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Condensation control on underside of metal roof

hotrod796 | Posted in General Questions on

I’m building a house using a steel post and beam structure. The floor of the attic space will be air sealed and insulated to R60 with blown fiberglass. I plan to place the steel roof panels directly on the purlins without decking or house wrap. The ends of the corrugated roof material will be left open with vents at the ridge to allow for ventilation. Denver, CO – Climate Zone 5, subtype B (dry).

Will I have a condensation problem? Is a layer of felt under the roof decking suitable for holding the moisture until it can return the air? I hesitate to use insulation due to cost.

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Replies

  1. JC72 | | #1

    Your comment is confusing because in the first paragraph you say no decking/housewrap and then mention roof decking in the second.

    I do not believe felt of any kind is designed to be self supporting so it'll sag and develop gaps which allow for water entry.

    Spend the money on roof sheathing.

  2. Expert Member
    Michael Maines | | #2

    Tony, in your second paragraph I believe you mean roofing, not roof decking. Decking is a synonym for sheathing.

    Your roof will likely cause condensation below, due to night sky radiation cooling the panels below the dewpoint temperature of the ambient air below. I agree with John; felt paper is not a solution. Conventional roof sheathing is one option, or you could use a Sarking membrane--a high-tech WRB made for this type of application. The one I've used is Pro Clima Solitex Mento Plus. But sheathing is more typical.

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