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How to protect roof deck under metal roof from trapped moisture

Jake Warnock | Posted in General Questions on

Hello, I’m building a home in northwest San Antonio, climate zone 2b, hot and humid.  I will be encapsulating the attic and would like to know if I should use closed or open cell foam(please explain reasoning for one or the other!) Under metal roof I’m planning on water and ice shield along eaves, rakes, hips, and synthetic underlayment over entire roof; do I want a “breathable” underlayment like GAF deck armor, or does that matter?  I will be using an air gap on top of the underlayment to allow air movement in event of condensation on underside of metal panels as I’ve read that is common or at least reason for concern.  Ultimately, I’m looking for answers on addressing/mitigating potential roof rot from moisture coming from inside the home and from moisture that would “attack” from above.  I’ve read several articles on GBA about this topic, but I just want to go through it in detail again.  It seems that it’s SUPER important to get this right.  I think I’ve answered the question about which foam type to use: unvented roof=closed cell as I don’t want moisture permeating through open foam to underside of roof deck.  The fear is that a detail isn’t perfect and humidity from bathroom or kitchen gets into the encapsulated attic; do i provide supply air into that space to fight humidity?  Thank you in advance for sharing your experience and best practices,

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  1. GBA Editor
    Martin Holladay | | #1

    I suggest that you start by reading this article: To Install Stucco Right, Include an Air Gap.

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