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Hardie Plank Directly Over Foam

2008gt500 | Posted in General Questions on

I’m laying out the details for my new house.  I realize I am not building the “greenest” house as I am more concerned with strength for storms, but I have learned quite a bit here.  The first floor will be ICF and the second floor framed.

My main question is about attaching HardiePlank directly over foam.  Hardie’s details typically show no rainscreen for lap siding.  This kind of makes sense to me as very little of the siding touches the wall, so it creates its own tiny airspace which likely helps with drying.  That said, they only allow you to attach it through 1″ of foam without furring.  I was hoping to use 2″ of EPS, but their 1″ limitation has me thinking I will shiplap polyiso for 1″ of foam on top of plywood sheathing.  I will set this up to be in plane with the ICF wall below for a seamless transition.  All of GBA’s articles recommend a rainscreen gap, so that is why I am asking the question.  Do I really need it in climate zone 2?

We built an ICF house 15 years ago in Orlando with the Hardie directly attached to the ICF and it is holding up great, so that is giving me the confidence to do it again.  In my mind, it seems like a better detail structurally to have the cement board hard against a solid wall so there is less opportunity for movement in hurricanes.  The lack of air space also makes it difficult for critters to make a home in there which is a plus on the water.

Secondary, has the dimensional stability of polyiso gotten better?  If it’s my main protection for the framed upstairs with little drying potential, I don’t want it shrinking much over the years.  I’m thinking ship lapping 1/2″ sheets should make it pretty safe.

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  1. Expert Member

    James Hardie has details for both, over 1" or less of foam, over drainable WRB's, and over furring strips. This pdf doesn't have the zones, but I'd suspect zone 2 is somewhere around where they've highlighted the need for at least a 3/8" rain screen, whether it's over insulation or not.

  2. 2008gt500 | | #2

    Thanks Kyle, I've read through that, but Hardie Plank lap siding is not one of the products listed on that guide. I probably went through their website about 20 times before I realized that. It seems their rainscreen recommendations are mainly for vertical siding, panels or their Artisan siding.

  3. Expert Member


    A rain-screen will be of no benefit on the ICF portion of the house. Whether it will be useful above depend some whether the foam is permeable enough to allow drying to the outside, or wetting from inward vapour drive.

  4. 2008gt500 | | #4

    I'm torn between EPS and Polyiso, but it seems like this may be my safest assembly. It does leave a unique detail at the top of the ICF since the furring strip is 3/4" back. I could add some sort of flashing detail there, or maybe the housewrap draping over the ICF is enough.

  5. 2008gt500 | | #5

    This was my preferred detail, but to accommodate the 1" requirement I was going to just extend the trusses by an inch.

  6. Expert Member
    BILL WICHERS | | #6

    I would put that "2 inches of EPS foam" in as two layers of 1" each, with staggered seams. This will perform better with less leaks.

    I prefer polyiso, but EPS will work too. Keep in mind that you want something like Type II or better, which is much less crumbly and will hold up better during installation.


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