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Spray foam for interior walls with external siding and rigid foam

user-6495937 | Posted in Energy Efficiency and Durability on


My house has existing vinyl siding with 1″ foam (not sure what kind yet) installed over the old clapboards and then the vinyl. Inside, there is no insulation in the 2×4 stud walls covered with crappy drywall. This is a coastal house in Zone 5 (Massachusetts).

The wind blows through all the exterior siding, mainly due to poor window installation. I will be replacing the windows and drywall, correctly flashing the windows. After that, my question is about the interior wall insulation. Can I just spray open or closed cell foam in between the studs and cover with drywall? My building inspector says anything is fine with him, no need to go to code (R21), he just wants anything better than nothing. I am hoping the spray foam will block any air leaks that blow through the exterior.



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  1. GBA Editor
    Martin Holladay | | #1

    Dana's advice is good. Open-cell foam is preferred to closed-cell foam for environmental reasons (most closed-cell foams have a blowing agent with a very high global warming potential, unless you use one of the new types of friendly closed-cell spray foams like Lapolla 4G).

    If you decide to use a closed-cell product like Lapolla 4G, you won't get a big R-value benefit when compared to open-cell foam, but the wall will still work, as long as the exterior rigid foam layer doesn't have a vapor-impermeable facer like aluminum foil or polyethlyene. (A small amount of outward drying is good for your sheathing, and unfaced rigid foam will allow a little bit of outward drying if it is only 1 inch thick).

  2. user-6495937 | | #2

    Hi Dana and Martin,

    Thanks for the advice! I do live north of Rockport in Newburyport and my guess is R4 for the external is a good assumption. I'll be removing some siding soon and will be able to validate. Still, I like the idea of the MemBrain + open cell. Have a great day!

  3. Expert Member
    Dana Dorsett | | #3

    Open cell foam is fine. Depending on the type of exterior foam it may be prudent to use 2-mil nylon (Certainteed MemBrain) under the wallboard (about 12-15 cents per square foot, installed), to give it some wintertime vapor retardency without reducing it's drying capacity. You could also use half-perm paint aka "vapor barrier latex primer" on the wall board, which would be cheaper, but would impede drying, unlike sheet nylon. The nylon has variable permeance, sensitive to moisture levels in the proximate air. In winter when the air is dry it'll be under 1 perm, but in the spring when the sun & outdoor temps are driving the winter's moisture accumulation out of the sheathing it goes above 5 perms at moisture levels high enough to support mold, which is more vapor open than standard latex paint. It's a good solution.

    The IRC prescriptive for dew point control at the sheathing when using higher permeance interior finishes in zone 5 for 2x4 framing would be R5 continuous insulation. EPS or aged XPS would only be about R4-R4.2, but most polyiso would be performing at or near R5 in your location. You might be just fine with R4 on the warm edge of zone 5, say on ocean-tempered Cape Cod or Nantucket, but it would be pushing your luck in Rockport/Goucester or further north.

  4. user-6495937 | | #4


    It's me again, somehow surviving in these single digit temps with limited insulation.

    I've decided to do a total upgrade, including the exterior walls. The windows all need to be replaced, so it makes sense. Plus the exterior polyiso is only 1/2" so even less useful. There's a few layers of shingles, too. It's a mess and will all come off, down to the T&G sheathing.

    Here is my total plan that I would love you to verify or optimize. I've assembled your wisdom from a number of threads but I'm sure I've missed something.

    The new plan would be:

    - New White Cedar Siding Shingles or Hardi-Plank cement board
    - 1/2"x2" furring strips for breathing
    -- new flange windows (Marvin Integrity) installed in boxes, air sealed, and flashed
    - Tvyek or other house wrap
    - 2" polyiso - Tuff R - foil on the outside
    - existing 3/4" tongue and groove siding

    Inside -->
    - 2x4 existing studs
    - Open Cell spray foam
    - Membrain cover
    - drywall

    Thoughts on this strategy?



  5. Expert Member
    Dana Dorsett | | #5

    The MemBrain would be a waste of money in this case, since you would have on the order of 2x the amount of exterior R needed for dew point control per IRC prescriptives, and the open cell cavity fill is inherently air tight. Be sure to put a bead of polyurethane caulk on the seams of any doubled-up framing, and where the bottom plate meets the subfloor, since the cavity fill doesn't seal those cracks. That's a lot easier & cheaper than a layer of MemBrain.

    The Tuff-R might also be serious overspending, given the square mile of 2" roofing polyiso in the local MA reclaimers' inventory, at ~1/4 the cost of virgin stock goods. The cost savings would more than pay for sending a truck from Newburyport to Framingham ( ) or Worcester ( ). Those are just the two largest vendors- there are others.

