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Can I really use 2″ of rigid insulation under fiber cement siding?

jkarch | Posted in Energy Efficiency and Durability on

I’m an architect in Wisconsin (climate zone 6B) and am planning to build a 300 s.f. addition to my house. I’d like to side it in fiber cement siding to match the existing house. According to my calculations, if I want to use rigid foam insulation on the outside of the 2×6 walls (with R-21 fiberglass in the wall cavities), I need at least R-11.25 in order to avoid condensation on the inside face of the sheathing. I was thinking we could use 2″ of polyiso. However, when I look at the installation instructions for Certainteed and Hardiplank fiber cement siding, both say that non-structural sheathing thickness should not exceed 1″. Does this mean that I can’t use rigid insulation and fiber cement siding together in my climate zone?

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Replies

  1. user-966377 | | #1

    Hi,
    We are working on a project here in Toronto, Ontario, Canada that is to have Hardieplank installed over external insulation - it can be done! The way we were told to install the siding (after discussions with Hardie, Tyvek, and the local building inspector) on min. 3/8" strapping - this can be ripped from any 3/8" building panel. Behind the 3/8" strapping is the Tyvek, creating an air cavity and drainage plane directly behind the Hardie, which prevents any moisture that penetrates the Hardie (and it will) from deteriorating any of the other building elements. The insulation is then behind the Tyvek.
    This is the preferred installation, but as we learned from Hardie and Tyvek either installation is acceptable and performs well.

    Hope this helps.

  2. GBA Editor
    Martin Holladay | | #2

    Julie,
    Hardie has several documents referring to methods of installing Hardie fiber-cement siding over vertical furring strips.

    http://www.jameshardiecommercial.com/cad-hardiepanel-wood-framing-furring.shtml
    "HardiePanel Siding Details - Wood Framing with Wood Furring Strips"

    http://www.jameshardie.com/pdf/install/install_panel-canada.pdf
    "HardiePanel® siding can be installed over furring strips (in accordance with local building code requirements)."

    http://www.jameshardie.com/pdf/USTB_2008_Oregon_Residential_Specialty_Code_Mid-Cycle_Amendment.pdf
    "When a drainage gap is required, James Hardie recommends creating the gap with one of the following methods: The use of furring strips (1/8” thick or greater)..."

  3. user-1037041 | | #3

    As far as I could tell, however, it seems that, like Julie says, Hardie (and Certainteed) only speaks to a maximum of 1" of rigid foam under the rainscreen. She wants to use 2" and I'm thinking of using 4" on my next project.

    Sounds like it's time for a call to Hardie. Donald, were your discussions with Hardie and Tyvek via telephone or email? Did you speak to their engineers? Did you receive written documentation regarding your use of their products and warranty information?

  4. user-1012653 | | #4

    All
    I just completed a home that used 2" of foam with strapping under hardie siding. The contractor too was questioning the detail, but he discussed it with his hardie rep and all was well. Just use the strapping with appropriate screw spacing for the weight and you are fine.

  5. GBA Editor
    Martin Holladay | | #5

    Julie,
    Here's an article with more information on furring strips:
    https://www.greenbuildingadvisor.com/blogs/dept/musings/fastening-furring-strips-foam-sheathed-wall

  6. bdrfab | | #6

    The Certainteed and Hardie recommendations are based on the assumption that the panels will be directly attached to the framing members. Furring strips are fine and can be used over any thickness of exterior foam. Martins links will answer questions from there.

  7. 7ddzyKuLnw | | #7

    The restriction of 1" solid insulation has more to do with structural support issues. The thicker the insulation, the thicker the furring strips have to be and the longer the fasteners required. As the fasteners get longer, it gets progressively more important to ensure that they can still carry the weight of the panels properly as the anchoring threads are further and further away or cantilevered from the actual load.

    With insulation thicker than 1 inch, I would consider using wood furring strips on galvanized steel zee girts or hatbars so that you have enough options for proper fastening. I would also consult a local structural engineer to make sure there is not undue force on the fasteners due to gravity load or wind load.

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