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Community and Q&A

Furring strip inside of exterior foam or nailbase Zip sheathing?

Ethan ; Climate Zone 5A ; ~6000HDD | Posted in Plans Review on

I am working on a home envelope renovation in climate zone 7 and we are trying to add insulation to existing exterior walls.

As shown in sketch, existing exterior walls have clear plastic sheeting (probably not “smart”) between studs and GWB at interior, fiberglass batt in 2×6 stud cavities, and plywood on exterior.

I am proposing 2″ of foam to exterior as well as nail base Zip for a total of 4″ of foam.Contractor wants to add furring strips between existing exterior sheathing and new foam.

He says this will help system dry, but shouldn’t air cavity be between exterior foam and siding (cedar or hardee board)?

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  1. GBA Editor
    Martin Holladay | | #1

    Your contractor is wrong. The foam layers need to be tight to the existing wall sheathing, with no air gap.

    The air gap belongs on the exterior side of the nailbase, between the WRB that you will install to protect the nailbase and the siding.

    If your contractor proceeds with his plan, the air gap will increase the chance that air currents through the deliberate gap or convective currents within the gap will undermine the thermal performance of the insulation. You definitely don't want any exterior air in the middle of your wall -- and I can't understand how your contractor thinks there will be any drying without air movement.

    For more information on this issue, see All About Rainscreens.

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