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slumping of siding from nailing through rigid foam

user-7005676 | Posted in Green Building Techniques on

Thanks for any assistance you can provide.

I am wanting to add additional insulation on the outside of our home. We are in zone 6. Our current walls are 2×6, 24 inch on center with 6 inches of fiberglass insulation, 7/16th OSB with tyvek. We have lived with cedar siding for 30 years and are now wanting to remove the siding and windows. Replace the windows and us LP siding.

I was looking at using 2 layers of 2 inch rigid insulation (staggered and taped) and 1×4 furring strips to hold the foam on. I understand there are two different LP SmartSide (76 and 38) siding for max 24″ on Center and Max 16″ on center, and I understand I should provide a minimum of R12 in rigid foam to prevent condensation (I used a poly vapor barrier)…sorry for the long explanation.

My question is, has anyone experience sagging of the siding because of the long nails through 4 inches of foam? Should I be concerned about slippage between the two layers of foam?

Thank you again for taking the time to reply.

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  1. Expert Member
  2. Expert Member
    Michael Maines | | #2

    Jeff, before you worry about sagging siding, I would worry about moisture accumulation in a wall with two vapor barriers. While it could be fine, and meets code, it is a vulnerable assembly. Although it costs more, I would recommend using rigid mineral wool on the exterior, to allow drying to the exterior. With mineral wool the R-value is not critical, as it allows the sheathing to dry freely to the exterior. (Though thicker is better, to keep the sheathing warm and dry.)

  3. Expert Member
    Dana Dorsett | | #3

    You don't long-nail through 4" of foam. The furring is attached to the studs with pancake head timber screws long enough to penetrate the studs by at least 1.5", and the siding gets nailed to the furring. With heavier siding ring-shank nails may be required for 1x furring, but with 2x furring it's not an issue.

    The maximum spacing of the pancake head timber screws depends on the heft of the siding and the foam thickness.

    Even though it's a moisture trap, as long as the sheathing & framing are dry (<<20% moisture content) when it's assembled, there isn't much risk to foaming over a wall built with interior side polyethylene. (There are tens of thousands of existence proofs in Canada.)

  4. Yupster | | #4

    "There isn't much risk" with the added caveat that water management details become much more important. A small flashing leak in a house I visited recently built with foam and interior poly caused a lot more problems than it normally would because the moisture had nowhere to go. So it stayed, swelled up the window jambs and the trim. I have no doubt when they pull the B&B siding off to fix the leak they will find OSB rot under the foam...similar problems without the double sided V.B. seem to take much longer to manifest, often never being a problem big enough to fix. Based on my own experiences, I'd say take extra care when using that assembly. It's not very forgiving.

  5. user-7005676 | | #5

    Thanks for all of the feedback, I down loaded a ton of information on proper nail length and spacing. I plan on using LP siding...recently found out that there are two sizes of lap siding, one used for 16" on-center and one for 24" on center, I'm 24" oc. I enjoy doing this kind of work, but find myself spending as much time planning and designing as doing.

  6. Expert Member
    Dana Dorsett | | #6

    It's better to spend as " much time planning and designing as doing..." than to spend as much time diagnosing FIXING as doing.

  7. KJGInMA | | #7

    I found this to be very helpful to understand why sagging (due to cantilever with multiple sheets of foam and long nails pulled by gravity) is usually not an issue if the cladding is not so heavy.

    In our house, we have 4 inches of polyiso in two layers, on top of 3/4 inch T&G pine sheathing. As mentioned, we used 7+ inch flat head screws and/or nails (but I prefer the log timber screws) on the strapping to penetrate 1.5 inches into the structural members. In my case I will have final layer as vertical shiplap, and plan to use 2 inch trimhead screws to hold it in place without piercing the foam/typar a zillion times.

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