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Proposed recycled polyiso wall insulation question

Zampano77 | Posted in Green Building Techniques on

To All,

I am remodeling a old house located in Rochester NY and the climate zone is 4 and 5 marine. The exterior is covered in vinyl siding installed over unfolded 3/8″ foil faced EPS which covers beveled cedar siding. I have demolished the interior and exposed the 2″ x 4″ walls cavities and was wanting to provide an air barrier without removing the exterior at this time. I am proposing a wall section of:

Drywall over 2″ wood horizontal strapping ( paper faced Iso or XPS between strapping as a thermal break to the exterior studs), OSB plywood glued and nailed to the inside face of studs for structural stability ( the house creaks a lot in our high winds), 2″ x 4″ wall cavity with 3” or the combination of 1 ½ and 2” paper faced ISO foam boards to equal a 3 ½” paper faced ISO, ¾” T& G sheathing, tar paper, beveled cedar siding, 3/8″ foil faced EPS, vinyl siding.

**An alternative would be a second interior 3 ½” wall staggered in front of existing wall instead of the horizontal strapping, with a layer of XPS on the interior of the existing wall(though still sheathed in plwood for the above reason).

In the future I would like to remove the vinyl to be able to remove the beveled cedar siding and install a 1” continuous layer of foam insulation, unfaced or paper faced, no foil, and then a good house wrap in order to detail the windows and flashing that are now non-existent .

Any review of the section and recommendations would be greatly appreciated.



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  1. Expert Member
    Dana Dorsett | | #1

    I don't quite understand the proposed stackup. It sounds like you currently have:

    outdoors| viny siding | 3/8 foil EPS | cedar siding | (#15 felt?) | (sheathing?) | fiber insulated 2x4 wall... which you want to add

    ... |1/2" OSB or plywood | 1.5-2" polyiso | 1x2 strapping | wallboard

    Yes, no, something else?

    Are the foil facers on the EPS perforated on some grid, or are they smooth & mirror-like as kitchen foils?

  2. Zampano77 | | #2

    I don't quite understand the proposed stackup. It sounds like you currently have:
    outdoors| viny siding | 3/8 foil EPS | cedar siding | (#15 felt?) | (sheathing?) | fiber insulated 2x4 wall.



    2" x 4" wall cavity with 3” or the combination of 1 ½ and 2” paper faced ISO foam boards to equal a 3 ½” paper faced ISO which you want to add
    ... |1/2" OSB or plywood | 1.5-2" polyiso | 1x2 strapping | wallboard
    Yes, no, something else?

    Are the foil facers on the EPS perforated on some grid, or are they smooth & mirror-like as kitchen foils?




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  3. Zampano77 | | #3 which you want to add
    ... |1/2" OSB or plywood | 1.5-2" polyiso | 1x2 strapping | wallboard


  4. Expert Member
    Dana Dorsett | | #4

    "The exterior is covered in vinyl siding installed over unfolded 3/8" foil faced EPS..."

    So, is the foil faced EPS perforated or not? (It matters, maybe a lot!)

  5. Zampano77 | | #5

    existing 3/8" eps is double foil faced and NOT perforated

  6. Jon_R | | #6

    Best practice for your climate is adequate external insulation (not an option) and/or a 5:1 exterior:interior perm ratio without creating a moisture trap. Intello Plus should get you closest to the latter.

    Other things that help are very good air sealing, low interior humidity during cold weather, cellulose fill and making sure that no room gets positively pressurized.

  7. Zampano77 | | #7


    If the 3/8 foil faced exterior foam was removed and replaced by 1" unfaced foam ( EPS/XPS) and a layer of building wrap, can one forget the Intello Plus ( am budget constrained and hoping to use reuse materials if at all possible), can the initial proposed section work?

  8. GBA Editor
    Martin Holladay | | #8

    Rochester, NY is in Climate Zone 5. It is nowhere near the border of Climate Zone 4 Marine.

    In that zone, a 2x4 wall with exterior foil-faced rigid foam will get into trouble unless the rigid foam has a minimum R-value of R-5. So the foil-faced 3/8-inch EPS is dangerous.

    Step one with this wall would be removing the vinyl siding so that you can beef up the R-value of the rigid foam layer.

    For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  9. Expert Member
    Dana Dorsett | | #9

    Unless you're going to strip the foil faced EPS out the wall stackup has to be designed to dry toward the interior, since each foil facer is less than 0.05 perms, with low/no reliable drying path toward the exterior.

