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Condensation from roof steel

h7Lqe8pifL | Posted in General Questions on

We have a Northern Canadian, log building and when the vapour barrier was installed after the 12″ of fibreglass insulation it was impossible to seal and tape because of the purlins in the roof structure.
We are having dripping problems from the steel roof (installed on 2×4 strapping, 2′ air space, then 12″ insulation and the then the interior vapour barrier) after temp changes from cold to warm enough to melt….is it condensation from the steel roof, not enough air space or combination???
We are considering removing the steel and sheating the roof and shingles…would this fix it?

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  1. Riversong | | #1

    The problem is no continuous air barrier (much more important than vapor barrier), air permeable insulation (fiberglass), and a cold condensing surface (metal roofing). Warm, moist air is rising up through the insulation from the stack effect, condensing and freezing on the metal roofing, and then thawing and dripping when the temperature rises.

    No, replacing the metal roofing with sheathing and composite shingles will not make a significant difference and will result in the rotting of the sheathing.

    You don't indicate what contains the 12" of fiberglass. I assume there are 2x12s "rafters" over the interior board ceiling. Typically, the air/vapor barrier would be installed over the ceiling boards and before framing the insulation cavities to allow continuity. It doesn't sound like this was done, or I'm not understanding your entire ceiling/roof cross-section.

    Your best bet might be to remove everything above the ceiling boards and install nailbase foam roof panels, as thick as you can find (you can get them with integral vent channels from Cool-Vent by Hunter Panels).

  2. h7Lqe8pifL | | #2

    Thank you for your suggestion! The roof truuses are laminated 12" and what we have is pine boards as interior ceiling finish, 6 mil poly (not taped or sealed), 12" fibreglass insulation, 2x4 strapping and then the steel roofing.
    Would we be better off to remove the steel and fibreglass and spray foam to seal and then re-roof rather than trying to do it from inside (for cost reasons)?

  3. Riversong | | #3

    I was suggesting an outside retrofit. I assume the ceiling boards are above the round roof purlins and the rest of the structure is built up from there. I'm not sure what you mean by laminated trusses. You could possibly spray foam from above, but it would depend on the framing details and the weather and that may be an expensive option.

  4. GBA Editor
    Martin Holladay | | #4

    Considering the weather conditions you are describing, the source of the humidity that condenses on the underside of your roof sheathing might be exterior air. This commonly occurs when the roofing is cold (or snow-covered) and the exterior temperature warms up. If the spaces between the 2x4 roof nailers are vented, then exterior air reaches the cold roofing, and condensation (and drips) result.

    Robert is right: you probably have an air barrier problem. Metal roofing should always be installed over an air barrier. If the fiberglass batts aren't installed against plywood, but are just butted up against the 2x4 roof nailers, you are asking for trouble.

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