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Community and Q&A

Interior Rigid Foam on Solid Masonry Walls–Am I Going to be Okay?

Mark Waldron | Posted in Energy Efficiency and Durability on

I have bought a home with solid masonry walls (CMU block interior wythe, concrete “crick” exterior wythe, plaster on the interior of the wall). I would like to add rigid foam insulation to the interior, with a gypsum board interior finish. The foam would be 1.5″ XPS (caulk around sheet perimeter, foam and/or tape between sheets) , then 3/4″ furring strips 24″OC, affixed with Tapcons to the block, with 3/4″ XPS between the furring strips. Attach drywall to the furring strips.
The home is near Dayton, Oh (extreme southern part of Zone 5).
I’ve read the helpful article by Martin Holladay regarding the issues/potential pitfalls of interior insulation of brick buildings (, and some other studies/articles on the subject. For my little project (a modest 1200 SF house), I just can’t justify the expense of having a consultant come look at the project to evaluate the bricks, etc. I’m not interested in maximizing the insulation, I’m okay with staying well clear of any danger zone (of keeping the bricks too wet/cold and inviting damage) if I can still significantly improve the efficiency/comfort of the present approx. R-2 solid walls..
My question: Is it likely this R-10 of interior foam will put the masonry at risk?
Mitigating factors:
– It’s not especially cold here,
– The exterior “bricks” are painted (reduced wetting/absorption?)
– The masonry appears to be in good shape. There are a few places where small bits of mortar are coming loose (house built in 1950), but things are generally in good shape. A chimney needed to be rebuilt, but it was due to its location at the eaves/lack of a cricket/and poor flashing, and water intrusion.
– The windows/lintels look like they are shedding water well
– I’m not adding much insulation. I see some of these projects much further north and with unpainted brick, adding R-20 or more to the inside of their walls and things are apparently fine.
Exacerbating factors:
– I’d guess that concrete “bricks” would absorb more water than fired clay bricks.
– The house is a Cape Cod style, with tiny roof projections.

Thanks for any opinions/ideas.


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  1. GBA Editor
    Martin Holladay | | #1

    What is a "concrete 'crick' exterior wythe"?

    Are these concrete bricks or clay bricks?

  2. Expert Member
    Dana Dorsett | | #2

    Freeze/thaw spalling of brick is a bigger problem in US climate zone 6 and higher, you're on the warm edge of zone 5. That's not to say spalling is unheard of in zone 5 (or even zone 4).

    The amount of direct wetting makes a difference, as does the amount of wicking that the paint allows (I'd hazard that it's a not very porous alkyd paint- maybe even leaded if it's old enough.) If the paint has held up and isn't flaking off, odds are you're in pretty good shape. But less wetting is always better- how deep are the "tiny projections" of eave & rake overhangs?

    Lose the XPS- it's R5/inch degrades to R4.2/inch over time, as it loses it's HFC blowing agents (which are powerful greenhouse gases.) Instead use foil-faced polyisocyanurate which is blown with much more benign pentane, and in your climate will perform at it's rated R value most of the time, even in winter.

    Polyiso performance drops when the average temp through the foam is below 40F or so, needs to be derated some if it's on the exterior side of an insulated assembly, but when it's the total insulation the average temp through the foam will be higher than that. The mean temp for Dayton in January is about 30F, so in a 68F house the average temp through the foam will be about 49F, a temperature at which some polyiso outperforms it's labeled value (which is tested at a mean temp of 75F for labeling purposes. See:

    So, the polyiso will run about R5.5-R6.5/inch (to XPS at R5/inch, eventually dropping to R4.2/inch) and foil facer next to a 3/4" air gap created by the furring gives the assembly another ~R1 that you wouldn't have with XPS.

    If the roof overhangs are deep enough to accomodate it, an alternative would be to coat the exterior bricks with a spray applied weather resistant barrier, and put 2" of 1.5lb density "Type-II" EPS (also blown with pentane), and finish it with a EIFS stucco material. Even though 2" of EPS it's only R8.4 it would perform about as well as R12 polyisocyanurate on the interior side due to the thermal mass advantage of having the masonry inside the insulation envelope.

