Hardie Plank Directly Over Foam
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.
GBA Detail Library
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