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Can I add foam insulation to the inside of a wall instead of exterior?

myuschak | Posted in Green Building Techniques on

I live in a three story end unit town house. I am getting ready to remodel the master bedroom. Our closet is on the north exterior wall and I was looking to remove the existing sheet rock on the north wall and install new insulation along with stryofoam insulation board.

Since I can’t install the foam board on the exterior, can I install it on the interior for additional insulation? And if so do I use unfaced insulation and the foam board will act as the vapor barrier?

Thank you
Mark Yuschak

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Since I can't install the foam board on the exterior, can I install it on the interior for additional insulation?"

    A. Yes.

    Q. "Do I use unfaced insulation and the foam board will act as the vapor barrier?"

    A. Unfaced insulation is probably best, although your studs can be filled with a wide variety of insulation types. You probably don't need a vapor barrier, although it's impossible to say unless you tell us your climate or your location.

    More on vapor retarders and vapor barriers:
    Vapor Retarders and Vapor Barriers

    More in interior foam versus exterior foam:
    Benefits of exterior foam vs interior foam

  2. Mark Yuschak | | #2

    I live in central NJ and it could be mixed humid or cold

  3. GBA Editor
    Martin Holladay | | #3

    On the interior of your wall, you need a vapor retarder, not a vapor barrier.

    Any type of rigid foam insulation qualifies as a vapor retarder.

  4. Riversong | | #4


    Yes - interior foam board is a well-tested method for improving thermal performance and comfort in a renovation.

    Among the several advantages of interior foam board in a cold climate is that it is placed on the correct side for a vapor retarder, and it is far less important that the foam be permeable to water vapor, thus allowing the use of less environmentally-harmful polyiso board.

    Another advantage of using foil-faced polyiso is that you can place horizontal strapping over it, thereby creating a radiant barrier space that will offer an additional R-3 just from the air space and keep the interior wall warmer in winter and cooler in summer because of the decrease in radiant transfer.

  5. jnarchitects | | #5

    How are electrical switches and outlets handled in this situation? Just padded in to account for additional wall thickness? Do you seal b/w all of the cut edges of the foam and the electrical boxes?

  6. GBA Editor
    Martin Holladay | | #6

    Either plan ahead and attach the boxes with the right amount of protrusion, or use box extenders.

    Yes, it's a good idea to seal the gap between the foam and the electrical box. Ideally, you should use airtight electrical boxes.

  7. Riversong | | #7


    If you strap over the foam, then electrical boxes can be moved out to the strapping. The strapping, holding the foam board tight to the studs, allows blowing dense-pack cellulose through holes cut in the foam which are then sealed with the cutout plugs and canned foam or foil tape if the foam is foil-faced.

    If no strapping, then I cut ½" plywood rectangles the height of an electrical box and the depth of the stud plus foam and nail or screw those to the side of studs at electrical box height. The boxes (plastic works best) are then screwed to the plywood extension plates.

    After electrical rough-in, the backs of the boxes are sealed with low-expansion gun foam, insulation batts installed (recycled blue jean, never fiberglass), foam board seams are taped and all edges sealed with gun foam, including around the electrical boxes (it's actually easier to do this if oversized openings are cut in the foam board, leaving a gap to fill with spray foam).

  8. Riversong | | #8

    Actually, the expensive air-tight boxes are not necessary with interior rigid foam because it's easy to seal regular plastic boxes with spray foam.

  9. Shawn | | #9


    I'm intrigued by your interior foam method, particularly with a remodel (garage conversion) where siding removal is not an option. Eliminating thermal bridging seems to be a big "win" for the interior approach.

    I'm curious why, when describing the batt installation, you say "recycled blue jeans, never fiberglass?" Is this a choice resulting from the desire to use a more "green" material, or is there a technical problem with fiberglass batts behind polyiso?

    Lastly, what wood is used for horizontal strapping? 1x3? 2x4? I assume it needs to be somewhat substantial if you're attaching drywall to it.

    Great discussion.

  10. Riversong | | #10


    Fiberglass is so air/vapor permeable, it's the reason that codes used to require a poly air/vapor barrier. The current code requirement is that fiberglass batts have an air barrier on all six sides, an often impossible requirement.

    Also the R-value of fiberglass decreases as the temperature falls below or rises above room temperature, in other words when it's most needed. Fiberglass is a known carcinogen and often uses formaldehyde as a binder - a primary sensitizer or trigger for multiple chemical sensitivity and asthma.

    Fiberglass batts are nearly impossible to install without voids and compression, which further undermines its insulating value. Fiberglass almost always becomes home to insects and rodents, with rodent waste and dead rodents accumulating over time. Fiberglass does nothing to reduce the flammability of a structure and may - with kraft facing - actually make the house more flammable. And fiberglass batts are often associated with in-wall condensation, mold and rot.

    If those are not enough reasons to avoid fiberglass, then their embodied energy and ecological footprint can be factored in, particularly when compared to the extremely low footprint of such alternatives as recycled blue jean batts or - better yet - dense-pack cellulose.

    Oh, yeah... 1x3 strapping is sufficient. The strapping is screwed to framing through the rigid foam, limiting the number of conductive penetrations, and the DW is screwed to the strapping.

