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Community and Q&A

Condensation on underside of roof deck

dfvellone | Posted in General Questions on

My location is area code 13420.
My new construction is currently unheated and closed in. No  insulation except for exterior wall foam sheathing. The rafters are open to the exterior at the top plate and the ridge is vented. When the temperature rises along with humidity the underside of the roof deck gets soaked with condensation. This happens in locations where the snow remains on the roof above, keeping the deck cold and enabling condensation to occur. Where the snow slides off the roof deck temp equalizes with exterior temp and the deck stays dry. My completed design is cape construction with 7/12 pitch and steel roofing.  5′ of the rafters – from the top plate to the intersection of collar ties – will get closed cell foam that will continue across the collar ties. Rafters get a 2″ vented space between the roof deck and foam. Ridge is vented and it’s height off the collar ties is 5′.
I don’t see this problem going away once the insulation is installed and the home heated, so besides insulating the underside of the roof deck are there any other solutions?

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that discusses condensation on wall sheathing, not roof sheathing -- but the issues are similar: "Condensation on Wall Sheathing During Construction."

    1. dfvellone | | #2

      Martin, thanks for the reply and link. Although the issue is similar - cold surface and warm, humid interior air condensing on that surface - in my roof circumstance nothing will change with completion of construction as in the wall situation; the attic space remains vented with warm and humid air entering during our more frequently occuring freeze/thaw cycles, and that humidity will condense on the underside of the deck. I've read about solutions involving spraying a thin layer of foam against the underside of the roof deck. Economy and reasonable budget issues aside, call me old fashioned but enclosing a roof deck to the extent that you'll never see a leak until the rot collapses is asking for trouble. I'm wondering because I'm using closed cell foam to what extent do I really need to ventilate the space? I'd prefer as much as possible to keep my roof cold and prevent dams (two dormers), but will r49 of closed cell foam adequately prevent enough heat loss so that a minimal amount of venting will suffice to prevent exterior thawing. Daniel

      1. GBA Editor
        Martin Holladay | | #3

        The type of condensation you describe -- attic condensation that occurs when there is snow on the roof and the outdoor air is warm -- is fairly common, especially in buildings with metal roofing installed over purlins (with no OSB or plywood). It is less common but possible if you have a full deck of OSB or plywood.

        If your attic is vented and unconditioned, this type of condensation is almost always harmless -- eventually the moisture evaporates without lasting effects.

  2. dfvellone | | #4

    Martin, thank you for the input. I suspect that I'm seeing it so frequently because of the drastic temperature swings that are becoming more frequent here. Deep sub-zero freezes one day and upper 40's and rain the next.

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