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Community and Q&A

Rigid Foam in Exterior Walls & Attic

Kartik Babu | Posted in Energy Efficiency and Durability on

Hello all,

I have to first thank all of the active members. The Q&A forums have been a trove of information as I gut and remodel my Cape Cod. I’m in Western PA in a zone 5 climate.

I recently purchased a foreclosed home that my wife and I are living in as we renovate. It is a 1965-built home with 2×4 stud walls and a finished attic with 2×6 (possibly 2×8) rafters. The home was vacant for at least one winter. Ice dams and water leakage have necessitated removal of the old fiberglass insulation and rotten framing around the windows and doors.

I have begun the process of stripping the siding, and replacing windows & and framing around the windows where it is rotted. I am considering Rigid Foam insulation in lieu of spray foam (mainly cost). I am considering the foil faced 2″ thick rigid foam from DOW, adding more layers to achieve the required R21 in the walls and R38 in the roof insulation.

My worry is if I need a vapor barrier stapled to the studs under the drywall? I will cut the foam to fit in the stud cavity (16″ O.C.) and seal edges with foam from a spray can. The manufacturer also recommends gap of 3/4″ between the drywall & , required to reflect radiant transmission.

First of all, is the 2″ or more foam board a sufficient vapor barrier?
Second, should I forget about venting the roof? The gable roof on this home (and several in my neighborhood) have no overhang and hence no soffit & ridge vent combinations. A gable vent exists, without a fan, so I fail to see how any air circulation takes place in the unconditioned portion of the attic. I would very much like to have a cathedral ceiling in the attic, so am considering eliminating the gable vent altogether.

I’ve read that spray foam directly on the roof sheathing does not adversely affect roof heat dissipation during the summer. But how does substituting spray with rigid foam affect this performance? Should I still keep a gap between the sheathing and the foam board and retain the gable vent? I could just raise the ceiling height in the attic and move the gable vent higher.

I’ll add drawings too if that will help. And provide feedback as the build progresses.

Thanks again in advance.

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  1. Jim Bannon | | #1

    in response to your first question, if you are using foil faced foam, the foil is a vapor barrier, regardless of the thickness of the foam. If you use the foil faced foam, you should NOT use a second vapor retarder. In general, poly sheeting between the studs and drywall is not advised in any case although some underinformed building inspectors are still looking for it.

    Regarding your attic and venting questions, it would be useful to know whether or not this will be conditioned space or not. Sounds like you are stil deciding that.

    If you are stripping the siding, this is a perfect opportunity to skin the exterior of the house with a minimum of 2" of rigid foam, then pump cellulose into your stud cavities. That would give you the a vastly superior wall system.

  2. Kartik Babu | | #2


    Thanks for the response. I have no idea how to put in my new windows if I were to add 2 inches to the exterior. If properly sealing the rigid foam once it is in the wall cavity will work, I would much prefer that method The iso foam boards I'm considering have a bonded foil layer on one side and poly on the other. I believe there is another version that has foil on both sides.

    The sheathing is 7/8 pine planks under really thick 3/32" asphalt paper WRB (celetex?) and is still solid except around the window openings. That asphalt stuff held up like a charm all these years. Whatever I take out will be replaced with the same or plywood. I plan on using Tyvek and a 5/8" rain screen separating the vinyl siding from the tyvek.

    As to my second question: See attached sketch.
    The attic currently houses two bedrooms and we will be adding a dormer to put in a second floor bathroom. When I bought the house, there was fibreglass in the rafter cavities. The empty space behind the knee wall upto the roof was on the insulated warm side. There was loose fibreglass above the ceiling drywall about 10" high,but settled in various spots to about 6". All the space above that all the way to ridge was uninsulated/unconditioned and vented by the gable vent.
    I want to make smart use of the space behind the knee walls for storage, so that space will have to remain conditioned.

