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Community and Q&A

Downside to exterior foam only?

user-6179159 | Posted in Energy Efficiency and Durability on

Heated outbuilding in Climate Zone 5
30x40x12 nominal size w/ 2×6 walls on trench footings (16″ OC)
Attic trusses upstairs with open stairway
Radiant slab for primary heat
Two 10×11 garage doors

Since this is going to be a garage, my preference is to not fill the stud cavities with insulation and have to cover them with drywall. That just makes future wiring tweaks a total PITA.

Wall code here is R19 or R13+5
Roof is R39

Building to R19, with the 2×[email protected] is really only R15 or so net (assuming R1 per inch of stud)

Seems like a minimum of 3″ reclaimed polyiso would beat the R19 wall most of the time, and 3.5-4″ would soundly beat it even in cold weather.

Is there a downside to this plan:
Siding over furring strips
over housewrap
over 3+” polyiso with taped seams
over 1/2″ OSB with taped seams
over 2×6 stud walls

The only issue I see is having the polyiso in contact with the ground, as I do want to insulate all the way down, so I’m thinking of putting EPS or XPS around the perimeter, just 1-2′ high and do poly up from there. Maybe that’s not even an issue if I flash it properly?

Truss designs will likely drive me to some foam over the sheathing, with only a 2×6 top chord in some places but that’s TBD at this point as I should be able to fur build inward.


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  1. GBA Editor
    Martin Holladay | | #1

    The approach you suggest has a name; it's called the PERSIST approach. For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.

    If you follow traditional PERSIST principles, you would want to install something that acts as both a WRB and an air barrier at the sheathing level. Taped Zip sheathing would certainly work.

    Needless to say, if building codes apply, talk to your local building inspector to make sure that he or she understands that 3 inches of polyiso is equivalent in performance to R-19 code requirements.

    It's a good idea to keep your polyiso above grade; switching to EPS or XPS for the portion that is in contact with soil makes sense.

  2. user-6179159 | | #2

    Thanks Martin. Very similar to what I mentioned, though I'm curious as to the need for the fully-adhered membrane on top of the OSB when it's then covered by multiple layers of foam. Purely an air barrier at that point?

    As an aside, fully adhered membranes has had me thinking recently.... my poured wall contractor uses a "spray rubber" (not sure what the actual product is) for foundation waterproofing. Wonder if it would be effective in sealing OSB as well. Though you'll probably give me a link to the article where you've written about the very topic ;)

  3. GBA Editor
    Martin Holladay | | #3

    When PERSIST was developed in the 1960s, Canadian researchers were convinced (a) that every wall needed a perfect vapor barrier on the warm-in-winter side of the insulation, and (b) there was a risk that rain might penetrate the rigid foam and threaten the sheathing.

    We don't worry as much about those issues these days. If you include a WRB on the exterior side of the rigid foam, and a ventilated rainscreen gap between the WRB and the back of the siding, there is almost no chance of water penetration. Moreover, the rigid foam is an effective vapor retarder. So I agree with your analysis -- all you really need at the sheathing level is a good air barrier.

  4. GBA Editor
    Martin Holladay | | #4

    I don't advise you to use a foundation waterproofing compound on OSB sheathing. But several manufacturers make liquid-applied water-resistive barriers (WRBs) intended for application on OSB. Here is a link to an article with more information: Housewrap in a Can: Liquid-Applied WRBs.

  5. Irishjake | | #5


    I'm building a PERSIST style home and detached garage/barn. In both structures - all my rafters are exposed with barnboard/shiplap on top (cathedral ceiling style). To keep these elements exposed I was forced to put insulation outboard of my WRB. It turns out it is one of the most durable ways to build a building (with a few important caveats of course), but it has been simple to detail the WRB @ the sheathing.

    I have radiant floor heat, large doors, and a R-46 walls in my barn/workshop/garage area, but you could easily do it with less R-value by withholding the interior insulation.

    Here are some considerations I might impart while you wade through the waters-
    Seek out reclaimed EPS for your garage. It is easily obtained, will save you a bundle of cash, doesn't thermally degrade in the cold, and will not be destroyed by moisture, unlike P-Iso. Make sure to use two layers at least and overlap/stagger the joints.

    When it comes to a liquid applied WRB for an entire building. I went with the Prosoco R-Guard system, which is extremely durable and has been exposed for months without failure. The only thing I can say about liquid applied WRB, is a consistent application is imperative to reduce having to go over sections several times. (We rolled ours via bucket and roller, and I had a few different folks doing it, which made the consistency element more challenging). I would expect that a sprayed applied application might be more consistent, but more expensive for certain.

    Drawbacks to liquid applied WRB on sheathing in my opinion are few, but considerable only if time is a consideration. Every nail, seam, edge, and penetration needs to be hit with one coat. Then your window and door openings get hit with another coat, and then the entire wall assembly gets yet another final coat over it all. Now all these coatings are similar in makeup, so they seamlessly integrate. If this is done from the ground after the walls are stood up, your time is consumed traveling over all that surface area.....several times. Your installation schedule can get buggered up too if rain or temps below 40 seem to haunt you.

    This was soooo time consuming for me, that on our house, we choose to use Zipwall, and Zip tape. (At the windows and doors we are still flashing and sealing with Prosoco, because of the exceptional performance it provides at these more critical areas.) It also turns out the Prosoco makes the Huber Liquid Flashing for Huber under private label, it's just less expensive via Prosoco.

    The ZipWall and tape are also fantastic, it sure goes MUCH faster than the liquid applied WRB. Every nail and penetration isn't sealed though, so I'm not sure it is truly comparing the same apples. The Zipwall WRB coating is extremely consistent and durable, as one would expect from a factory applied product. Time is a premium for me, so getting the walls primarily sealed and taped up on the ground, before they are stood up, is an obvious home run.

    The ZipWall sheathing is OSB, so it is definitely subject to swelling from moisture infiltration at the edges if the stacks of sheets aren't covered prior to installation. We got more than five inches of rain here recently, and some of the sheets on one of our stacks swelled quite a bit, as OSB can do.....

  6. Chaubenee | | #6

    I used osb and taped the sheathing seams with 3M all weather tape. From inside I caulked the seams on the osb where they were visible. I also caulked my stud bays. Then I used Tyvek Drain Wrap and on top of that 2.5" reclaimed EPS and 3/4" furring strips. I used the same 3M tape to tape the seams of the foam. To this I attached Hardi siding. Inside the walls I had dense pack cellulose. Bottom line was using osb and 3M tape saved me thousands of dollars over using the Zip system.

  7. user-6179159 | | #7

    Thanks Brad and Joe,

    Zipwall looks nice, convenience for a premium, but my walls are already framed and trusses in progress, so we're past that point.

    My plan for the OSB is to do what Brad mentioned. 3M tape the outside seams, and caulk/seal the joints from the inside, too. Flashing tape at the sill as well.

    Polyiso is readily available as reclaimed, and the R-to-Thickness of it certainly is appealing in not having to make the walls "overly" thick to get a suitable R-value. I'm wondering if XPS/EPS over Polyiso would make sense. The outer styrene would keep the Polyiso warmer, thus minimising the temperature effects, resulting in a higher winter R-value. Hmmm.....

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