  11. Shawn | | #11


    I'm still not convinced...


    Thanks for the thoughtful reply. It seems like the "dream" solution is polyiso with blown-in cellulose.

    I wonder if there's a similar way to handle the remodeled vaulted ceiling?


  12. Riversong | | #12

    Yup, same way.

  13. Shawn | | #13

    Think it's worth creating a ventilation method with the roof, or just dense-pack the cellulose between rafters, then cover in polyiso?

  14. GBA Editor
    Martin Holladay | | #14

    According to most building codes, your proposed insulation method requires a ventilation channel between the top of the insulation and the roof sheathing.

    To learn more about insulating sloped roofs, see Creating a Conditioned Attic.

  15. lanthony2020 | | #15

    You answered my questions. But, the new Eco-Touch that is on sale right now is dirt cheap and un-faced advertises an R-14 in 2x4" channels? Is this new fiberglass insulation better than the old stuff in any other way than, increased R?

    I am remodeling my old farm house in Eastern South Dakota. Temps can reach -30 F and -120 F if you factor wind. I have been thinking of many methods including yours above. I also was thinking of extending the studs only with rigid foam and filling with mineral wool or now, this new and supposedly better fiberglass. I am planning on triple pain windows and want to get the walls right and avoid condensation.



  16. Expert Member
    Dana Dorsett | | #16

    You realize this is a nearly three year old thread, right?

    Higher density fiberglass is more air retardent than low density fiberglass, enough to make a difference. But any batt solution has to be installed perfectly to hit the rated performance.

    The difference between R13 and R15 cavity fill has a remarkably small impact on whole-wall performance after factoring in the thermal bridging, but R15 rock wool or R15 "cathedral ceiling" fiberglass is remarkably more air-retardent, approaching that of wet-sprayed cellulose.

    But no degree of air retardency on batt insulation or blown is a substitute for air-sealing with goop (acoustic sealant type caulk, or can-foam). When you have the cavities open, meticulously caulk or foam seal the studs to the sheathing, any electrical & plumbing penetrations (including where the power is run through studs), the stud plate to the sub-floor, and between doulbed-up top studs.

    Adding 2" foam to the interiors of 2x4 studs to be able do accommodate rock wool R23s or cathedral-ceiling R21-22 fiberglass works. You'll still need ot use air tight methods on the gypsum, and a "smart" vapor retarder like Certainted MemBrain or Intello Plus under the gypsum to protect the sheathing.

  17. lanthony2020 | | #17

    Thanks, this was very informative. As a new member and just searching for answers I want to thank you for your advice via an "old link."


  18. lanthony2020 | | #18

    how about R15 Roxul in the 2x4s, Murphy wall 1x3s and put 1 inch foil facing in toward Roxul?

  19. GBA Editor
    Martin Holladay | | #19

    First of all, I'm guessing that your meant to write "Mooney wall" instead of "Murphy wall." (I'm not sure what a Murphy wall is, unless it is a wall with a Murphy bed.)

    Most Mooney walls use blown-in insulation, so that you get added R-value from the depth of the horizontal interior strapping. It's possible, of course, to install batts between the horizontal strapping if you want to do that type of fussy work (although it would be hard with 1x3s). It's also possible to give up the R-value, and just use the strapping depth as a wiring chase.

    I don't recommend that you install a foil vapor barrier on the interior side of your wall. (Perhaps you were thinking of installing foil-faced polyisocyaurate, but you didn't write that.) An interior foil vapor barrier has several disadvantages -- most glaringly, the fact that it prevents inward drying -- and relatively few advantages.

  20. lanthony2020 | | #20

    Thanks again. I really want a lot of bang for the buck. I am doing this house one area at a time and really want the max "R" and minimal thermal bridging. You are right, it is "Mooney." Sorry, the old memory is not as good as it once was.

    So I am down to 2x4 mineral wool + 1" foam OC pink lapped foam board. Or fir stripping out with foam board strips and adding 5.25" mineral wool or Roxul? I worry about the attachment of dry wall on the latter, with the 1.5-2" foam strips. What do you recommend?



  21. lanthony2020 | | #21

    Should I also use "Spray Seal on all the seams before roxul? There is the new spray seal in the 5 gallon buckets folks are using if their airless is powerful enough. Sort of makes the space "spray foam" like? I do not know if i want to get this involved as this first area is the only area that needed to be taken down to the studs. The other exterior "out" walls, I was thinking about adding (glue/special foam screw) lapped pink board again and re=drywalling?


  22. GBA Editor
    Martin Holladay | | #22

    There are too many variables to provide a single answer to your question about "the most bang for your buck." What you want is a wall that is as airtight as you can get it. You can achieve airtightness with caulk, with canned spray foam, or with one of the new sprayable caulk products that you refer to.

    Once you've paid attention to airtightness, the next thing to think about is R-value. The higher your R-value, the better. There are so many choices -- cellulose, mineral wool, fiberglass, and rigid foam -- that it is hard to come up with a simple answer to your question. Choose the type of insulation that you are comfortable with, make it as thick as you can afford, and do a careful job of installation.

  23. lanthony2020 | | #23

    Thanks Martin.

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