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What is considered best practice for insulating the interior of a block wall during new construction?

Yakmanfl | Posted in Energy Efficiency and Durability on

The homebuyer is trying to urge a tract builder (in North-Central Florida -hot-humid) to do more than their minimum standard practice. The as-designed block wall would be stucco-block-airgap 3/4″ furring and “radiant insulation” (foil) over the furring then drywall. The homebuyer is interested in adding rigid foam or spray foam and has concerns about condensation and mildew.

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  1. GBA Editor
    Martin Holladay | | #1

    A continuous layer of rigid foam -- as thick as the homeowner can afford. Thicker is always better than thinner.

    Even better: Install the rigid foam on the exterior of the wall.

  2. Yakmanfl | | #2

    Thanks Martin,

    My advice to the homebuyer was the same. His concern was in some details. I recommended continuous 3/4 - 1" foil faced (for radient heat transfer reduction) poly-iso board cemented to the block, taped at the seams with foil tape and with 3/4" furring strips over the continuous layer of foam, anchored by tapcons into the block. Horizontal 1x4's at cabinet top height for stronger anchoring. Gasketed drywall screwed into the furring. All electrical box penetrations to be sealed with foam. His main concern was potential condensation and mildew problems behind the drywall. So the main question - for the hot-humid region does the technique I proposed a risk of that problem? I have not encountered enough case studies to answer that.

    Thanks again for your help,

    Green Energy Options

  3. user-869687 | | #3


    First, the wall you described as standard practice seems certain to condense moisture at the outer face of the foil, and then drip down to the baseboard. The back of the drywall will be relatively cold (with A/C running) and could easily fall below the dew point when it's hot and humid outside. Your proposed upgrade makes sense, except that it's only R5 or R6. That's not much. Isn't there a code minimum?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "For the hot-humid region does the technique I proposed [have] a risk of that problem?"

    A. No.

  5. Yakmanfl | | #5

    Thanks to both Martin and TJ.
    Sorry about the delay in acknowledging your help. I was kayak camping and out of touch. TJ, believe it or not FL code for block walls has only required R4. And until this year, that radiant foil was allowed to meet the R-4 requirement. This project highlights the challenge of getting a tract builder (who is building the house with his own money and does not want to inconvenience his process) to modify standard practices without making all the trades (framing, window and door hanging, electrical) change their practices in too great of a way. Otherwise, a greater R-value for the walls would be preferable. As an interesting aside, the preferred approach, exterior foam insulation is not allowed here due to potential termite infestation.


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