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Insulating a 100-year-old barn roof

ejc_farms | Posted in General Questions on

Hello all – My wife and I are in the process of restoring and insulating a 100 year old timber frame barn in Climate Zone 4a.

The barn has a 10:12 pitch metal roof over 1x skip sheathing.  With the goal of installing a new insulated roof while keeping the framing exposed to the interior, one suggestion proposed to us is to leave the existing metal in place, and to install vented nailbase roof insulation directly over top of the existing metal. New metal panels would then be installed over the nailbase. The interior would be unvented.

Is this a viable approach or should the existing metal be removed? What additional control layers, details and/or precautions would be important?  Moisture and condensation management are my primary concerns.

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  1. GBA Editor
    Martin Holladay | | #1

    The main problem with the suggested solution is air barrier continuity. How are you going to connect the wall air barrier -- which will probably be either the interior drywall or new taped wall sheathing -- with the ceiling air barrier -- which will be the new nailbase panels?

    That junction at the eaves is a mess, with old metal roofing introducing all kinds of air leaks.

    In order to prevent massive air leaks at the eaves, the existing metal roofing will need to come off. Even then, you need careful detailing at the eaves for air barrier continuity.

  2. ejc_farms | | #2


    Thank you so much for the quick reply. If the metal roof is removed, would the skip sheathing need to be covered with OSB and taped, or is there another suitable air control membrane that could rest between the skip sheathing and vented nailbase foam?

  3. GBA Editor
    Martin Holladay | | #3

    I think that the best air barrier above the skip sheathing would be taped OSB -- perhaps Zip sheathing.

    If you want to do it all with one product, I suppose you want structural insulated panels (SIPs), not nailbase.

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