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To ventilate or not to ventilate?

Greg Smith | Posted in Energy Efficiency and Durability on

Greetings all,

I am in the planning process for a new post frame (pole shed) building that will be used primarily as a wood shop. Planning to start construction within the next month.

The building will be insulated (R50 attic floor and R22 walls) and heated in winter – northern boundary of zone 6. When not in use, the building will be kept mid 40’s. Warm up to working temps will be air-tight wood stove drawing outside combustion air.

Exposed metal roof with no decking – panels will be attached directly to roof purlins. Standard procedure is to install Insulation directly on ceiling panels. Is there value in this case in calking joints and overlaps of the ceiling panels? Or would a vapor barrier between ceiling panels and attic make sense? I would assume that the metal panels makes a heck of a vapor barrier on their own when potential gaps are blocked.

I am down to two builders and one of them is emphasizing that the building needs soffit/venting in order to ventilate the attic since it will be insulated and heated, the other claims it isn’t necessary to ventilate.

My thought is that venting isn’t necessary for this building (based a good bit on reading about attic venting here and Building Science site), but I would greatly appreciate any input since this isn’t quite the same as venting or not venting a home.

Thanks for any thoughts/ideas!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Greg,
    What type of material are you using for these "ceiling panels"? It sounds like these are some type of metal panels -- can you describe them?

    If your building will have a vented unconditioned attic, you can use a wide variety of materials for your ceiling, including drywall, OSB, or plywood. Whatever material you use for your ceiling, you need to include an air barrier under your insulation.

    It's usually a good idea to ventilate an unconditioned attic. Soffit vents and a ridge vent won't hurt, although you may have plenty of air flow up there already, even without soffit vents and a ridge vent, depending on how you detail your purlins.

  2. David Meiland | | #2

    Agree, clarification needed on what these ceiling panels are. No matter what you do, you need to take into account the possibility of condensation on the underside of the metal roof. I've been hired a few times in the past to remove a metal roof installed over purlins and re-install it over a solid deck with either felt or I&WS over it, because the original installation was dripping condensation.

  3. Greg Smith | | #3

    Thanks Martin and David,

    The ceiling panels will be interior steel, that's why I was wondering if they would act as vapor barrier if the overlaps were sealed.

    The purlins will be installed vertically on top of 6' OC trusses 24" OC.

    Assuming the ceiling is tight I would anticipate a dry attic area?

    Thanks again

  4. GBA Editor
    Martin Holladay | | #4

    Greg,
    Steel ceiling panels are an effective vapor barrier, but they aren't an effective air barrier.

    If there is some way to use EPDM gaskets effectively at the seams of the steel panels -- and I doubt that there is -- you could try to seal them that way. Otherwise, I would advise you to switch to taped drywall, which is an effective air barrier.

    If the nature of your shop work requires a steel ceiling, you could install a European air barrier membrane on the underside of your ceiling joists before you install the steel panels.

    As I said in my earlier response, it's still a good idea to ventilate your attic. Are there big air gaps at the rakes and ridge? If so, you're fine. If you have a tight ridge cap and trim at the rakes, you might want to include soffit vents and a ridge vent to get a little air flow up there.

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