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Hybrid interior wall insulation system

Al_Ray_T | Posted in GBA Pro Help on

My background is custom home construction/historic preservation and restoration. I had a pre-fab 2 story garage/studio erected (Amish company out of PA.)  on my property. 28 x 32 slab, 2 courses of concrete block perimeter. 1st floor framing is 2 x 6, 2nd floor is attic trusses w/14 ‘ open space (7’ ceiling height) the length of the building.   I furred out the gable truss triangles on each end to create 2 x 6 bays for insulation.
At this point with all my “stuff” moved in I can’t bring a foam contractor in to shoot 2″ of polyiso  in the walls.  It would be a nightmare to empty the building at this point.  My plan is to use 2″ of fiber-faced (not foil) polyiso foam board in the walls initially, so I can at least work comfortably in the building this winter, and then run all the wiring and then fill the remaining 3 1/2″ with Ruxol or hemp insulation or fiberglass. I’d cut the foam sheets 1/2″ light on all 4 sides and spray foam that 1/2″ gap for an airtight seal. The exterior is sheathed/sided with a one layer T-111 type  product which came pre-painted. I’m in zone 6 or 6a in zipcode 12461.  These are my questions:
* Is 2 ” of foam correct to abate the dewpoint inside the wall cavity once I have the entire 5 1/2″ filled?
*Is fiber-faced and not foil faced polyiso the correct choice?
*Should I treat the interior side of the sheathing with a vapor retarder or barrier?
* I’m thinking no polyethylene between the wallboard and the insulation to allow for moisture to migrate to the interior. Correct?
* Best technique/product for attaching the polyiso boards to the interior side of the sheathing?
*The top chords of the roof trusses are 2 x 8s.  Best way to insulate that area? Pack it with polyiso foam boards (fiber-faced) tight up against the roof sheathing?  There is a ridge vent, and ventilated soffits.

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Replies

  1. FrankD | | #1

    If you use foil-faced polyiso, then 2" would meet the IRC requirement (at least R-11.25) for using a class III vapor retarder, which can be a coat or two of latex paint. But if you use fiber-faced polyiso, it might be too vapor permeable (> 1.5 perms), so the IRC would require a class II vapor retarder instead. Kraft-faced batts would be a good choice.

    Of course, those requirements are for houses occupied by a bunch of people who are cooking and showering. You have more leeway with a typical shop that is occupied only part of the day by one or two people, provided you're not using unvented propane heaters or doing something that puts a lot of moisture in the air.

    Since you already have roof vents installed, I'd leave a decent vent space. You could put a 5-1/2" batt in the cavity and attach foil-faced polyiso to the underside of the top chords, taping the seams for air tightness.

    1. Al_Ray_T | | #2

      I had suggested fiber-faced polyiso in the wall cavity thinking that the foil-faced with foil on both sides might somehow screw up the movement of moisture, but I don't know the mechanics of it and would defer to your recommendation of using foil-faced polyiso. Then does the need for a craft-faced batt to fill the rest of the cavity limit me to fiberglass, or are there other type batt insulations available with craft facing?
      Re the 5 1/2" 2nd floor ceiling batts, are you recommending unfaced insulation of any specific type?
      And, would 1/2" polyiso boards on the underside of the rafters be adequate, given this space is not a residence?
      I assume those plastic air guides in the rafters bays should be used as well to discourage the insulation from laying against the underside of the roof sheathing.
      Thank you for the advice on this project.

      1. Expert Member
        BILL WICHERS | | #3

        The foil facers on a piece of polyiso don't cause a problem with a double vapor barrier for the polyiso itself. For the purposes of a wall ASSEMBLY, polyiso is a vapor barrier because of that foil facer. Fiber faced polyiso can be better in some wall assemblies since it is somewhat vapor permeable whereas foil faced polyiso is not, because the foil facer is a good vapor barrier.

        If you were to use two sheets of foil faced polyiso with fluffy stuff in between, THEN you could have issues because of the double vapor barrier, but the double vapor barrier in this case would be each of the two SHEETS of polyiso, not the two FACERS on each sheet.

        The purpose of vent baffles is to maintain an air space above the insulation and below the roof sheathing. By code, this vent space needs to be at least 1" deep (more is better).

        The kraft facer on fiberglass batts is a vapor retarder. In assemblies where a vapor retarder is required, you can still use any kind of unfaced batt (mineral wool, denim, etc.), but you'd need to provide a seperate vapor retarder. This is what I like to do myself: I like to use mineral wool batts in the walls, but I then put a layer of Certainteed's MemBrain, which is a smart vapor retarder, over the inside of the studs behind the drywall. Since the layer of MemBrain is a vapor retarder, you can use unfaced batts in the wall.

        Bill

      2. FrankD | | #4

        I'm not recommending foil-faced over fiber-faced; either would work fine. They just have different vapor retarder requirements according to the IRC. I haven't heard of any other insulation type coming with Kraft facing, but not even Kraft-faced fiberglass is sold in my area, so I don't know for sure.

        If I'm going to make a recommendation, it will be to fill the wall cavities with 5-1/2" batts and then apply polyio in full sheets on the interior of the studs. I think that will take less time than cutting and foaming the pieces in place. A 5-1/2 batt + 1" continuous polyiso will outperform the 2" cut-and-cobble + 3-1/2" batt. Even a 5-1/2" batt + 1/2" polyiso would do better. If the polyiso was foil-faced, it wouldn't need another vapor retarder, and you could tape the seams for a good air barrier. Otherwise, you could install poly sheeting for a vapor barrier and seal the edges for an air barrier.

        Any type of batt will work thermally, but a denser batt and stiffer material like mineral wool will be easier to install without falling out right away. Or use Kraft-faced batts so you can staple them in place as you go. A layer of taped foil-faced polyiso underneath would make for a pretty good air barrier. Try to seal around all the truss webs as best as you can, but the vent space will take care of any moist air that does leak through. I can't say how much insulation would be adequate. You'll have to make a judgement call based on how often you'll be using the shop and how much you'll heat it. It would make sense to have the roof R-value be the same or slightly more than the wall R-value.

        The roof vent baffles aren't really needed if you're careful not to push the batts in too far and block the vent. With 5-1/2" batts in a 7-1/4" space, that's not likely to happen. I don't really like the plastic baffles because they block vapor movement, and the whole idea of the vent space is to dry out moisture. Supposedly the plastic ones are okay if you leave big gaps between them, but I'd prefer a material that is more vapor open.

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