    Fiber faced polyiso is typically less than 1 perm, and if you're stacking up 5-6" of it even the polyiso itself will be less than 1 perm (even if you peeled the facers), which would compromise drying path from the sheathing toward the interior. If there were sufficient exterior R for dew point control at the sheathing, and you were diligent about air sealing it would be fairly low risk, but with only 3/8" EPS (R1.5-ish) on the exterior it means the sheathing will have a very low average wintertime temperature, and would tend to hang on to too much moisture.

    The safest thing (and perhaps the easiest) would be to pull the siding now, get rid of the foil facers, and put the 1.5-2" of reclaimed polyiso on the exterior. You would then have the opportunity to inspect (and correct if necessary) the window & door flashing details, and better air-seal the structural sheathing. With even 1.5" of exterior polyiso you would have sufficient dew point control at the sheathing for an interior-drying stackup, and it would then be relatively safe to cut'n'cobble low permeance foam in the stud cavities if you chose to, but the boost in thermal performance of filling the cavities with polyiso instead of R13-R15 fiberglass or rock wool is tiny (about R1 "whole wall" after factoring in the thermal bridging of the framing), and the installation labor difference would be large.

  10. Zampano77 | | #10

    So the exterior foam would act as a stop to thermal bridging and there would be no need to add foam over the interior of the studs between the 2" x 2" straps?

    Also, with the exterior foam correctly detailed and installed, with a bldg wrap, you could or would forgo interior air sealing such as spray flash or cut'n'cobble low permeance foam as it becomes redundant?

  11. Expert Member
    Dana Dorsett | | #11

    Even with exterior foam you would still need/want to air seal both the structural sheathing and interior wallboard, but minor air leakage in the wallboard into the insulated cavities would then have little to no moisture consequences for the sheathing.

    Caulking the sheathing to the framing inside every stud bay with polyurethane caulk (and some can-foam for any bigger holes) can make the sheathing the primary air barrier. Be sure to caulk between doubled-up top plates and where the bottom plate of the studwall meets the subflooring, etc. Pay attention to air sealing the plumbing & electrical penetrations (at every stud, for lateral runs), not just at the electrical boxes. Making the framing & sheathing air tight is far less labor & material than air sealing cut'n'cobbled rigid foam with cases of can-foam & miles of tape.

  12. Zampano77 | | #12

    To All,

    Thanks for the very good advice.

    I do have a problem of I have sprayed a portion of a wall ( 20% of wall area) with a 1/2" flash coat of 2 part closed cell foam. I did not like the results ( part my workmanship, part the cost) and was moving toward the cut'n'cobble low permeance foam in the stud cavities as a work around. With the strategy you just explained, can this be rectified by a low permanence foam on the outside or does it become nil in the equation?

  13. Zampano77 | | #13

    Sheathing on this house is 1' x 6" x 3/4" T and G . Caulking all of the seams becomes a lot of caulk, that is why I went to spray foam and then to possibly the cut n cobble.

    I have used Denarco gaskets for the drywall in the past. Just looking for the best value option to stop air infiltration on an older house especially prior to entering the cavities. I can caulk all of the T and G sheathing if I have to, just looking for the best option before I do something that throws a wrench in the works ( my spray foam comes to mind)

  14. Zampano77 | | #14

    Add on to initial post:

    Correction: This house is in Climate Zone 5.

    I am trying to used recycled foam products and I am not set on any type, ISO/ EPS/XPS etc , I just want to make sure that what I propose and then build does not cause any vapor transmission problems in the future.


  15. Expert Member
    Dana Dorsett | | #15

    A flash-inch of closed cell foam to seal the t & g sheathing stud bays would run about 1-perm, give or take, which is sufficient for seasonal drying, yet would be low enough to protect against wintertime moisture drives in a zone 5 climate. You can then fill the rest of the depth R13s (unfaced or kraft, but not foil), and defer the exterior foam until you're ready for that project.

    Alternatively (and greener), you could install all open cell foam in the stud bays and use 2-mil nylon (Certainteed MemBrain) under the wallboard, or half-perm "vapor barrier latex" primer on the wallboard. The 3.5" of open cell foam has less total polymer than 1" of closed cell, and it's blown with water rather than HFC245fa (a powerful greenhouse gas), and it air seals better than a flash-inch of closed cell too.

    The vapor permeance of the exterior foam hardly matters when the stackup can dry toward the interior- it's all about having sufficient R-value to keep the average wintertime temperature at the sheathing warm enough to not take on moisture from the interior air. With R8-R10 on the exterior you would have PLENTY of dew point margin for a 2x4 wall with fluffy fill (or open cell foam). With 2" of reclaimed roofing polyiso it would be about double the performance of the 2x4 wall with cut'n't cobbled or fiber fill + 3/8" EPS.

