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Community and Q&A

Low-slope roof insulation questions

yoder | Posted in Energy Efficiency and Durability on

Q1. Can existing loose fill cellulose be dense-packed later?

Q2. On a vented low slope roof with loose fill cellulose on the attic floor, will rigid foam above the decking create issues?


I just read Martin H’s article on this (Insulating Low-Slope Residential Roofs) and live in a Philly row home. Our “attic” slopes from about 4′ to about 8″ between top of ceiling joists and the bottom of roof joists. We had a min. of 8″ of cellulose (r30) blown in( not dense packed) over minimal existing insulation and we have a couple mushroom vents on the roof. So it’s currently a vented low sloped roof but probably not adequately vented at the moment.

We’re planning to install solar pv and considering a roof tear-off and replacement with tpo. I understand they like to install tpo over rigid foam and I’d love to benefit from that insulation but I want to prenvent condensation conditions.

Would it make sense to dense pack the inaccessible part of the ceiling from above after the roof tear off so that portion of the roof would be unvented and benefit from the rigid insulation? Can that be done?

I’m thinking I should then just add venting on the high side of the attic and rely on the loose cellulose for insulation since Bruce Harley states in the article that he doesn’t advise dense packing an entire attic with significant changes in fill depths because it can be hard to dense pack the deeper sections. I don’t expect the rigid insulation to be of any benefit on the vented portion of the roof, would it create issues?

Anything else I’m missing?

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  1. GBA Editor
    Martin Holladay | | #1

    The only way you are going to get any benefit from exterior rigid foam is if you convert your roof to an unvented assembly. If you are introducing outdoor air into your attic, you aren't getting any benefit from the rigid foam.

    A well-designed unvented assembly in your climate zone (Zone 4) would require a minimum of R-15 of rigid foam above the roof sheathing, along with R-34 of fluffy insulation on the interior side of the roof sheathing (and in direct contact with the roof sheathing). To make that happen, you will need to block off your vents and move the fluffy insulation (at least on the "high side" of your attic) to a new location -- something that is easier to do with batts than with cellulose.

    I can't really come up with any good recommendations for a roof that is kinda, sorta, half-vented and kinda, sorta, half-unvented.

    For more information, see Combining Exterior Rigid Foam With Fluffy Insulation.

  2. yoder | | #2

    Thanks Martin,
    I'm trying to square your answer with this quote from Bruce in the article:

    Harley still endorses limited use of the dense-packing method, as long as a list of conditions is met: “For a common rowhouse in Chicago or Philadelphia, in climate zone 5, in a building with effective code-compliant venting of the attic space, we have seen pretty good results from an approach that includes dense-packing the lowest part of the the attic,” Harley said. “But never more than one-third of the total attic area."

    Isn't the dense packed area unvented ad the rest vented?

    I understand your prescription for the unvented roof as:
    1.installing r15 rigid on the exterior
    2. removing venting
    3. dense packing the inaccessible portion to ensure fluffy/sheathing contact
    4. removing existing cellulose where accessible and installing r34 fluffy in contact with sheathing

    Correct? I'd also need attic wall insulationn as well wouldn't I?

  3. GBA Editor
    Martin Holladay | | #3

    I've always been a little leery of the approach described by Bruce Harley, but you're right -- Harley is describing an attic that is half-vented and half-unvented. Proceed at your own risk.

    If you want to go 100% unvented -- as described in the 4 points in your most recent comment -- you can. If you want to do that, I would advise that you build a partition wall in the attic to divide the dense-packed section from the section with fiberglass batts installed against the roof sheathing.

  4. Expert Member
    Dana Dorsett | | #4

    In zone 4, adding 7" of rigid polyiso above the roof deck and under the TPO, and sealing up the roof vents, insulating the exterior walls, and just leaving the R30 in place would get you to code-min performance on a U-factor basis. That may be cheaper than screwing around with dense-packing part of the roof and not another, etc. Code max is U0.026 which is about R38.5 "whole assembly", which includes the performance of interior & exterior air films, roof decks, ceiling gypsum, etc.

    At 7" polyiso derated to R5.6/inch the foam layer is already R39.2.

    R39.2 > R38.5, code compliance is certain, Q.E.D.

    The roof deck, underlayment & TPO is good for another R1. The air films above the TPO & below the roof deck add another R1, so you'd surely be north of R40 whole-assembly, and well under U0.026 without having to rely on the R30 on the attic floor or the interior finish ceiling for anything.

    With R39 above the roof deck and R30 on the attic floor the temperature in the attic should average above the dew point of the conditioned space air, no need to remove any of it.

    You can probably get away with only 6" of polyiso (R33.6, derated), without fudging the performance numbers of the other layers in the stackup TOO much, if the inspectors will allow it, or maybe no fudging at all if the roof deck is 2x planking.

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