    If you want the easy air-sealing of a foil facer, tape the seams of the reclaimed foam with housewrap tape, and add layer of half-inch or one inch Tuff-R (staggering the seams) with the rest, or factory seconds foil-faced goods from Green Insulation Group (they always seem to have some in stock, but Nationwide might too.)

  6. user-6495937 | | #6

    Awesome Dana ... so I'll remove the Membrain and I will go with the reclaimed polyiso. GIG seems to have lots of options but I'll contact Nationwide as well.

    Replaying the GBA videos on rigid insulation, it seems that the foil polyiso acts as the water barrier, so Tyvek is also probably overkill.

    So here is my updated wall strategy:

    - New White Cedar Siding Shingles or Hardi-Plank cement board
    - 1"x4" furring strips for breathing and siding attachment
    -- new flange windows (Marvin Integrity) installed in boxes, air sealed, and flashed
    - 2" polyiso - foil on the outside, everything wrapped as rainscreen
    - existing 3/4" tongue and groove siding
    Inside -->
    - 2x4 existing studs
    - Open Cell spray foam
    - drywall

    Thank you!

  7. Expert Member
    Dana Dorsett | | #7

    Keep the Tyvek and use the crinkled DrainWrap version, if you're mounting the windows "innie", with the glass roughly co-planar with the sheathing rather than "outie", with the glass roughly co-planar with the siding. Innie mounting would have the flashing dumping bulk water at the outside of the sheathing, so it needs a drainable WRB to lap properly onto to manage wind-driven rain.

    Even if the windows are mounted outie installing flat (crinkled isn't necessary) Tyvek on the exterior of the foam is cheap insurance. This would be more important in houses higher wind areas (I know it never really blows during rainstorms in Newburyport, right? ;-) ), and houses that don't have deep roof overhangs (like most Capes.)

  8. user-6495937 | | #8

    Okay, awesome! The plan was for "outies" and I'll keep the Tyvek as originally planned. Because yes, the wind comes right off the ocean and the window-blown rain is very intense! Great feedback!!!

  9. user-4704465 | | #9

    Having built and renovated in a coastal environment for the last 30 years, I've learned that Tyvek isn't the right material to use under cedar shingles. White Cedar shingles last about 20-30 years depending on exposure, roof overhangs, etc., and by the time we strip them off, there isn't much functionality left in the Tyvek. Shingles are self venting and very wind and weather resistant; they don't seem to need air space behind them, and in any case, furring strips horizontally 5" on center are more of a liability than a benefit. Many of us are going back to old fashioned felt paper, which is often in good shape after 20-30 years. One builder friend of mine in coastal New Jersey uses cedar breather mesh on sidewalls- he says it reduces cupping. I haven't tried it, but then, I haven't had too much trouble with cupping, either. I have used the crinkle wrap tyvek behind vertical t&g siding; it's a nice forgiving product to install, but I'm not sure about how well taping the seams works. And the jury is out on what the acids in the siding do to it over time. Hope some of this is helpful.

  10. user-6495937 | | #10

    Great comments! But alas, leads to more questions! I searched the archives at GBA and this articles surmises my question:

    So my original plan (like the GBA videos) was to use 1x4 vertical strips to install siding, which will work for Hardi-Plank or other board-based siding.

    Does this change is I used Happy Valley cedar panels ( Of course, they say just nail into the plywood and "code approved building paper". But nothing about over rigid foam ...

    Now with cedar shingles, I see the 1x4 furring run horizontally not vertically. So hence I appreciate your comment ... does not appear shingles over foam is a good idea due to too much work, issues, etc.

    Sounds like a Hardi-plank (or similar a siding that installs in horizontal fashion) is a better strategy with exterior foam?

  11. Expert Member
    MALCOLM TAYLOR | | #11

    You have any number of options, although some require more work than others.

    Cedar shingles or shakes can be installed on horizontal furring on top of a layer of vertical furring. Going this route maintains a good drainage plane and drying potential. I agree with John that cedar shingles do dry quite well on their own, but one of the chief benefits of backers like Cedar Breather or wood rain screens is the capillary break they provide - and which you don't get when you install them directly onto your sheathing.

    If you want the look of shingles, but would prefer something easy to apply over foam, consider using shingle panels as John suggested. They can't be applied directly to foam, but it isn't desirable to attach any cladding that way, and they work well on vertical furring @ 16" oc.

  12. user-4704465 | | #12

    I've grappled with the issue, as I've tried to move myself and my clients into more high performance building envelopes. But I haven't found anything that gets me away from two layers of sheathing under shingles- one on the framing for strength and sheer, and one over the insulation for a nailbase for the shingling. If you're not in a high wind engineering situation, you might be able to use one of the nailbase sheathing products, like the zip system insulated panels.

  13. user-6495937 | | #13

    Hi John,

    Okay, it seems you like the following for an ocean house like mine:

    - Cedar shingles
    - felt
    - plywood
    - tyvex
    - 2" polyiso
    - existing T&G sheathing


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