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Do minimum thickness of foam rules still apply for the partition wall between an insulated attached garage and the house?

Rick Van Handel | Posted in Energy Efficiency and Durability on

Climate zone 6 location near Green Bay, wi. Partition wall is advanced frame 2×6 wall construction with planned 2 lb density blown fiberglass. I would like to use 1″ foil faced polyiso on the cold side of the wall to limit thermal bridging and also tape the panels for a primary air barrier.

The “cold” part of the garage is insulated 2×8 framing with 2×4 mooney wall strapping for a center cavity depth of 8.75″ of 2 lb density blown fiberglass. Exterior sheathing is zip taped cdx. The foundation walls are r-10 continuous foam, and the garage floor is also r-10 foam. I doubt the interior of the garage will ever drop below freezing, so I was hoping I could cheat a little bit on the foam thickness for the partition wall.

Thoughts?

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Replies

  1. Charlie Sullivan | | #1

    I think it will work fine if you mostly keep the garage doors closed. The only downside would be that if a future resident decided to keep the garage doors open most of the winter it would not stay above freezing in there.

  2. Expert Member
    Malcolm Taylor | | #2

    Our code considers the wall between an attached garage and a residence an exterior wall, with the same requirements as any other. Might be worth looking at how yours views it.

  3. GBA Editor
    Martin Holladay | | #3

    Rick,
    The wall is definitely an exterior wall as far as the code is concerned.

    If I were building the partition wall, I would detail it as an exterior wall -- to future-proof the house (in case a future owner decided to demolish the garage, and to protect the partition wall from moisture in case the garage doors were left open,as Charlie mentioned).

    If you want to install rigid foam on the exterior side of a 2x6 wall, the rigid foam needs a minimum R-value of R-11.25 in Climate Zone 6 or R-15 in Climate Zone 7. The upcharge from 1" thick foam isn't that much.

    For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

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