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Community and Q&A

Polyiso on both sides of sheathing?

sleaton | Posted in Energy Efficiency and Durability on

So I have insulation challenges. I am pretty set on my walls, I have 2×6 construction. I am in the Austin area, and plan on using polyeurethane to seal all sheething joints, then following that up with 2in of polyiso on the exterior. I have considered using a STO liquid over the sheething, I think they even have one that you stick the insulation to, so am pricing that. Then a rainscreen, with cemplank over that.

So the questions.
If I have polyiso on the outside, do I dare put it in the inside cavity ? I have a real good deal on it, and figured I might use it most everywhere. Or I might just do open cell in the cavity. My concern with polyiso on both sides is that if moisture ever did get in there, it would not be able to get out.

On the roof, I was thinking 3 layers of the 2in polyiso would put me at r36, much closer to where I need to be, and then I might spray a light sprayfoam over that just to make sure it is airtight and no gaps might leak etc.

Thoughts ? Advice ?

I know it would be nice to put polyiso on top of the roof, but at 100square paying for the sheething twice just isn’t in the budget.

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  1. Expert Member
    ARMANDO COBO | | #1

    I design high-performing houses in TX, and IMO, you are spending unnecessary money with these assemblies.
    I would install taped Plwd/OSB and 2” taped Polyiso. You only need 1/2”, and 1” is better. However, with 2” polyiso, you need to make sure you rainscreen and cladding attachment is perfect, or you’ll have problems later on. Some municipalities in the south, and some cladding, require engineering for such attachments. The taped polyiso can become WRB if taped with approved tape. You could also cover the polyiso with a WRB roll, cheaper than liquid or sprayed and less labor, and then your rainscreen and siding. (Just in case, you need to make sure you have 7.25” brick ledge if using brick or stone anywhere).
    NEVER install closed cell on both sides of the sheathing. Instead of open cell inside the wall cavities, dense packed cellulose is better option and costs less. Make sure is installed at 4 ls/f3, and rolled!
    On your roof assembly, 3-2” polyiso is R40. If you tape you sheathing and staggered polyiso seams, you don’t need open cell foam under the roof decking (assuming you do a good quality job taping)

  2. sleaton | | #2

    I can look just the 2in was cheap and readily available. On the roof I was not clear . I will put the polish on the inside of the attic. I know on top is best but double decking it is not in the budget

  3. user-2310254 | | #3


    With cut and cobble, it is my understanding that maintaining the air seal is a little iffy. (The framing moves over time.) If your roof is fairly simple, it might be better to attach it to the bottom of the rafter and then pack with air permeable insulation. What are the dimensions on your rafters?

    If you don't mind loosing a little floor space, you might also be able to use this type of approach on your walls. It would be simpler than installing the foam on the exterior.

    Hopefully, some other members will post on whether my suggestions are worth considering.

  4. GBA Editor
    Martin Holladay | | #4

    1. Installing polyiso on the exterior of your wall sheathing is a good idea.

    2. It rarely makes sense to install polyiso between studs (a method called cut-and-cobble). It would be especially silly if you already have polyiso on the exterior, because polyiso inhibits drying. If you have exterior polyiso, use a vapor-permeable insulation between the studs -- a type of insulation that is intended to be installed between studs like cellulose, mineral wool, fiberglass, or open-cell spray foam.

    3. When it comes to your roof, your statements are confusing. In your most recent comment, you wrote, "I will put the polish [= polyiso ?] on the inside of the attic. I know on top is best but double decking it is not in the budget." If I read that correctly, it sounds as if you are thinking of installing polyiso on your attic floor.

    If that's what you are thinking, it doesn't make much sense. You are trying to use an insulation material for a purpose to which it is not suited. Cellulose is ideal for attic floors, so that is what you should use there.

  5. Expert Member
    Dana Dorsett | | #5

    Putting any high R/inch foam between the studs is pretty much a waste of foam. You'll get about twice the performance out of bumping up the exterior foam thickness by a half-inch (another R3 "whole wall" ) than you would installing polyiso between the studs instead of R20 batts or open cell foam. When thermally bridged by studs the R6/inch foam adds less than R2 to the whole-wall performance. See:

  6. sleaton | | #6


    I am looking at cut and cobble for the roof. polyiso between the rafters, with a layer under the rafters in areas where they are not very deep, and to help with bridging. I am looking to do a hot roof, conditioned attic. I have 2x8 rafters in a lot of the roof, 2x6 in other areas. The local insulation contractors all want to spray 5.5 inches of open in the roofs and call it good. That is no where near code, and I am not happy with that, which is how I started looking here. The other option I could do is give up on conditioned attic and do buried ducting. I have a vault upstairs 90 ft long, but I could still do the traditional roof if I needed.

  7. GBA Editor
    Martin Holladay | | #7

    I don't recommend the cut-and-cobble technique for unvented roof assemblies. There have been reports of moisture accumulation and sheathing rot in unvented roofs insulated with the cut-and-cobble method.

    For more information, see Cut-and-Cobble Insulation.

  8. sleaton | | #8

    I don't really see a good option here then without cut and cobble and having enough insulation.
    I guess I could
    1. scrap the idea of encapsulation and do the old fashioned roof and tons of insulation on the deck over the rooms.
    2. put a vent channel in, put poly over the rafters, and blow in insulation over that.
    3. I could do open cell, and that would be doable, and code compliant for most all of the house if I overspray to get enough depth. Closed cell just is way too costly for that size of a roof.

  9. sleaton | | #9

    My roof is a bit complicated, I have two dormers, have a 12/12 that joins an 8 /12 and also it joins a lot of 4/12. I was thinking that open cell spray foam was the most cost effective easiest option. I figured that along with an aipril aire system was a good option, but I know there is a lot of debate about open cell in roofs even down here in Austin. I am really kind of stuck, venting this roof could probably be done fairly well I would have to take a good look at the dormers

  10. GBA Editor
    Martin Holladay | | #10

    Q. "I am really kind of stuck."

    A. If your house is already framed, you are -- in the sense that it's hard to change the dormers now.

    The best way to address this type of roof is with an adequately thick layer of rigid foam above the roof sheathing, followed by a second layer of sheathing. If your budget can't afford doing it the right way, you may have to accept some type of compromise.

  11. sleaton | | #11

    agreed on the compromise. I E those are the 3 ideas I had that are available to me at this point.

    Do all external insulation options require the sandwich methodology ? or if the foam is thin enough can it be roofed over with longer nails ? everything I have seen is talking about quite thick layers which would for sure require another deck over the insulation

  12. GBA Editor
    Martin Holladay | | #12

    If you choose to install screw-down (through-fastened) metal roofing, the roofing can be installed on 1x4 or 2x4 purlins, 24 inches on center. The purlins (which are installed parallel to the ridge) can be installed above the roofing foam. The purlins are fastened to the rafters below with long screws.

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