    Martin: I took "crick" to mean "c" oncrete b "rick"/

  3. GBA Editor
    Martin Holladay | | #3

    I don't think that the concerns over freeze/thaw damage to clay bricks apply to concrete.

    In other words, I don't think you have to worry.

  4. Mark Waldron | | #4

    - "crick" was my typo, sorry. The exterior wythe is "concrete brick"--they are the size of standard fired bricks, but are made of concrete with rough aggregate.

    Thanks for the input, I appreciate it. I was planning on XPS to allow modest drying of the masonry to the interior, which would seem to be of benefit in avoiding freeze/thaw damage to the masonry. I'd even thought of using EPS for its higher perm rating for this reason. Of course, there will be none of that with foil-faced polyiso. Am I wrong about the importance of allowing the masonry to dry to the interior?
    An aside: Has polyisocyanate gotten less expensive lately? I recall it used to be significantly more expensive than XPS, but now my local Menards has them both at approx the same price. If I went with 1.5" thick polyiso and the reflective face/ 3/4 air gap between furring strips, I'd be at about R10, which would be great. But I do worry about that need for drying the masonry to the interior.

    Yes, I would love to put the insulation outside the masonry, but it's just too pricey for my budget. The eaves and rakes project about 2" beyond the brick face, so there's not much room there. I have the skills to do drywall and the material is inexpensive, but I don't have the skills to do a high-quality (i.e durable, weather-resistant, moisture managed, good looking) stucco/EIFS job, and the materials would be too expensive. Plus, the brick exterior will hold up to errant baseballs, falling tree limbs, and other hazards of neighborhood life (with kids). If I can get these walls to R6-R10 at reasonable cost and keep the walls from crumbling, I'll be satisfied. I know it won't meet current building codes code (or feature in GBA's "greatest hits"!) but it will be miles ahead of the present situation.

    Thanks a lot for the informed opinions!

  5. Expert Member
    Dana Dorsett | | #5

    At 2" XPS is a Class-II vapor retarder, orders of magnitude too vapor tight to deal with any direct wetting of the brick. The brick HAS to dry toward the exterior, or vertically up the CMU wythe into the attic.

    If you only have 1.5" of interior foam depth to work with, you'll get an honest R10 out of foil faced polyiso with the air gap included. That is roughly the whole-wall performance of a typical 2x4/R13 framed wall, which is WAY better than nothing! The brick & CMU give you another R1.5-R2, so it's really not terrible.

    If you can find 1.5-2" reclaimed roofing polyiso it will be cheaper than virgin stock foil faced goods, and a bit more rugged too. At 2" it'll be an honest R11, at 1.5" it would be R8+. These folks in Akron/Canton area seem to have a stock of 2": (advertised here: )

    For something closer, run this search every week or so, see if something comes up.

    Gluing 2" polyiso to the brick, & wallboard to the foam with foam board construction adhesive and using crown molding kick-board (maybe even chair-rail) Tapconned to the brick for a more robust mount would run about about the same depth as the 1.5" XPS + 0.75" furring + 0.5" wallboard proposed.

  6. Mark Waldron | | #6


    I don't think thate concerns over freeze/thaw damage to clay bricks apply to concrete.
    In other words, I don't think you have to worry.

    Thanks. If concrete blocks (and the mortar) are less susceptible than fired clay bricks/mortar to freeze/thaw spalling, then that is good news indeed.

    I appreciate the very thorough and helpful reply. I'll have to check out Craigslist for potential sources of polyiso insulation. I've really been surprised at the moderate price of brand new polyiso at retail building supply stores. For example, Menards near me sells faced 1" polyiso for 39 cents PSF, while 1" XPS is 53 cents PSF.
    In the project I'm doing, the exterior walls on two of the bedrooms have already been insulated with 1" >faced< EPS behind panelling and stained OSB (i.e. not a suitable thermal/ignition barrier). I was all set to tear down the EPS due to the facing/lack of drying, but if it vapor transmission to the inside is not needed, then (depending on what I find when I tear off the panelling/OSB), I might just leave the faced EPS on the wall and add another layer of something else between the furring strips, add drywall and call it done (on those walls). That EPS may cost me about R2 vs 1" polyiso, but also save me some time, materials cost, keep a few sheets of EPS out of the landfill, etc.