    The ceilings on this floor are low (7'6"), and if not cathedral I want to at least raise them another 10-12". Which leaves only about 3 feet from the ceiling drywall till the ridge beam (It is a fairly steep roof). I can leave this space unconditioned.

    The sketch I've attached shows the current setup. (I've gutted one room on the 2nd floor already). I think I might keep this configuration, but raise the ceiling and collar ties. Having the space behind the knee walls conditioned will allow me to build storage into the walls. Right now each room only has a tiny closet.

    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    First of all, I'd like to put in another vote urging you to install a continuous layer of rigid foam on the outside of your sheathing. If you are stripping the siding, this is the ideal time to perform this work -- it is in fact a rare opportunity. The thermal performance benefits will be huge, because the foam will address air leakage and thermal bridging, and will reduce the chance of condensation that could damage your sheathing. If you cut up the foam into thin strips and insert them between the studs, you won't get much benefit from the foam, and each stud bay will need to be laboriously sealed at the edges with canned spray foam.

    If you don't know how to install windows in a foam-sheathed wall, you have two choices: hire a contractor who does, or learn how to do it right by studying these details on GBA. You could start with these two articles: Nailing Window Flanges Through Foam and ‘Innie’ Windows or ‘Outie’ Windows?

    If you want to insulate a sloped roof, you can either include a ventilation channel between the top of the insulation and the roof sheathing, or (if you do it right) you can omit the ventilation channel. It's your choice. More information here: Creating a Conditioned Attic.

  4. Kartik Babu | | #4

    Thanks Martin. Is there a need for Tyvek style homewrap under the exterior foam sheathing?

    I will be using the existing gutters and fascia, as these were newly installed about 2-3 years ago along with a new roof. I suppose z flashing is a good idea where the foam meets the 2x6 fascia board?

    I will provide some detail drawings for feedback.

  5. GBA Editor
    Martin Holladay | | #5

    Every wall needs a water-resistant barrier (WRB). You can either install housewrap as your WRB, or use the rigid foam as your WRB. If you use housewrap as your WRB, the housewrap can be installed under the foam or over the foam -- your choice.

    Here are two articles to help you through the decision process:
    Using Rigid Foam As a Water-Resistive Barrier

    Where Does the Housewrap Go?

    I hope your rigid foam is not proud of your fascia! That would be bad. If that's the case, you definitely need to extend your roof overhang.

  6. Kartik Babu | | #6

    I'll have to think out loud for a bit. The fascia is a 1-5/8" thick 2x6. I can downgrade to 3/4" foil faced polyiso foam, that will provide continous R5 layer and leave 7/8" on the fascia. If I add 1x4 strapping for a rain screen, and add 5/8" vinyl siding: that will leave the exterior cladding proud of the fascia by 3/8".

    I have thought about insulated vinyl siding, but then I would obviously have to omit the rain screen behind the siding. In addition, insulated EPS Vinyl siding only goes up R3. However, this may be one of those compromise solutions. What is the general opinion on insulated siding on this board?

    Tyvek makes DrainWrap which is supposed to aid water drainage, but I'm not sure if that adds a small air gap between the siding and the WRB. Wouldn't such a gap lower the temperature of the OSB, which is what we're trying to avoid in the first place?

  7. Bob Dazel, AIA, LEED GA | | #7

    Kartik: I may be a few days late to the dance here but I agree with much of what the other comments are suggesting. You will have far better performance results bu utilizing an exterior continuous insulation over the existing sheathing rather than spending all that money to get something else less effective into the cavity. Not knowing exactly what your final exterior wishes to look like but perhaps an Exterior Insulation and Finish System (EIFS) cladding with a traditional stucco or specialty finish like brick, limestone or granite would meet your final aesthetic needs. EIFS claddings offer everything you are looking for as a single sourced solution which include air / weather barrier, flashing, continuous insulation, durability, architectural detailing and pleasing aesthetic. If you would like additional information, I would be pleased to help.

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