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R-25 exterior rigid foam in Maryland, USA

James Eric | Posted in Green Building Techniques on

I was given 50 sheets of 4×8 R-25 rigid foil faced foam that was left over from a commercial building project. I also have 35 sheets of 4×8 R-10 blue board. I live in Maryland just outside of Washington, DC. I plan to add an addition on to the house this summer. The existing house is all masonry with no insulation. I would like to use the 2″ R-10 board to cover the existing brick section and them add cement siding and new windows,

How can I use the R-25 in the new addition? I would like to use it under the siding and on the roof but I am not sure if it is a good choice in my region. Can I leave the stub bays empty? Do I need house wrap, etc? Please point me in the right direction.

Thanks,

James

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    James,
    You mentioned "stud bays," so I'm guessing that you may be mistaken when you describe your house as "all masonry."

    There are two possibilities: either you have a house with wood-framed walls and brick veneer cladding, or you have all-masonry walls (for example, concrete block walls faced with brick).

    If you have wood-framed walls with brick veneer, then you don't really want to install insulation on the exterior of the bricks unless you can stop air from flowing between the bricks and the wall sheathing. Such walls usually have weep holes at the bottom -- sometimes, but not always, large enough openings to allow for significant air flow -- and sometimes have an air exit path at the top.

    If you have all-masonry walls, they will be easier to insulate. In this case, you would want to attach the rigid foam to the masonry with special plastic fasteners or adhesive, and then install vertical furring strips using long TapCon screws, followed by siding.

    For information on the types of plastic fasteners used to attach rigid foam to a concrete or masonry wall, see How to Insulate a Basement Wall. (Look for the paragraph with the heading that reads, "Briefly, how are basement walls insulated on the interior?")

    For information on vertical furring strips and siding, see How to Install Rigid Foam Sheathing.

  2. Expert Member
    Dana Dorsett | | #2

    I'm assuming the addition is stud-framed, not necessarily the original house.

    If it's a solid masonry wall and not a cavity-wall or brick veneer (as described by Martin) you can put the 2" foam over the exterior brick using furring and 4" TapCons 24" o.c. or tighter (with fender washer to keep the furring from splitting) to be able to support the weight of the siding, and it would perform better than an insulated 2x4 studwall. You need to pay attention to window & door flashing details, and may need to cut in some metal flashing to make it drain properly, but it's do-able.

    It's possible to do it with a brick-veneer walls too, but the details get more complex. With rainscreened fiber cement and a couple inches of foam you cansometimes even fill the cavity with fiber insulation, (though low expansion injection foam or perlite may be "safer" from a moisture migration point of view) as well as insulating the stud bays. A lot rides on the wall stackup, roof overhangs, flashing details, and the foundation details.

    R25 foil faced foam would be fine under siding or roofing, and sufficient R for dew point control that you could then fill the rafter bays with cheap fiber (blown or batts) with no need for interior side vapor retarders. R15 over the roof deck is all it takes in a MD climate, but R25 gives you plenty of margin for derating if the foam is 4" polyiso (which will under-perform it's labeled-R in mid-winter in a sheathing application) rather than 6" EPS.

  3. James Eric | | #3

    Thanks for the replies. The original house is brick with 4" block and furring and plaster. The addition will be wood framing. If I use the R25 attached to the framing can leave the stud cavity empty and what about house wrap?

  4. GBA Editor
    Martin Holladay | | #4

    James,
    Q. "If I use the R-25 attached to the framing, can I leave the stud cavity empty and what about housewrap?"

    A. Since your existing house has no framing, I am assume that your latest question concerns your planned addition.

    If you want to insulate the walls of your addition with exterior rigid foam, there is no need to install any insulation between your studs (as long as the exterior foam is thick enough to meet your local code requirements for minimum R-value). The method is called the PERSIST method. For more information on the PERSIST method, see Getting Insulation Out of Your Walls and Ceilings.

  5. James Eric | | #5

    Thanks.

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