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Metal roof underlayment

joe2baba | Posted in Energy Efficiency and Durability on

I am building a very tight house borderline zone 3-4. The walls are 2×6 probably going with dense pack cellulose. Zip sheathing with foil faced 1/2″ polystyrene facing out to 1/2″ air gap rain screen.
The roof is metal over an OSB deck. Part of the roof is over an insulated attic (12″ cellulose ) the other part is over a vaulted ceiling. The rafters are 2×8. The vaulted ceiling is going to be closed cell foam.

The roof will be metal (white) screwed to 2×4 purlins. This gives me plenty of ventilation under the metal.
My question is what underlayment should i use under the metal? I thought initially I would use a reflective foil to keep the roof deck cool. I think that might be overkill. However I am looking at Tyvek Protek 120 and the cost would be comparable to the foil. Is there a problem over the vaulted ceiling with condensation under the underlayment? I have never had a problem with metal roof leakage so am not too worried about that. I don’t want to trap moisture under the underlayment on the deck though from condensation. My gut tells me I have plenty of air flow under the metal.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    Q. "Is there a problem over the vaulted ceiling with condensation under the underlayment?"

    A. With metal roofing, the usual location of condensation is above the underlayment, not under the underlayment. Here is the mechanism: if a warm morning follows a cold night, warm air enters the ventilation channels between the underlayment and the metal roofing. Moisture in the warm air condenses on the underside of the metal roofing. The condensation then drips onto the underlayment.

    This problem won't lead to roof leaks, as long as the underlayment is carefully installed. The moisture usually evaporates before it finds its way into the house.

    You wrote that "My gut tells me I have plenty of air flow under the metal." Ironically, however, the air flow is the source of the problem as well as one of the solutions to the problem. You can't solve the problem with air flow, since the air flow brings in the moisture in the first place. That said, the advantages of the ventilation gap outweigh the disadvantages.

    It's also possible to have condensation (or moisture accumulation) at the underside of your roof sheathing, especially if you have done a bad job detailing your cathedral ceiling insulation. The best way to prevent that type of problem is to have a good air barrier at the ceiling. If your cathedral ceiling is insulated with closed-cell spray foam (not open-cell foam), then you'll probably be OK.

    For more information, see How to Build an Insulated Cathedral Ceiling.

  2. Peter L | | #2

    Martin,

    Is this type of condensation an issue that can cause any leaks or OSB rotting issues?

    Condensation like this occurs on all metal roofs that have an air gap (purlins) between the metal roof and the OSB membrane?

    If so, is it better than to screw the metal roof directly onto the OSB substrate, therefore removing the air gap?

    Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    Peter,
    I have already answered your questions.

    Q. "Is this type of condensation an issue that can cause any leaks or OSB rotting issues?"

    A. As I wrote in Comment #1, "This problem won't lead to roof leaks, as long as the underlayment is carefully installed. The moisture usually evaporates before it finds its way into the house." In case this is unclear: the moisture evaporates before it reaches the OSB.

    Q. "Condensation like this occurs on all metal roofs that have an air gap (purlins) between the metal roof and the OSB membrane? If so, is it better than to screw the metal roof directly onto the OSB substrate, therefore removing the air gap?"

    A. As I wrote in Comment #1, "the advantages of the ventilation gap outweigh the disadvantages." The type of condensation I described doesn't happen every day -- just during certain weather conditions.

  4. joe2baba | | #4

    Thanks for the response. Any input on what underlayment to use. Would the radiant barrier work?

  5. GBA Editor
    Martin Holladay | | #5

    Joe,
    A radiant barrier will add about R-2 to the R-value of the roof assembly, as long as the radiant barrier faces an air space. So yes -- the radiant barrier will work.

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