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Best insulation combination for a 2×8 wall?

Boris Rubinstein | Posted in General Questions on

hello have a question building a home in zone 5 , exterior wall of house is 2×8 with zip sheathing taped
was hoping to get R30 + from the wall using a combination of interior and exterior insulation
1) with zip sheathing do i still need and exterior membrane ?
2) interior insulation was planing on using fiber glass or maybe go up to mineral wool
3) exterior insulation because of zip do I still have to worry about vapor permeability , was looking at using xps or mineral wool on exterior what would be the better combination
4) planing on using fiber cement panels on exterior
hope to get advise on building the best combination

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Replies

  1. Nate G | | #1

    Don't use 2x8s; instead stagger 2x4s and make a double-stud wall. Fill it will dense-packed cellulose or fiberglass, or mineral wool batts if you want to make people renovating in the future not hate you. Then you don't need to worry about external insulation.

  2. GBA Editor
    Martin Holladay | | #2

    Boris,
    Q. "With Zip sheathing, do I still need an exterior membrane?"

    A. You can visit the Zip sheathing web site for more information. Zip sheathing is a water-resistive barrier (WRB), but some builders still use housewrap as a WRB, even over Zip, to make it possible to lap the WRB with flashing. For more information, see All About Water-Resistive Barriers.

    Q. "Exterior insulation -- because of Zip do I still have to worry about vapor permeability?"

    A. Zip sheathing is OSB. If you add exterior rigid foam, you still have to follow the minimum R-value rules provided in this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    Q. "I was looking at using XPS or mineral wool on exterior."

    A. Green builders try to avoid using XPS because it is manufactured with a blowing agent that has a high global warming potential. For more information, see Choosing Rigid Foam.

    If you use exterior mineral wool instead of rigid foam, you don't have to worry about following any minimum R-value rules for the mineral wool layer (because mineral wool is vapor-permeable). For more information, see How to Design a Wall.

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