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Exterior Mineral Wool Insulation in Hot-Humid Climate

dgotsch | Posted in Energy Efficiency and Durability on

Should I be concerned about dew-point on the surface of ZIP sheathing behind exterior mineral wool in central Texas?

I live near Austin TX, and I’m replacing my siding. There was no sheathing underneath so I’ve sheathed the house in ZIP on the second floor and will do MgO panels with PolyWall’s AlumaFlash on the first floor for increased water + insect resistance. I would like to add 2″ of continuous exterior insulation (followed by rain-screen and likely Hardie planks). I’m trying to decide between 2″ of XPS and 2″ of Thermafiber’s RainBarrier (https://www.homedepot.com/p/Owens-Corning-Thermafiber-RainBarrier-CI-HC-2-in-x-24-in-x-48-in-Mineral-Wool-Insulation-96-Pieces-1096865/319282702)

The way I see it, if water ever gets between XPS and ZIP it will stick between the two (mostly) water proof surfaces causing damage to the OSB if it finds any holes/scratches.

On the other hand, if I go with the mineral wool which allows a lot more moisture through (great for drying), in the summer months humidity could condense on the surface of the ZIP sheathing behind the wool since it should be colder than outside air thanks to the insulating properties of the mineral wool.

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Replies

  1. Expert Member
    KYLE WINSTON BENTLEY | | #1

    I think you'd be fine with rockwool. They've got a doc on your climate that may be useful:

    https://www.rockwool.com/syssiteassets/o2-rockwool/documentation/brochures/commercial/exterior-wall-solutions-for-hot--humid-climates.pdf

    1. dgotsch | | #2

      Thank you, that answered a lot of my questions. Although I'm still confused about dew-point in such a system.

  2. Expert Member
    AKOS TOTH | | #3

    Condensation will only happen on a cold surface. In cooling dominated climates this your interior drywall, exterior insulation or not, this won't change. Adding exterior insulation (permeable or not) will lower the sheathing temperature a bit but not enough for it to matter.

    Your assembly is hard to justify in our much colder climate. I can't see there being a good reason for exterior mineral wool insulation unless your existing walls are not insulated at all. Your money is better spent on getting good exterior shading for your windows and more efficient HVAC equipment.

    The important detail to get right is air sealing, which it sounds like you are well on the way by re-sheathing the house and taping the seams. Make sure to figure out how to tie your wall sheathing air barrier into your foundation and to your ceiling air barrier for air barrier continuity.

    Outside of that, a rain screen is a good idea for both siding paint durability and adding extra drying capacity to your sheathing.

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