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Thickness of furring strips with thick exterior rigid foam vs. siding manufacturer requirements

Leigha Dickens | Posted in Green Building Techniques on

I am experimenting with thick (>2″ XPS ,EPS, or polyiso) rigid exterior foam applied to wood framed walls for zones 6 and 7 as an alternative to a double stud wall, as a building scientist for panelized home manufacturing company focusing on high performance for extreme climates and weather conditions (including high wind and seismic zones within cold climates.)  I don’t live in a cold enough climate zone myself where thick exterior foam is done residentially, so I must rely on asking those who do when I have a question.

I notice that the example furring strip materials shown in cold climate wall details and guides are relatively thin:  plywood or 1x materials. (I have poured through many such guides, including the resources and blog posts here on the subject, and they have been great!)  However, I keep running into siding manufacturer stated requirements of 1 1/4″ siding fastener penetration into structural wood. I’ve seen this with the common fiber cement or composite lap products that we use (Allura Lap and LP Smartside being a couple of examples.) Plywood or 1xs would not be thick enough to provide 1 1/4″ structural wood penetration. So I am curious to know if others building with thick exterior foam have run into this issue and what actually gets done about it in the field.  Are builders merely sticking to siding products whose manufacturers don’t require this thickness? (like…vinyl?) Are they sizing up to 2×4 furring strips when they have to? (I’ve never seen an example of someone who’s done that, and it seems unweildy.) Are they making calculated decisions to disregard siding manufacturer requirements, as those requirements are probably overly conservative? I would appreciate what perspective on this folks who live where thick exterior foam is a more common practice can offer.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Leigha,
    This issue was discussed in my 2010 article, "Fastening Furring Strips to a Foam-Sheathed Wall."

    There are three issues:

    1. Code requirements for attaching siding.

    2. Requirements established by siding manufacturers.

    3. Engineering recommendations.

    All three issues are discussed in my article. In some cases, and engineer's analysis (usually involving a recommendation to use ring-shank nails) can convince a siding manufacturer or building official to bend the rules.

    1. Leigha Dickens | | #3

      Martin,

      Thanks! I had seen many articles here but not *that* one, and with 71 comments it's got a lot in there to look at. I have a structural engineer comrade and I am translating this through him, so I'll pass it on to him as well. We've begun trying to speak more directly with the siding manufacturers and it sounds like that's worth doing more of. Still curious for what others have done.

  2. Matt McLagan | | #2

    Leigha,

    I've deliberated over this issue for most the past year. Originally I planned to use Hardiepanel siding over a 1x4 furring, Zip, double stud wall.
    I'd carefully planned the framing/furring spacing to ensure the seams & battens of the siding would be uniform and spaced nicely.
    I threw this idea out and decided to use Zip-R. Now my wall system will be LP Smartside 38 series board & batten, a required 1mm non-compressible drainage wrap, Zip-R sheathing, 2x6 framing.

    LP Smartside 38 series only requires at minimum 7/16" penetration into structural sheathing.
    See the top bullet point: https://lpcorp.com/resources/product-literature/technical-notes/tn-028-vertical-siding
    It requires a minimum 1mm drainage gap behind it. I had planned still to use a rainscreen, and my framing spacing, but this was missed during framing. I expected it might not translate perfectly, which is part the reason I switched wall systems and siding.
    1mm obviously isn't as robust as a 3/4" rain screen, but it is something.

    I can't speak for other LP products, but they all have similar lb/sq.ft. when I had looked. I would wager there may be technical bulletins or details from LP that specify similar fastener depth requirements.

    1. Leigha Dickens | | #5

      Hi Matt,

      One cold climate builder told me he uses 1x4 furring strips over 2" foam over CDX sheathing with LP Smartside's LAP product, yet when I looked at the installation instructions for their 38 Series Lap Product (https://lpcorp.com/resources/product-literature/installation-instructions/lp-smartside-strand-substrate-lap-siding-application-instructions-english) I saw that pesky requirement for 1 1/4" penetration. But as noted in Martin's article there may be much more subtlety to that if one actually talks to them, with fastener type and wind speeds going into account. I'll check out the info on the vertical panel product, it would be interesting if it were different.

      1. Leigha Dickens | | #7
        1. Brendan Albano | | #8

          As you've discovered, some (many?) siding manufacturers have a separate set of installation instructions for installing their siding over furring strips that is different than the standard installation instructions.

  3. Ryan Magladry Ottawa, Ontario | | #4

    we are installing Maibec wood siding on 1x3 furring strips over 2" EPS.
    The furring strips are screwed using 5" screws run into the studs. Siding then stapled to furring strips. Rock solid, not technical backup, plans were reviewed and approved by code officials and engineer.

    1. Leigha Dickens | | #6

      Hi Ryan,

      Thanks for the anecdote! I will check out Maibec wood siding.

  4. Expert Member
    Akos | | #9

    Hardie provides fastener schedule for thinner sheathing:

    https://www.jameshardie.com/JamesHardieMainSite/media/Site-Documents/TechnicalDocuments/Reports/ESR-2290.pdf

    With 16" OC spacing, to hit their fastener spacing, you would have to use slightly wider plywood strips with 2 ring shank nails per batten.

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