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Community and Q&A

Exterior insulation and cement siding over concrete block

Lee Meyer | Posted in GBA Pro Help on

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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  1. GBA Editor
    Martin Holladay | | #1

    I vote for Approach #1: "Don’t worry about the penetrations."

  2. Lee Meyer | | #2

    Wow! Thanks so much for the quick reply. Would you have any preference for the rigid insulation?

  3. GBA Editor
    Martin Holladay | | #3

    Polyisocyanurate is the most environmentally friendly type of rigid foam, and it works particularly well in hot climates.

  4. Lee Meyer | | #4

    Thank you again. If you don't mind, one final question. Would it matter whether it was foil faced or not?

  5. GBA Editor
    Martin Holladay | | #5

    If you plan to install vertical furring strips for a rainscreen installation (creating a 3/4-inch-deep air space between the polyiso and the back of the siding), then you will get slightly better thermal performance if you specify foil-faced polyiso for the outermost layer of polyiso (rather than a different facing).

  6. Lee Meyer | | #6

    That was my plan. Thank you so much for your help.

  7. Aj Builder, Upstate NY Zone 6a | | #7

    Lee, a good install of rigid foam requires that you pay attention to installing it like it has to hold air like a balloon or water like a tub or whatever... if your edge of sheet is not airtight it is possible that the entire installation of material is doing nothing. Example... put a small whole in a it a drain... let me know how long it takes for the water to leave... seconds... same for a balloon.

    I have seen some nightmare installs of foam sheets. And if installed over an airspace it is crucial to design and install taking into account air movement in and out of the airspace both to keep the foam insulating and to deal with fire concerns when the airspace is vertical and what the airspace is made of and what it connects to above and below.

    Concrete block... fireproof... good.... horizontal furring...good....

    Also if you can find a local who has built what you desire and all works great... that person is your best to work with. We (free advice at internet blog sites) should be a second reference source not primary.

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