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Anyone back-vented Hardie lap siding over exterior insulation?

Bob Alf | Posted in Energy Efficiency and Durability on

At this point we plan on building a 2×4 wall with dense pack cellulose (r-12), then 1/2″ plywood, 2 layers of 2″ fiber faced polyiso (r-26), WRB, 1×3 strapping and then siding. Can we use Hardie lap or Hardie panel siding? Likely a question for Hardie but they will likely say they want direct contact with sheathing and if foam, then no more than 1″ thick. Their install instructions imply direct contact with sheathing but I don’t see any restrictions against fastening to strapping….but maybe I missed that. Also, i’m guessing that strapping 1′ O.C. would make the most sense to reduce/eliminate deforming of the slightly floppy fiber-cement? Anyone done anything like this? Maybe this has been addressed but I didn’t find it when doing searches thru the QA section. Oh, and I would assume strapping desirable since wind blown rains will get behind the siding and it’ll never drain/dry if siding is tight to sheathing on a high-r-value wall…?

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  1. GBA Editor
    Martin Holladay | | #1

    Here is some information from Hardie:
    "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)
    • The use of water resistive barriers enhanced for drainage."
    "HardiePanel® siding can be installed over furring strips (in accordance with local building code requirements)."

  2. Bob Alf | | #2

    You guys are the best! Thanks for such FAST response.

  3. homedesign | | #3

    I agree that Fiber cement siding can be kinda "floppy"
    furring at 12" OC might be overkill ... I have seen "wavy" siding when spacing is 24" OC

    "warning imperfections in the substrate can telegraph thru fiber cement siding"

  4. wjrobinson | | #4

    John is right, I once fiber cemented a large poorly framed project, pointed out the wall flaws to the PM, was told to side. All the flaws seen before siding showed right through nicely.

  5. user-659915 | | #5

    If your studs are 16" on center won't your furring need to be the same? - or am I missing something?

  6. Bob Alf | | #6

    James- I would be screwing 1x3 furring into the 1/2" plywood sheathing that is behind the 4" of foam and of course there would be 2x blocking at the bottom (inset to 1st layer of foam) which would help eliminate downward pull of foam/furring/siding that are dependent on 5" long screws that could otherwise bend downward. I realize slightly better to screw furring into the framing but assumed the frequency of furring (1' OC) would compensate plus give better siding support. Thoughts?

  7. user-788447 | | #7

    Hi Robert,
    A local (S Minneapolis) project recently completed a Hardie lap install over 1x furring 16"o.c.
    I haven't seen it since the siding has been completed. Might be worth your while to swing by.
    Address listed on their blog:

  8. Bob Alf | | #8

    Thanks J- I didn't realize Tim's project used Hardie....I'll check it out.

  9. user-869687 | | #9


    I don't think a 5" screw will be long enough, even where you don't expect to hit framing. Here's an economical 6" roofing screw: Note that they recommend a minimum 3/4" plywood for holding strength. Aim for framing wherever possible.

  10. user-869687 | | #10

    John B, that looks like a seismic X-brace that the contractor was hoping would be invisible.

  11. user-1037041 | | #11

    I'm gearing up to do pretty much exactly the OP's wall build. I found this at the Fastenmaster website:

    Now if only I could find the specific gravity of 3/4" plywood. I'm considering using plywood rips as my strapping to help avoid splits. I'll be predrilling of course, but I think the ply might be the way to go.

  12. Irishjake | | #12


    What zone are you in? If you are in a colder or very cold climate, there are many that are veering away from Hardie for many reasons. Long term performance and product issues being the first......

    There is a special Hardie Lap siding document out there, that specifically instructs installers on installation of it's product over exterior insulation. The IRC/IBC does too.

    You will want the screw to go into the stud a minimum of 1.5". That is an industry standard.....Check out TruFast SIPTP screws. Reasonable cost and good performance.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    If Robert hasn't finished the job by now he has bigger problems than which screws to use. His question is from 2012 :)

  14. Irishjake | | #14

    Malcolm.....ahh yes thanks!

  15. Expert Member
    MALCOLM TAYLOR | | #15

    Even if it's too late to help Robert, your answer will be very useful for others with the same problem. I certainly enjoy all your posts!

  16. Irishjake | | #16



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