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A layer of 1″ XPS foam between fiberglass batts?

stoneyrunner | Posted in Energy Efficiency and Durability on

I live in MN and am working on a complete interior remodel. The exterior walls are 2″x6″ w/ R19 fiberglass batt insulation, some with Kraft back & some with poly on the interior side. I would like to try & at least double the R value, but due to other design constraints would like to keep the additional interior intrusion to a minimum (exterior insulation is not possible).

Therefore, I’m thinking of installing 1″ XPS foam on the interior wall followed by a 2″x4″ wall filled w/ R13 fiberglass batts without any additional vapor barrier. My thinking is that the foam breaks up any thermal bridging & drastically reduces air movement while contributing to the overall insulating value.

My question is will I have condensate issues? My initial thoughts are that since since the more impermeable layer is more in the middle of the thermal envelope, thus kept warmer than the dew point, that any moisture that does develop on either side of that vapor barrier will be able to dry seasonally through the permeable exterior and interior layers.

This approach in theory gives me a total R-value of 38 while only adding an additional 4 1/2″ to my interior space. Does is sound like a reasonable approach?

Thanks much to any who take the time to give me some thoughtful advice!

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  1. GBA Editor
    Martin Holladay | | #1

    In most cases, your planned assembly will be fine. It would help to know what type of exterior sheathing and siding you house has, however. (The more vapor-permeable, the better.)

    There are a few tricky areas to consider when adding insulation on the interior side of walls. These areas include partition intersections and rim joists. If you can access these areas and improve the insulation levels there, it will improve your results.

  2. stoneyrunner | | #2

    Thank you Martin. The exterior cladding is 1/2 OSB, Tyvek housewrap, & vinyl siding on all but the south wall which has 3/4" OSB, 1" XPS foam, & vinyl siding. How do you think the S wall would fair under this scenario?

    As to the the probelm areas, I am able to access a fair number of those to seal up & insulate.

  3. Expert Member
    Dana Dorsett | | #3

    You will get better performance out of 1" foil-faced polyiso than with 1" XPS in that stackup, and it would be greener too, due to the much lower impact blowing agents used. Foil faced poliso is also easier to air-seal, which can be done reliably with 2" FSK (purpose made aluminum duct-tape.)

    The 2x6/R19 wall with half-inch OSB and half- inch gypsum has a "whole wall" performance of about R13 after factoring in the thermal bridging of the framing. Adding an inch of polyiso would bring that up to R16-R16.5. If you made the interior-side studwall stud spacing 24" o.c. and use unfaced R15 rock wool or fiberglass you'd be at about R28 whole-wall. With 16"oc. spacing and R13s it would be about R25.

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