  16. Zampano77 | | #16

    The EPS you speak of would be sheet goods or does it need to be sprayed in. If sheet goods, can it be sealed at edges with closed cell spray foam. Am still trying to stay away from the sprayed close cell over the entire cavity due to cost, thus I was pushing for a cut'n'cobble air barrier to cut costs.

    How thick does an cut'n'cobbled unfaced layer EPS foam board need to be to act as an air barrier when inside the wall cavity and up against the sheathing. Still looking to not use the closed cell spray foam exclusively as my air barrier so I can use what is left to seal irregular areas as well as the mud sill in the basement.

    TO ALL: Thanks for all of the very useful info.

  17. Expert Member
    Dana Dorsett | | #17

    The only EPS I'm referring to so far is the pre-existing 3/8" foil faced underlayment mentioned in your first post. I wasn't recommending cut'n' cobbled foam as an air barrier.

    Assuming there isn't 1001 nail heads poking through the t & g you can make cheap & reasonable air barrier out of #15 felt cut 4" wider & 4" longer than the stud bay, then stapling it to the sides of the studs near the sheathing, leaving enough of a flap to caulk it to the stud, re-stapling over the bead of caulk to make it lie flat to the stud. It's a lot of detail work, but from a cash outlay point of view it's a lot cheaper than closed cell foam if you're discounting DIY labor.

    Unfaced 1" EPS sealed in place with can-foam could work too, as long as you used 2-mil nylon on the interior. Type-I EPS (1lb density) is pretty easy to break, and is as vapor-open as interior latex, but Type-II (1.5lbs density) would be under 3 perms @ 1" and is a little more rugged for handling. Reclaimed 1" XPS would be about 1-1.5 perms, and a bit more rugged still, and might be the better choice if you're keeping the foil faced EPS on the exterior rather than bumping it up with 1.5-2" polyiso for a rew years. If you compress R13 fiberglass into 2.5" space it delivers about R10, so with 1" of cut'n'cobbled EPS or XPS (R4 or higher) you'd have sufficient dew point control at the foam/fiber boundary to avoid moisture accumulation in the fiber.

  18. Zampano77 | | #18


    Thanks for the post, it is becoming much clearer now!!

    Question: 2-mil nylon on the interior, not sure what it is- could you expound? I will still also create a service cavity of some sort, insulated with horizontal batts,on top of the OSB interior cladding from the 1st post. Does the OSB layer ( described in 1st post) in any way affect this proposed wall assembly?

    As for the thickness of EPS/ XPS foam in the cavity compressing the batts, I have no problem furring out the wall to keep a 3.5 inch void for the R-15 Batts I now own. Wall thickness is not a problem.

    Is there a reason/ need for an interior wrap ( Intello, etc) or any other materials other than:

    SR l ?? ?? l batts in service cavity l OSB l batts l EPS/XPS foam or 15 lb felt l 3/4 T and G l Felt l cedar siding l 3/8"Foiled EPS l Vinyl

    Thanks again,


  19. Expert Member
    Dana Dorsett | | #19

    If you fur out the stud edges with 1" polyiso strips you'll more than double the R-value at the framing fraction, and the perfect depth for the R15s.

    The 2-mil nylon is just a thin plastic sheet that goes directly to the interior side of the batts & studs, under the wallboard. If you added an inch of foam to the stud edges you'll have to use longer nails or screws for the wallboard (or in your case, continous OSB?) but at only 1" of additional depth that's not a big deal. Here's a short info-mercial:

    If you ARE installing a continuous layer of OSB detailed as an air barrier ( rather than just a few shear-panels to stiffen up the walls) you can skip the sheet nylon. OSB is also a variable-permeance vapor retarder, that would allow the exterior sheathing to dry toward the interior. But if you're insulating the 1.5" deep service cavity too it's cheap insurance to add a the layer of nylon sheet directly behind the drywall, covering the service cavity fiber.

  20. Zampano77 | | #20

    I like the ISO furring strip idea but I will need to furr the studs in wood as I am trying to stiffen house with the OSB. It is also acting as an additional air barrier with the permanence you spoke of, would this 2nd air barrier be a problem within the assembly? Finally, I am building the service cavity on top of this wall for two reasons.

    1.)To allow for thicker walls and create deeper window sills and jambs.
    2.) Be able to easily straighten the old wavy and out of plumb walls with shimmed horizontal purlins

    The wall spoken of dries to the inside? How does the 2 mil poly work in this case?

    Appreciate your advice when able.

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