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Community and Q&A

Can foil-faced polyiso be used on the interior side of a CMU wall in South Florida?

cevhh | Posted in Energy Efficiency and Durability on

I am building a house in South Florida that has the following Wall Assembly (starting from exterior and working in):

2″ Stone Veneer
Hydro-Ban Waterproofing
8″ CMU Wall
3/4″ Hunter Foil Faced Poly Iso
1 1/4″ Air Gap
Metal Stud

A NYC Architect hired by the Owner believes that the home should NOT have 3/4″ Foil Faced Poly-Iso adhered to the inside face of the CMU, but instead should be unfaced Poly Iso claiming that the Foil Face is acting as a Vapor Barrier and therefore the Moisture Drive is coming through the Veneer, Hyrdroban, CMU Wall and then condensing on the exterior face of the Poly Iso.

The point I made is (1) that we do NOT tape the Poly Iso so therefore it cannot be considered a true Vapor Barrier; the Laws of Thermodynamics allow the grains of moisture to move to the driest air and so they will move through to the joints of the Poly Iso and then move to the interior of the home and disappear; (2) the Dew Point is never cold enough on the exterior side of the Poly Iso to be below the Dew Point to allow the moisture to condense on the Poly Iso; (3) even if this were to happen, the Hydroban is sealing up the wall on the outside limiting the vapor drive to next to nothing.

We have done extensive work in Palm Beach Island building VERY expensive homes and this has been the way we have done it… never having any issues… perhaps we have been doing something wrong and just not had an issue.

Does anyone have specific South Florida experience with a CMU Wall using Foil Face Poly Iso? I want to do what is right and have no issue removing all of this and installing unfaced polyiso but I want to hear from someone who is a Building Science type person or can point me to the right person?

Your quickest response is greatly appreciated!

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  1. Expert Member
    Dana Dorsett | | #1

    The exterior face of the polyiso is at the same temperature as the CMU and if the exterior foil is at the dew point of the air in the CMU cores or outdoors, the condensation would be happening in the CMU, not the foil facer. The CMU is highly tolerant of moisture- so is the foil facer so...

    DO tape the polyiso! It is INDEED a vapor barrier whether or not it's taped, but you want to keep exterior moisture from reaching the interior side facer, which would be a potential problem. Sealing the seams with a flexible sealant as it goes up, and taping the interior side facer to prevent any air from passing through, bringing outdoor humidity with it pretty much eliminates the risk even under extreme conditions.

    UNFACED polyiso would potentially load up with moisture from the CMU that wicks up from the foundation, and taking a serious performance hit!

    There will be times and conditions when the CMU is below the outdoor dew point and takes on moisture, but it's really "who cares?". Concrete is used even in ridiculously wet conditions (such as the bottom of the bay, holding up bridges,in dams holding back rivers, etc) and it lasts for centuries even when fully saturated.

    I'm not clear how the polyiso is being affixed to the CMU in your stackup description, or what the 1-1/4" of air between the polyiso and CMU is about. Empty wall cavities are thermal bypass and fire spread paths- what's proposed might not meet fire safety codes. Putting the studs right up against the polyiso would keep it in place if it's fasteners or adhesive ever failed.

    What is the vapor permeance of HydroBan when applied to CMU? Just because it rejects liquid water doesn't necessarily mean it can't pass water vapor.

  2. GBA Editor
    Martin Holladay | | #2

    First of all, can you tell us your name?

    Dana is 100% right.

    1. It's perfectly OK to use foil-faced polyiso.

    2. You definitely want to tape the seams of the polyiso.

    3. The foil-faced polyiso is a vapor barrier.

    4. It's a good thing that the polyiso is a vapor barrier, because you don't want to invite outdoor humidity into your home.

    5. There won't be any condensation on the polyiso because the polyiso has R-value. The first condensing surface (the exterior foil facing) won't be cold -- it will be hot.

    Don't trust any advice provided by the NYC architect.

    My only other comment is that 3/4-inch polyiso is pretty minimal insulation, so (a) it wouldn't hurt to check whether such a thin layer of insulation meets the local building code requirements, and (b) you might consider increasing the thickness of the polyiso. It will never to cheaper to add thicker insulation than right now.

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