    Spalling may be unlikely, but you want to do the things to reduce risk anyway:

    1) stop bulk water leaks
    2) block ex-filtrating air
    3) balance water vapor movement (ie, more external perms than internal).

    I'd use EPS for it's slightly better summer drying performance. Or unfaced polyiso.

    Thanks. In order:
    1)Bulk water: There don't >seem< to be any problems now, but I gotta say I'd be a lot happier with even modest eave overhangs. The house needs a new roof and I'm going to explore the idea of adding 1' overhangs--it would give me a chance to add soffit vents (the roof now has zero intake ventilation, which I suspect is one reason the shingles failed early). It would be a lot easier and cheaper just to tuck a vent strip under the second row of shingles at the bottom, but having even a small overhang would reduce water on the brick/windows, give a little shade to the wall at mid-day, etc.
    2) Air exfiltration: I'll be conscientious about caulk/foam when I put up the interior insulation and drywall.
    3) Vapor movement and summer drying: If I had a cavity wall (studs/fiberglass) inside of the masonry, I'd definitely want some drying to the inside to avoid a huge issue in the summer. But with rigid foam right up against the masonry (and caulked at the edges), will I have a problem in the summer if the insulation has impermeable faces? Clearly, I used to think so (why I planned to remove the existing faced EPS), but maybe not. Aside from my "gut" desire to get water out of the wall, if the masonry's interior surface remains a bit more wet in the summer (with a Class 1 vapor retarder than it would be with a Class 3 vapor retarder), will I really have an issue?

    Thanks again for the information and insight.

  7. Jon R | | #7

    I expect the risk is quite low - I wouldn't remove any existing non-permeable foam.

  8. Jon R | | #8

    Spalling may be unlikely, but you want to do these things to reduce risk anyway (for other reasons):

    1) stop bulk water leaks
    2) block ex-filtrating air
    3) balance water vapor movement (ie, more external perms than internal).

    I'd use EPS for it's slightly better summer drying performance and good R/$ value. Or used unfaced polyiso.

  9. Expert Member
    Dana Dorsett | | #9

    "1" >faced< EPS behind panelling and stained OSB (i.e. not a suitable thermal/ignition barrier)."

    Half inch (7/16" actually) OSB or plywood (painted or not) qualifies as an ignition barrier- not need to add anything or to rip it out, unless you want to bring it fully up to code for R-value.

    Impermeable foam on the exterior of an insulated 2x4 wall keeps summertime moisture problems (the moisture drive off the masonry into the cooler wallboard & studs) mitigated needs to be at least R5 to avoid wintertime moisture accumulation. An inch of foil faced EPS isn't quite enough, but adding even 1/4" fan-fold XPS siding underlayment between the EPS and studwall gets you there, as would adding an additional layer of half-inch rigid foam (any type.) If you want to leave the OSB in place and not install additional foam you'll still be OK from a dew point perspective if you use low density R11s, not R13 or higher in the studwall, but the thermal performance would be slightly less than code.

    When re-roofing, rather than building structures to extend the overhangs and add venting it's probably better to add 4-6" of reclaimed roofing polyiso on the exterior held down with a nailer- deck through-screwed to the structural roof deck with pancake head timber screws. With the foam and nailer deck extended beyond the current drip line & trimmed level with the ground, even a 12:12 pitch would add about 6" of additional overhang. Lower pitches would add even more. If foam is held down by 2x4 furring through-screwed to the rafters, mounting the nailer deck on the furring it would add about 8.5" of additional overhang to 12:12 roof, and the nailer deck would be vented. With 4" of exterior polyiso & R20 batts between the rafters would meet current IRC code performance on a U-factor basis, and would have plenty of dew point margin for the structural roof deck. With 4" of exterior foam you could add up to R30 on the interior and still have sufficient dew point control, assuming the rafters are deep enough to accomodate R30s (2x10s), and it would beat code minimum on both R-value and U-factor basis.

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