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Community and Q&A

minimum exterior foam, climate zone 6

climbing_carpenter | Posted in Energy Efficiency and Durability on

Outside of my wheelhouse to perform the calculations, currently do not have access to PHP.

Will function as a shop space, existing garage with shiplap siding/ sheathing and open stud bays.  Our goal is to keep the stud bays open (no cavity insulation).

What’s the minimum thickness foam required to keep the shiplap warm?  Not as concerned with energy savings as it’ll be a part time shop space.  We’ll run an airgap over the foam.

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  1. Expert Member


    Here is what Martin wrote in his article on minimum foam:

    " order to keep the inside face of the sheathing warm enough to prevent "condensation," the ratio between the insulation outside of the sheathing and the insulation inside of the sheathing matters. The more insulation inside of the sheathing, the colder the sheathing.

    If the stud bays are empty, you really don't have to worry about minimum foam thickness. There really is no minimum in that case -- other than the existing code requirement for wall insulation. That's the beauty of the PERSIST system."

  2. climbing_carpenter | | #2

    Thanks Malcolm!

    No enforceable code! I typically build with zone 6 in mind.

    I would assume that there is a minimum thickness of exterior foam with zero interior insulation? We’d see frost on the walls with 1 inch of foam in January, and warm, moist interior air as the sheathing itself would be frozen. The PERSIST system would merely allow the sheathing to dry?

    1. Expert Member
      Deleted | | #4


  3. Expert Member
    PETER G ENGLE PE | | #3

    It takes only a little bit of foam to keep the sheathing warm/dry. 1" will do nicely. In an unconditioned and uninsulated building the sheathing will definitely be wet and frozen in winter. And if you only warm up the building long enough to do work in it, the walls won't dry out ever. Any moisture generated by interior activity will condense out onto the sheathing and the cycle continues. But add 1" of foam and the dynamics change quote a bit even if the building is only used occasionally. For one thing, heat from the ground becomes a significant factor, as does the small amount of solar gain you get in CZ6. The mass of materials inside becomes great enough to stretch out the time factor of cool-down after use of the building. More insulation is generally better and you should balance the amount of insulation with the amount of time the building will be used now and in the future. You typically only have one opportunity to add insulation to the exterior, as its lifespan is generally longer than your ownership of the building.

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