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Real Life Drying Potential Difference of Unfaced EPS vs Faced EPS

JohnJones171 | Posted in Energy Efficiency and Durability on

Hello all. I am selecting between two products, ~2 inch faced GPS (perm 0.5, R10) and unfaced 2.5 inch type 2 EPS  (R 10, estimated perm 1.2-1.4 as not actually tested by the manufacturer). Application will be poured foundation basement walls and Rim joists space in Zone 5 (Toronto). Due to code, the unfaced EPS will require a smart barrier as the perm>1.

Anyway, got me thinking, out of pure curiosity-how much actual drying would the unfaced EPS in this case allow vs. the faced GPS.? In other words, despite the permeability being almost 2.5x greater, 2.5x an immeasurably small amount is still a small amount. Are their any practical examples or anecdotal examples of what this would translate to in reality, i.e. the equivalent extra vapor pass through on some arbitrary square foot size results in what amount of equivalent liquid water over the course of a year or something like that.

This question is more applicable to the Rim joist area, as I realize concrete doesn’t matter if stays wet, but I imagine some drying ability inwards in the Rim joist is probably a good thing. My home is brick veneer, with what I believe to be some sort of black poly behind the brick as the weather barrier, tied in to weep holes.


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  1. JC72 | | #1

    Remember a perm rating revolves around water vapor rather than bulk water.

    1 perm represents 1 grain of water vapor per hour, moving through one square foot of material at one inch of mercury.

    1 grain of water condensed = 1 droplet.
    7,000 drops per 1 lb water.
    1 lb water = 15.34 fluid oz.
    1 inch of mercury is .49 psi.

    I can't think of an analogy.

  2. Jon_R | | #2

    More inward drying can be useful (1 and 2), but it is easiest to just comply with code, focus on the far more important air sealing and not worry about it. For modeled numbers, you can run WUFI.

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