This is usually necessary to ensure that there will be solid nailing for the asphalt shingles.)

    Clues to the presence or absence of plywood sheathing can sometimes be determined by looking for penetrating nails in the attic. The thin galvanized 4d nails used for shingles look different from the larger diameter 6d or 8d nails used for plywood sheathing.

    If the penetrating nails don't provide a clue, take a screwdriver up there and a good flashlight. Choose an area between the skip sheathing, and carefully pry (split) the wood shingles. Make splinters. Pull out the old splintered shingles. Do this work without penetrating the roofing. Pull out the splinters with pliers. See what you find above the wood shingles.

    All of this said, there is no reason you can't install spray foam on the underside of your skip sheathing. Nor is there any reason that you can't install metal roofing later. When you hire a roofer to install the metal roofing, the roofer will evaluate the existing sheathing from above, and will advise you whether you need new plywood, new OSB, or perhaps just 1x4 or 2x4 purlins, or even nothing (if there is sound plywood up there already).

  2. Dana1 | | #2

    It might be cheaper & easier (and greener) to add 4" of rigid foam above the roof deck, under the metal roofing, and install cellulose between the rafters, as opposed to deepening the rafters and installing the 3.5" of closed cell polyurethane (which would run ~$3.50 per square foot, to 4" polyisocyanurate at ~$2.50 per square foot.)

    If you are doing this in stages with years between the initial installation and the re-roofing, you can protect the sheathing with as little as 1" of closed cell foam (about a buck a square foot), with cellulose for the rest, and protect the cellulose from excessive moisture cycling with 2 mil nylon "smart" vapor retarder (eg Certainteed MemBrain) on the interior side for about 15 cents per square foot. An inch of ccSPF is R6-ish, between 0.8--1.2 perms , the boundary of Class-II vapor retardency, and the remaining 4" for the cellulose would add up to about R15 for a total of R21, with 28.6% of the total-R on the exterior side of the fiber layer. With the additional thermal performance of the sheathing & shingles it's probably over 30% outside the inward side of the closed cell foam.

    In your climate zone (5A) it takes ~40% of the total R on the exterior for dew point control, but with the hygric bufffering capacity of cellulose you'd probably still be OK without the MemBrain. But installing the vapor retarder would be cheap insurance. It is relatively vapor open (10+ perms) when at high humidity, so if the cellulose were ever humid enough to need to dry toward the interior, it would. When the cellulose dry 2 mil nylon behaves a Class-II vapor retarder (less than 1 perm) dramatically impeding the rate uptake of moisture in winter. With ~30% of the total R exterior side of the foam/fiber interface there would be a limited number of winter hours dwelling below the dew point, but not few enough to be reliably safe without the vapor retarder. With the vapor retarder it's pretty safe.

    With 4" of polyiso above it would add up to R45 at center cavity (R41 with the polyiso derated for temperature), but it would still meet code on U-factor basis due to the R20+ thermal break over the rafters, and the additional thermal performance of the intervening layers of skip sheathing, cedar shingles & plywood or OSB. And even fully derated, the 4" polyiso would be above the 40% minimum needed for dew point control at the moisture susceptible wood layers.

  3. drewbug | | #3

    Thanks for all the information!

    As directed, I ripped apart a section of wood shingles between the skip sheathing. Photos attached, along with one of an unused shingle I discovered.

    I also peeked under the drip edge and I think I can see another layer of asphalt shingles beneath the top layer. My understanding is that this is pretty inconsequential aside from the additional tear-off work when the metal roof eventually goes on.

  4. GBA Editor
    Martin Holladay | | #4

    I'm not sure from the photos what you've learned (nor have I learned whether it matters). If you aren't a roofer, you may be gathering more information that you need (although it never hurts to know how many layers there are on your roof).

    Did you find any plywood or OSB sheathing?

    The basic answer to you question is: If you want to install closed-cell spray foam, you can. When it comes time to install metal roofing, trust your roofer.

  5. drewbug | | #5

    What I think I learned is that there is more board sheathing above the wood shingles. While I'm not a roofer, I am extremely motivated to learn and eventually do the re-roofing myself.

    I really like your hybrid solution, Dana. The metal roof is indeed still at least several years away. Is there any reason I'll be limited to 4 inches of exterior polyisocyanurate?

    You both have been so helpful, it's truly people like you that give me hope for the future of our world. Is there a better way than getting a GBA Prime membership for me to say thank you?

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