Exterior insulation and cement siding over concrete block
I’m planning an extensive renovation of a 1950s cinder block home in central Florida. I’ll be taking off the roof and taking down all the interior walls, leaving a bare, uninsulated concrete block wall, with no finishes either on the inside or outside.
My current question concerns insulating the exterior walls. I’ve looked at a lot of the material on Building Science, and would like to do what they have dubbed the “500 year wall.”
If I understand correctly, this involves (from the inside out): latex paint, drywall, furring, concrete block, a unitary Class 1 vapor barrier, air barrier and drainage plan, rigid insulation, a drainage cavity and finally a brick veneer.
The only variation is that I want to use cement fiber board instead of brick (for purely aesthetic reasons), and this seems to cause an issue that I’m having trouble figuring out.
In particular, one way or another, I will have to use some kind of fasteners to hold the siding in place. I think the same is true for the rigid insulation, although I imagine that could be affixed with adhesive (even in a hurricane prone area). And those fasteners are going to pierce the vapor/air barrier and drainage plane.
At least in theory, that seems like a problem, primarily from the standpoint of the potential for moisture laden air being driven into the house.
I had several thoughts on what to do about this:
• Don’t worry about the penetrations, on the theory that the wall will dry to the interior and the moisture driven into the house will be minimal.
• Apply some sort of sealant to each penetration, which seems completely impractical.
• Make the rigid insulation another vapor barrier by using foil-faced polyiso, although the penetrations in this barrier will line up precisely with the penetrations in the barrier on the concrete block, so I’m not sure it will help that much. Plus I was wondering whether there would be problems putting a vapor barrier behind the polyiso, effectively sandwiching the foam between two barriers.
Being a relative newbie to all this, I would really appreciate any thoughts from the many experts who frequent these forums.
Thanks in advance.
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