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Rigid Insulation on the interior

PgJHtb9449 | Posted in Energy Efficiency and Durability on

This could be a newbie question or just so utterly wrong that even newbies don’t ask it, but I’m remodeling the upstairs of our older home (60-70 years with 2 x 4 walls). Living in Wisconsin we have fairly high winter heating bills. I was considering adding rigid insulation (xps) between the vapor retarder for the fiberglass batts and the drywall as opposed to furring out the walls to add more insulation. From the limited amount of reading that I’ve done the vapor barrier is already meant to be less permeable then my exterior so the wall should “air out” to the exterior. Plus the rigid insulation should help counter the effects of thermal bridging which furring out the wall won’t accomplish. Other than needing longer than normal drywall screws what would be the issue with attaching rigid insulation behind my drywall to shore up my insulation value and to limit the problems caused by thermal bridging. After all my vapor barrier should be less permeable then the rigid insulation anyway. From what I’ve read a one inch thick piece of xps has a perm rating of around 1.0 and is considered semi-permeable.

Any thoughts would be welcomed.

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    People do it all the time. I've done what you suggest on several homes.

    Some people don't like the feel of the foam under the drywall and prefer to install horizontal 1x3 strapping over the foam, but I've always had success without the strapping. (I used 1-in. thick rigid foam).

    Of course, be sure you relocate your electrical boxes, or install extenders.

  2. user-659915 | | #2

    One minor note of caution: code-related dimensional issues. If your renovation is extensive enough to require permits the loss of an inch against an exterior wall could put you out of compliance with code in certain situations - stairway widths, bathroom fixture spacing etc. Of course with an older home these elements could be out of compliance already......
    It would take an extremely fussy inspections department to pick up on this, but it does happen. If in doubt bring it up with the inspector early. They almost always respond well to a pro-active collaborative approach from the homeowner or builder.

  3. Riversong | | #3

    Joe,

    If you can swing it, I would recommend adding 2" of XPS on the interior and horizontal strapping (which also gives something to attach electrical boxes to) before rehanging drywall. If you have R-11 batts in the walls now, the additional R-10 will bring your walls up to current code standards for your climate zone.

    I would also recommend taping the joints in the foam board and sealing the edges and any penetrations (including electrical boxes) with spray foam to create a good air barrier which is essential with fiberglass - both for moisture control and energy efficiency.

    If you also seal up other air leakage points and repair any weatherstripping, you should have dramatically lower heating bills. You might consider an energy audit and blower door test to see where the weak spots are.

  4. PgJHtb9449 | | #4

    Thanks a lot to everyone who responded. This is the exact information I was looking for.

    Thanks,
    Joe

  5. user-993901 | | #5

    I have a similar situation on my older home but am considering installing 1" xps on the interior of the second story roof slope. Our climate requires a .5 perm vapor retarder so I have a couple of concerns. 1.) The existing condition has 2x4 rafters with R-13ish faced insulation with no circulating air channel. It has seemingly done fine over its 85 years (although it gets stifling hot up there in mid summer) so by adding the rigid and retarder am I changing the assemblies nature enough to cause problems (moisture) that dont presently exist? 2.) have you had any problems with the rigid insulation compressing and causing nails to pop or joints to crack and would the 1x3 strapping help this in any way?

    Thanks

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