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Musings of an Energy Nerd

Walls With Interior Rigid Foam

When remodeling an older home, it sometimes makes sense to install rigid foam on the interior side of exterior walls

Some remodelers install rigid foam on the interior side of exterior walls. While the best place to install rigid foam is usually on the exterior side of the wall, that approach often can't be used on an existing house.
Image Credit: Image #1: Iso R Plus

There are two main ways of reducing thermal bridging through studs: you can build a double-stud wall, or you can install a continuous layer of rigid insulation on one side of the wall.

Most builders who install a continuous layer of rigid insulation use rigid foam (polyisocyanurate, expanded polystyrene, or extruded polystyrene); a small minority of builders use semi-rigid panels of mineral wool.

Builders who install rigid foam on the walls of a new building usually install the foam on the exterior side of the wall. There are several reasons for this:

Unlike new home builders, remodelers can’t start with a blank slate. If a remodeler wants to reduce thermal bridging through studs, the exterior foam option is usually prohibitively expensive — especially if the siding is in good shape. So many remodelers ask, “Why can’t I put the rigid foam on the interior side of the wall?”

The answer is, you can.

Back to the 1980s…

In the 1980s, most cold-climate builders were taught that walls needed an interior vapor retarder, and needed to be able to dry to the exterior. When a few pioneers started installing exterior rigid foam, some people thought the practice was risky.

“Rigid foam is a vapor retarder, so it belongs on the interior,” some old-time builders opined. “Exterior rigid foam is risky, unless you live in Florida. It’s a wrong-side vapor barrier.”

Eventually, researchers proved that these builders were wrong. (To learn more about the history of vapor barrier requirements and the origin of unjustified concerns about wintertime vapor diffusion, see “Do I Need a Vapor Retarder?”)

Exterior rigid foam works very well indeed, as long as the foam is thick enough to keep the interior face of…

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22 Comments

  1. Mark Fredericks | | #1

    Thanks
    Thanks Martin for yet again summarizing another green building question so well.

    You've got the following typo under What type of rigid foam is best?
    "because XPS contains contains both"

    Interior foam board seems like a great solution for remodeling dilemmas where exterior foam isn't a practical option, but I'm still unsure how the exterior should look, for this to be a safe assembly. I can think of my 1950's house with solid board sheathing, tar paper, and original wood shingles. This would seem safe enough, but this was all covered with 1/2" EPS, and wide plank color-lock siding. No rain screen gaps to encourage drying.

    This is not uncommon for the older homes in my area to have several layers of siding accumulate over the years. Can you comment more on when interior foam shouldn't be used? When does the exterior stack up create too much risk?
    Thanks!

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Mark Fredericks
    Mark,
    Thanks for catching the typo.

    There is no simple answer to your question about exterior layers.

    Unfaced EPS that is 1/2 inch thick is fairly vapor-permeable (about 4 to 12 perms), so that's not much of an issue. If the EPS has a plastic or foil facing, of course, the permeance could be much lower.

    Multiple layers of wood-fiber products, even when painted, are also (usually) permeable enough not to cause problems.

    If you install interior rigid foam, the interior foam layer reduces air leakage, and just about stops outward diffusion in the winter. That helps keep the wall assembly dryer.

    As long as the wall isn't getting regularly soaked by wind-driven rain, and as long as the wall has good flashing, I wouldn't worry too much. If the sun hits the wall, the wall will dry. North walls (and walls shaded by foliage) tend to stay a little damp, however.

  3. David Hicks | | #3

    foam for furring out studs
    Martin -- it's a little off the topic of your blog, but I wonder if you had an opinon on the method described by Stephen Bonfiglioli in the April issue of FHB, whereby he furred out the studs to the interior with foam to create a thermal break.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to David Hicks
    David,
    My first reaction to the idea of "Bonfiglioli strapping" is that this method seems quite labor-intensive.

    But one thing I've learned over the years is: there is no way to discourage a builder who has found a technique that he or she is comfortable with. If Stephen Bonfiglioli likes his system, that's great. It works for him. That makes it a good system.

    There are many, many ways to build a wall: the Mooney wall (2x6 wall with interior horizontal 2x4 strapping, all filled with cellulose), the double-stud wall, the Larsen truss wall, the Klingenberg wall, the wall with exterior foam, and the wall with interior foam. Now we can all consider the idea of Bonfiglioli strapping.

    Choose a system you like, and build it.

    .

  5. R Miller | | #5

    vent
    Martin, did you notice the vent stack on the exterior wall in the FHB article? That was surprising.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to R. Miller
    I agree. Locating a DWV pipe in an exterior wall is usually a bad idea -- especially if you are an energy-conscious builder, and especially if the wall has no exterior rigid foam.

    .

