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Standing seam condensation issues over uninsulated roof in (occasionally) conditioned space?

Adam_nw | Posted in General Questions on

Hi everyone,

I live in Seattle, Climate Zone 4C.  I am designing a complete roof re-build for a detached garage that I share with my neighbor (CMU wall in the middle).  The proposed roof is a simple gable (2.5/12), with Zip System 5/8″ roof sheathing, and standing seam metal on top.   At the same time the new roof is put on, I will be converting my half of the garage (about 180s.f.) into a workshop with continuous rigid insulation at the existing concrete walls and floor.  We are not planning any above-deck or cavity insulation in the roof system, as my neighbor’s half of the garage will be completely unconditioned and my half will have a small unit-heater that I will only use when I am actively working.  In other words, the rafters and the bottom of the zip will be exposed on the inside.

My question is this: when I do turn on the heat in my half of the garage and (potentially) leave it on for stretches of time during a project, do I need to worry about moisture moving through the Zip and condensing on the underside of the cold metal roofing?  If so, would I be better off simply doing 5/8″ CDX, taping the seams, and then installing a vapor impermeable membrane such as I&WS on the entire roof deck? (drying to the inside, nothing passing through)

I recognize I might be overthinking this, but with the current astronomical price of steel I want to do everything I can to ensure longevity for this roofing system. 

Thanks in advance for anyone who chooses to weigh-in!  Cheers,

-Adam

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Replies

  1. Jon R | | #1

    I've seen references to a solid metal deck with tight joints = .64 perms (of drying to the exterior). Zip allows about 1 perm of wetting from the interior. With good air sealing, your very intermittent heating and lack of moisture sources, my guess is "safe".

    1. Adam_nw | | #2

      Hi Jon,

      Thanks so much for your response, it's greatly appreciated. Would you mind expanding on your thinking a little to help me fully understand. Are you saying that the OSB portion of the Zip could absorb any moisture in the air during heating (up to 1 perm) and that, even if some vapor did pass through the Zip membrane, it would be dissipated by outward drying of the metal roof itself (through seams, etc.)? Would this be further complicated if the roofing manufacturer requires an underlayment over the zip?

      Thanks in advance for your time. Cheers,

      -Adam

      1. Jon R | | #3

        If .64 effective perms is correct, then there is almost as much upward drying as wetting from below. This is good. A permeable underlayment would allow this, a near zero perm underlayment would block this drying to the exterior. With your minimal wetting, it's not clear how important this is - but a cold side vapor barrier is a move in the wrong direction (in Winter).

        https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/171-Pinon.pdf
        https://www.jm.com/content/dam/jm/global/en/commercial-roofing/Resources/RS-7380_VaporRetarders.pdf

        1. Adam_nw | | #5

          Hi Jon,

          It seems there is some disagreement on these forums as to whether or not a standing seam roof actually dries at all to the exterior. I don't have an educated opinion on this matter. But for the sake of argument, let's assume the proposed metal roof is completely impermeable. Would this assembly have issues then during periods when the heat is turned on in the garage? What if the heat is on for several days?

          I guess what I'm having a hard time understanding is what it would take for water vapor to pass through the Zip OSB, through the Zip membrane, and to the underside of the roofing? What is 1 perm of wetting, relatively speaking? And given that there will be rigorous air-sealing of the entire roof, and no regular sources of interior moisture other than ambient conditions, is significant wetting of the sheathing from the inside even a strong possibility? Yes, I will be heating occasionally and--yes--warm air carries more water vapor. But does this even matter in a space where there are few, if any, sources of moisture introduction?

          I have a strong suspicion that I am really overthinking this, but I don't have the knowledge base to be certain. Can you help a (relative) building science noob understand the actual risks here?

          I really appreciate your time and engagement on this issue. Cheers,

          -Adam

        2. Adam_nw | | #6

          See new question below.

  2. Adam_nw | | #4

    Hi Jon,

    It seems there is some disagreement on these forums as to whether or not a standing seam roof actually dries at all to the exterior. I don't have an educated opinion on this matter. But for the sake of argument, let's assume the proposed metal roof is completely impermeable. Would this assembly have issues then during periods when the heat is turned on in the garage? What if the heat is on for several days?

    I guess what I'm having a hard time understanding is what it would take for water vapor to pass through the Zip OSB, through the Zip membrane, and to the underside of the roofing? What is 1 perm of wetting, relatively speaking? And given that there will be rigorous air-sealing of the entire roof, and no regular sources of interior moisture other than ambient conditions, is significant wetting of the sheathing from the inside even a strong possibility? Yes, I will be heating occasionally and--yes--warm air carries more water vapor. But does this even matter in a space where there are few, if any, sources of moisture introduction?

    I have a strong suspicion that I am really overthinking this, but I don't have the knowledge base to be certain. Can you help a (relative) building science noob understand the actual risks here?

    I really appreciate your time and engagement on this issue. Cheers,

    -Adam

    1. Jon R | | #7

      1 perm allows only slow moisture movement (wetting or drying) via diffusion.

      I don't have a definitive answer. I'd add "does the Zip accumulate moisture when the building isn't heated?". Some solar heating, a high perm concrete floor and night sky cooling might mean yes.

      1. Adam_nw | | #8

        Thanks Jon, you've been immensely helpful. As an aside, I will be installing a vapor barrier on the concrete floor, covered by rigid insulation and a floating plywood floor. Concrete walls will get rigid insulation as well. Final question:

        If this were your garage, would you go for the assembly I've proposed above? Or would you consider using plywood for the roof-deck instead (higher perms, easier drying), with taped seams and a permeable underlayment for the standing seam?

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