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Rigid Insulation Between Double Studs

Bowler222 | Posted in Green Building Techniques on


I was wondering if someone could check me on my proposed wall assembly and provide recommendations if I’m off base…

I”m planning on utilizing a double stud wall assembly, two rows of 2×4 studs at 24″ O.C. with a 3″ gap between. The two stud bays will be filled with Roxul comfort batt. The exterior will be sheathed with 1/2″ plywood and covered with housewrap. The siding will be lapped fiber cement board. Interior is latex painted gypsum drywall.

Now for my question: Is there a problem with filling the 3″ gap between the two stud walls with XPS rigid insulation? I would tape the inside seams and apply sealant where it abuts the top and bottom plates to form my air barrier. I’d like to reserve the inner row of studs as a utility chase as to eliminate worry about penetrations in my air barrier. I realize that this much XPS is nearly (if not) a vapor barrier but I figure the wall can dry away from the rigid to both the interior and exterior. I would still like to utilize the air tight drywall approach as a belt and suspenders effort but the XPS would be my primary air barrier.

Any issues with this approach? Should I use EPS instead as it is more permeable? Incidentally, I intend on purchasing recycled XPS from Insulation Depot which doesn’t seem to be a whole lot more expensive than Roxul.

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  1. wjrobinson | | #1

    Complicated idea. Less complicated builds to me would perform more to my desire. Ten ideas are not my idea of a good build. Less is more.

  2. GBA Editor
    Martin Holladay | | #2

    I don't think that your proposed wall will have any moisture problems, but it isn't particularly buildable. Introducing a layer of rigid foam between the two rows of studs complicates the wall framing. It also complicates the cellulose installation. Talk to your cellulose installer and think through the order of construction.

    Finally, if you are willing to invest in the expense of a layer of rigid foam, why not install it where it belongs -- on the exterior side of the wall sheathing, where it can perform the important task of helping to keep the sheathing warm and dry?

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