  7. Malcolm Taylor | | #7

    David
    A few companies make thermally broken studs - rstud and Nordic Industries come to mind. They don't seem to have caught on. It would be interesting to compare their prices once you factor in Mr. Bonfiglioli's time to make his homemade versions.

  8. User avater
    Steve Robertson | | #8

    Vapor barriers revisited
    At first I thought this might be a cold climate only technique. But you mentioned that this technique is fine even in climates with inward solar vapor drive during the summer. After reading your articles on vapor barriers as well as Joe Lstiburek’s, I am slightly confused.

    I’m assuming that all poly-iso panels are vapor barriers (Class I). In Joe's BSD-106 paper, figure 6 shows a brick veneer and typical 2x4 wall construction. My understanding is that the vapor barrier was eliminated from this wall by design. Are you saying that it is OK to add the poly-iso panels under the drywall in this type of wall?

    Would this rigid foam technique be allowed for an unvented cathedral ceiling? That is could the poly-iso be placed between the drywall and the rafters with batt insulation between the rafters?

    Thanks for this blog. Hopefully I will understand vapor barriers after this.

    Links to Martin’s blogs and Joe’s paper:
    Do I need a vapor retarder?
    Insulated cathedral ceiling
    Cut and cobble insulation
    BSD-106 Understanding vapor barriers

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Steve Robertson
    Steve,
    I don't see any reason why you couldn't install rigid foam on the interior of the wall shown in Lstiburek's sketch. Because rigid foam has R-value, it isn't as dangerous as polyethylene. Air-conditioned poly can be a condensing surface, but the outward-facing layer of rigid foam wouldn't get as cold as the poly.

    No, you can't install interior rigid foam on an unvented cathedral ceiling (unless you are following the cut-and-cobble technique, which I don't recommend). However, interior rigid foam is often used on vented cathedral ceilings.

    For more information, see How to Build an Insulated Cathedral Ceiling.

  10. Roman Stankus | | #10

    polyiso board facing
    Martin - can one use a fiber faced polyiso board rather than foil faced board - the fiber faced board is easier to obtain - (standard low slope roofing insulation)? Is this an air sealing issue?

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Roman Stankus
    Roman,
    Yes, you can use almost any type of rigid foam in this location, including polyiso with fiber facing. I don't have advice about the best tape to use, since I haven't ever had to tape this product. In my experience, however, you can tape almost anything with Siga Wigluv tape.

  12. Chris Stratton | | #12

    Moisture concerns about sandwiching interior plywood
    Hi Martin -

    I'm retrofitting a house in southern California with uninsulated stucco walls and no sheathing. My plan is to use the "sticks" in the stud bay corners method you describe to create an air gap behind the stucco/building paper and then cut and cobble polyiso in the 2x4 stud bays. So far so good I think. (Yes I am aware of the labor intensity of cut and cobble.)

    But the structural engineer has specified that these need to now be shear walls since there is no shear bracing otherwise. So inboard of the studs will go 1/2" plywood. I'm concerned about thermal bridging through the studs and through the plywood, and some simulations I've done seem to substantiate these concerns. So I am considering adding another 1" of polyiso inboard of the plywood, then drywall, and affixing the drywall by running screws through the polyiso into the plywood/studs. (Do I need furring between the polyiso and drywall?)

    Should I be concerned about sandwiching the plywood in between the interior continuous polyiso and the cut and cobbled polyiso in the stud bays? Will the studs provide sufficient pathway for any moisture in the plywood to get out? Thanks for your help.

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Chris Stratton
    Chris,
    I assume that you have read this article: Insulating Walls in an Old House With No Sheathing.

    Q. "Should I be concerned about sandwiching the plywood in between the interior continuous polyiso and the cut and cobbled polyiso in the stud bays?"

    A. I don't think there is much reason for concern. There shouldn't be much vapor drive through this wall assembly -- either inward vapor drive or outward vapor drive -- that might lead to moisture accumulation in the interior plywood sheathing.

    That said, there is no reason to choose a very low-perm material if it isn't necessary. What type of polyiso are you talking about -- foil-faced polyiso or a fiber facer? For the interior layer of rigid foam, the best choice would be either polyiso with a fiber facer (not a foil facer) or EPS. These types of rigid foam are somewhat vapor-permeable.

    -- Martin Holladay

  14. Chris Stratton | | #14

    response to Martin
    Martin,

    Yes I have read your article about insulating a house with no sheathing. It was very helpful. Thank you for writing and for referring me to it. I'll go with fiber-faced polyiso for the wall insulation, per your recommendation.

    Thanks again,
    Chris

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Christopher Merino
    Christopher,
    Q. "Does one need insulation in the wall cavity if installing foam board on the inside?"

    A. No, there is no need for insulation in the wall cavity, as long as the insulation plan meets local code requirements. (In most cases, retrofitting insulation in an older home will not require compliance with modern energy codes, but regulations vary from town to town, so it's usually wise to call your local building office if you are uncertain of your local regulations.)

