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Insulating a new concrete block (CMU) house

user-484462 | Posted in Energy Efficiency and Durability on

Like all questions (at least here in Citrus County, Florida) this question is also has to consider building codes.

I am planning to build a block home. I would like to take advantage of the thermal mass capabilities by insulating the exterior rather than the interior. Keep in mind I am building on a shoestring (and a worn one at that). I would like to put foam panels on the wall with horizontal furring strips (to attach metal siding), or foam that can be stucco’d over. Will I need tyvek too? Open to suggestions for method and materials.

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that discusses your question: Using Rigid Foam As a Water-Resistive Barrier.

  2. Expert Member
    Dana Dorsett | | #2

    IRC 2015 code minimum in your area with the foam on he exterior of a block wall would be R4. That could be 1" or more of EPS, or 3/4" or more of polyisocyanurate. (see the mass-wall column for the zone 2 row.)

    With furring + siding foil faced polyiso or EPS would have the additional performance of the foil adjacent to the air gap, but it means you'd have to use siding, whereas EPS can be finished with an EIFS stucco, usually for less money than siding.

    If you have a local source for used / reclaimed roofing foam, Type-VIII (1.25 lbs per cubic foot) , Type-II (1.5lbs) EPS are pretty commonly used in commercial roofing applicatoins, and will typically cost less than 1/3 , sometimes less than 1/4 the cost of virgin-stock goods which would make even 2" roofing EPS cheaper than a code-min 1". Roofing foam doesn't come with foil facers, but sometimes reclaimers will have foil-faced polyiso (often factory-seconds), if that's the way you want to go with it.

    These folks are in Atlanta, which probably isn't worth the drive/shipping unless you need a LOT of it:

    Keep running this search- other surplus deals will crop up, but you have to be able to move on it when you find the right quantity an size at the right price:

    In slab-on-grade-hurricane-country there are many advantages to insulating at the roof deck rather than at the attic floor, which keeps any mechanical systems & ducts in the attic fully within the thermal & pressure boundary. Vented attics are more prone to roof deck failures under high wind condtions. Using 3" of reclaimed roofing polyiso above the roof deck and R20 batts (or 5.5" of half pound open cell spray polyurethane) between 2x6 rafters can sometimes be about the same money as a code-min R38 on the attic floor. With only 3" the center cavity R would be slightly under R38 , but with the R17-ish thermal break on the framing the performance exceeds that of R38 between floor joists.

    And with the ducts & air handler inside of a conditione attic the actual cooling LOAD is lower without the parasitic load of the duct gains, which allows you to go with smaller AC, by half-ton, often 1-ton or more: See:

    If you have better than code walls & windows, the parasitic load of ducts in a hot attic can even be half the total cooling load or more.

  3. GBA Editor
    Martin Holladay | | #3

    Below is a link to an article on installing exterior rigid foam. While the article assumes that the house has wood framing and OSB or plywood sheathing, the steps are similar for a concrete block wall. You'll just need TapCons instead of ordinary screws to attach the furring strips. (The rigid foam can probably be held to the block wall with construction adhesive temporarily, until the furring strips are installed.)

    Here is the link: How to Install Rigid Foam Sheathing.

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