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Our rigid foam sheathing is not thick enough?

user-6122944 | Posted in Energy Efficiency and Durability on

we are in zone 5 ( but just had a month of about -20C!), and are in the process of building a cottage. from the chart that i found thanks to a different question on this forum, i see that our rigid foam thickness is probably not enough. it is polysterene with aluminum foil backing, R5 value. R22 on the inside ( 2X6 construction). The foam is in place, windows and doors are installed, the siding is not on yet. how to mitigate?

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  1. GBA Editor
    Martin Holladay | | #1

    Are you planning to include a rainscreen gap? If so, have you installed your vertical furring strips yet?

    If you haven't yet installed any furring strips yet, you should probably add an additional layer of rigid foam with an R-value of R-2.5 or R-3. Then you can install vertical furring strips on the exterior side of the second layer of rigid foam.

    The tricky details concern the windows. How to ensure that your windows are properly flashed under these circumstances depends on the details of your window installation. Did you include flashing on the rough sill of your window rough openings? If so, you should describe the sill pan flashing.

  2. brendanalbano | | #2

    Will you have a rainscreen gap between your foam and your siding? If not, adding one will probably help things dry out when they get wet.

    A few more details will probably help others give better answers:

    - What type of siding?
    - What type of insulation in the 2x6 cavities?
    - Are the windows flashed to the outside face of the foam board, or two the sheathing?
    - Are you using a weather-resistive barrier (house wrap), or are you using the face of the foam as the drainage plane?
    - If you're comfortable sharing, a more precise location (city or zipcode or something) probably will help people five more specific advice, but it's not required.

    If you are able to time/budget-wise, there are probably clever ways to add another inch of insulation to the outside without having to redo things (maybe mineral wool might be a good choice if your flashing is already finished? or maybe just the same foam as the first layer?), but if not, you'll probably want to add some form of vapor retarder on the inside, like Certainteed Membrain, or a half-perm paint.

  3. user-6122944 | | #3

    type of siding: cedar shakes- was thinking of nailing right through the foam ( there is a 3/4" plywood backing)
    type of insulation in the 2x6 cavities; batting
    windows are flashed to the outside of the sheathing
    location: west kootenays
    hoping to use foam face as the drainage plane, no rainscreen gap

  4. GBA Editor
    Martin Holladay | | #4

    You can't nail cedar shakes directly to rigid foam. Some type of air gap is required.

    What material did you use to flash the rough window opening sills?

  5. GBA Editor
    Martin Holladay | | #5

    GBA readers,
    I had to look it up, so I might as well share what I learned. The West Kootenays are a region in southeasten British Columbia, Canada.

  6. AlanB4 | | #6

    I didn't know you were user-756436

  7. brendanalbano | | #7

    As I understand it, your windows being flashed to your sheathing is somewhat counter to your goals of using the face of the foam as the drainage plane.

    You might need to draw a detail of that connection to clarify exactly what you've already built in order to get the best advice as to how to proceed.

    A rainscreen gap is a good idea everywhere, a particularly good idea in places with lots of rain, and as Martin says, a good idea for cedar shakes. And aren't they a code requirement in BC? Or is that location specific? Anyways, all signs point to rainscreen ;)

  8. GBA Editor
    Martin Holladay | | #8

    Every week, a new web site glitch. Our current problems: newly registered GBA readers are being assigned the screen name "N/A N/A", while my screen name (at least for visitors who aren't GBA Prime members) now appears as "user-756436" instead of "Martin Holladay."

    Hopefully, these glitches can be fixed.

  9. Expert Member
    Dana Dorsett | | #9

    First, don't panic. A week of -20C overnight lows doesn't matter, it's the seasonal binned hourly mean temperature (not the lows) that determines how much wintertime moisture loads up in the sheathing.

    If you can't add another 3/4" of foam, half-perm interior latex primer (as Brendan suggested in #1) and air tight drywall interior will make it pretty moisture-safe from a vapor drive point of view, if somewhat less resilient to bulk water intrusions.

    Getting the bulk water management details such as window flashing perfect is the most critical thing to get right.

    (Any backcountry skier/ski mountaineer who likes it steep & deep would want to know about the Kootenays! Great snow, without that cheap low-oxygenated air that plagues the best skiing in the US Rockies. :-) )

  10. user-6122944 | | #10

    sorry, i am pretty kootenay-centric, should have clarified
    Dana, right on! we had an epic dump of snow in the last 48 hrs by the way. the passes, public transport and schools were closed. everyone headed to the hill instead :-)
    thank you everyone for your help. i will get the window flashing detail.
    we were planning airtight drywall and vapor resistant paint for interior and the flashing details will be good.
    Martin, we indeed wondered whether nailing through the rigid foam was ok, but thought the plywood backing would make it acceptable. what is the main concern?

  11. user-6122944 | | #11

    the cabin photo is attached

  12. user-6122944 | | #12

    window flashing- resisto membrane, the window opening is flashed to the foam, not the sheathing. the windows are siliconed onto the foam
    a rainscreen gap is code in coastal BC, but not in the kootenays- it's not quite as wet here.

  13. user-2310254 | | #13

    Tatiana. You will have a better and safer structure with an air gap behind the shakes. It encourages drying. See this article for more detail:

  14. cedarshim | | #14

    hi there, im a builder out here in Quebec,so im used to those minus 20c! two points i would like to make,
    1. why not a rain screen? for the relativity small investment (150/200 lengths of 12'furring box or two of nails and your labor) you would be getting all that extra and beautiful protection from that kootney climate, all that extra drying out gap between your foam and the back of your shingles no rot no moisture stains ventilation is so important. I have yet to build a house out here without one.
    2. Definitely you don t want to listen to anybody who is telling you to install cedar shingles or shakes directly on to the foam. Not sure how thick the foam is but if you have 1" or more just fastening the shingles wood be iffy. again its so important for the back of those shingles to breath. When i do shakes,i strap(install furring on the exterior wall vertically on each stud, and then strap across those horizontally spaced the distance of my shingle courses. that might be extreme, but definitely you need to install horizontal strapping on your walls to receive the shingles. I use a 1/2 inch crown Hitachi stapler and stainless steal staples(so you don t end up with black corrosion streaks all over your pretty shingles) to install the shingles . If installing shakes i would consider using ringed siding nails (again stainless)
    Your house has a great shape and is going to look top notch with the siding .Its a lot of work so you only want to do it once . Good luck.

  15. cedarshim | | #15

    Steven, if i had read your post first i could have saved my breath!

  16. user-6122944 | | #16

    Thank you for your help. I guess rain screen it is. but first,I'll add a layer of 3/4 to 1" foam. the challenge being the windows,especially since i want to side it with cedar shingles ,3/4" for the vertical, and another 3/4" for horizontal backing and about 1" of shingle thickness...

  17. GBA Editor
    Martin Holladay | | #17

    If you're lucky, you'll be able to unstick the outer edge of the Resisto peel-and-stick from the rigid foam at the window sills. Then maybe you can slide some metal flashing or some type of peel-and-stick product under the Resisto. This new metal flashing or peel-and-stick will need to lap over the new layer of rigid foam that you plan to install.

    -- Martin Holladay

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