  16. Christopher Merino | | #16

    interior rigid foam
    Does installing rigid foam insulation over interior wall studs negate the need for insulation in the wall cavities themselves? May they be disregarded?

  17. Ian DeGraaf | | #17

    interior foam plan and a service cavity?
    Thanks for this excellent resource. I am currently remodeling an 1890s house in climate zone 6. My wall from exterior looks like this:
    8" lap steel siding
    15lb felt / original
    original 1" x 6" horizontal sheathing
    true 2x4 studs / balloon framing

    Unfortunately, I did not consider installing insulation on the exterior when I resided a few years ago, so now my plan is to add this to the interior wall based on what I have read in multiple articles on this site.

    air seal the interior of the walls in the joist bays
    add insulation in the joist bays likely fiberglass or rock wool. R 13 likely
    2" of polyiso board on the interior of the studs = R 12 total
    2x strapping for both holding polyiso boards in place, attachment for interior wall coverings and to create an electrical chase.

    Another option would be to simply build out the studs by 2" vertically and I could use the Roxul 2x6
    R23 bats and then another 1 or 2" of polyiso on top.
    By my math this would only give me an additional R2 (23 in wall + 2" rigid @ 12 total)

    I am open to any feedback on this.

    Also has anyone built an electrical service chase on top of the interior rigid foam? It seems that the strapping for wallboard could work perfectly as the framing for this service cavity as well.

    Thanks in advance

  18. user-7287490 | | #18

    Martin,

    Not sure if this is still an active thread, but I am following up on a point you made stating this approach should not be used in a cathedral ceiling room. I am reinsulating several rooms with flash and batt approach (zone 5). These rooms are not vented. I would like to add rigid insulation to the underside of the roof to secure batts and reduce thermal bridging from rafters. Given your comment I am reconsidering this approach. Any guidance is greatly appreciated,

    Ryan

  19. User avater GBA Editor
    Martin Holladay | | #19

    Ryan,
    For an unvented flash-and-batt roof assembly to be safe, you need to pay attention to the ratio rules.

    A standard flash-and-batt job for a roof in Climate Zone 5 requires (according to most codes) that the closed-cell spray foam layer have a minimum R-value of R-20. That can be achieved with about 3.5 inches of closed-cell spray foam. (For more information, see "Flash-and-Batt Insulation.")

    If you are adding more insulation on the interior side, though, the ratio changes. Note that the ratio rule (for a roof assembly in Climate Zone 5) is that a minimum of 41% of the total R-value of the roof assembly come from the outermost layer of air-impermeable foam insulation (which in your case would be the closed-cell spray foam that has already been installed).

    For more information on the ratio rules, see "Combining Exterior Rigid Foam With Fluffy Insulation."

    So you need to do some math to see how close to the borderline of the 41% minimum you're already at.

    1. user-7287490 | | #20

      Martin,

      Thank you for your response. I split r23 roxul batts and put over close cell foam to fill the bay, and intended to apply foam board over that to secure batts and help with thermal bridging. With batts plus board I would be within the 41% rule (50+ percent).

      Based on feedback from another thread it appears I must use a relatively permeable layer to enclose the batts (unfaced EPS) due to the likely occurrence of condensation on interior closed cell foam. I have not been able to source unfaced EPS yet, and have started looking at other materials. If you have thoughts on this I would greatly appreciate it,

      Ryan

      1. User avater GBA Editor
        Martin Holladay | | #21

        Ryan,
        As long as the rafters are dry when you install the interior rigid foam -- and there's no reason for me to think they'd be wet -- then there really isn't any downside to using interior rigid foam with a low permeance rating. Plenty of people successfully install foil-faced polyiso on the interior of cathedral ceilings.

  20. akjone02 | | #22

    Mr. Holladay,
    Thank you so much for all your articles on these topics. They've helped me a great deal.

    We're building a new home in zone 6 and I planned for 2 inches of exterior XPS and R-23 rockwool cavities. Our builder feels strongly we should do 1.5 inch foil-faced polyiso on the interior instead of exterior foam. His reasons are that it's hard to attach siding through foam, and pests (ants specifically) often nest in it. He recommends the same change for the basement (no exterior foam, all interior).

    I've read several of your articles on walls and insulation and I'm concerned this moves our dewpoint to inside the stud cavity. We will have no exterior vapor-barrier so he says (and it looks like you agree in the article above) that it's OK if we have frost or condensation in the wall because it can dry to the outside during the summer.

    I also have concerns about putting the walls (both stud and basement) outside the insulation envelope and therefor subjecting them to bigger temperature swings.

    Do you have thoughts on doing interior foam in new construction like we are doing, as opposed to a remodel?

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