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Exposed insulweb/blown cellulose?

EngineerNate | Posted in General Questions on

Follow up to my recent attic questions.

I’m considering building baffles out of 2″ GPS foam that go from top to bottom spaced 1.5″ from the , but rather than putting batts below it, I’m thinking it may be easier to fur down 10-12″, staple insulweb to the bottom, and blow as much cellulose in there as I can.

My question is, in the space behind the knee walls, would it be acceptable to leave this assembly exposed or would it need a facer of some sort?

Is there a better way to hang insulation that’s thicker than the rafters that’s vapor open? Layers of foam with staggered seams is easy to do but I’m worried about trapping moisture, and the cost/labor adds up. At some point a few inches of open cell blown over the baffles starts to look preferable.


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  1. charlie_sullivan | | #1

    I found your question a little confusing but let me see if I have this right: You are insulating at the roof in the "devil's triangle" area behind the kneewalls, and you are using 2" GPS as a baffle, 1.5" below the roof decking?

    And then the question is what is needed to cover the insulation, probably blown cellulose, where it faces the attic.

    In general, you want a good air barrier there. Less important for cellulose than for fiberglass or mineral wool, but it's still a good idea. It could be a simple as Tyvek, or you could go with a smart vapor retarder to mitigate any risk of 2" not being thick enough to prevent condensation. Intello is strong enough to use to hold back the insulation, although it's easier to blow insulation in when you have air permeable netting. Or you could opt for a rigid material, likely drywall, given the high prices of wood sheet goods these days.

    Then there's the question of whether you should have something that is fire resistant. I'm not a code expert, but I don't think code would require that.

    1. EngineerNate | | #2

      Got it! I wasn't sure if an air barrier was necessary on the interior if the baffle system ran all the way across the "devil's triangle" from bottom to top. I want to insulate along the roofline rather than at the floor. I would like this space to be conditioned for future installation of an ERV, it's the perfect size and placement to snag the master bed/bath and kitchen.

      I've considered trying to hang rockwool or other batts over the 2" baffles as well but it doesn't seem any easier than framing to allow enough blown in below the baffles and would be considerably more expensive.

      Another thought I had was to face it with a full covering of a second full coverage layer of foam on the interior side with the joints either taped or froth kitted air tight. I just worry about too many vapor barriers in the assembly. (Multiple Foam layers + synthetic underlayment)

  2. EngineerNate | | #3

    I've attached an image showing one possible assembly I thought up. To create enough depth for the r value needed, I would take. 2x10 or 2x12 TJI and using a skilsaw, slice one flange flush with the web on one side and glue/screw this to the existing rafter. The offcut could be used as the spacer for the opposite side of the rafter. Foam then goes between the TJI against the upper flange, creating the ventilation channel. Below that, blown in held in place with something still to be determined, or even dense fiberglass or mineral wool batts.

    Any thoughts?

  3. Expert Member
    Akos | | #4

    Make sure the foam you use is vapor open, you want to allow for vapor to move through the insulation into the vent channel. Best bet is go with either thin plywood, fiberboard or just plain housewrap or insulweb stapled up instead of foam.

    In cold climates, with a vented roof, you need a warm side vapor retarder. This could be faced batts, 6mil poly or one of the fancier smart vapor retarders.

    1. EngineerNate | | #5

      I'm in climate zone 4a, if that helps.

      I was wondering if I'd be better off making the baffles out of 1/2" osb sheathing and just putting a continuous layer of GPS or Polyiso on the interior side. That way I'd have something solid to blow the insulation in through and only one reap vapor retarder/barrier on the internal side.

      1. charlie_sullivan | | #6

        Yes, 1/2" OSB would alleviate concerns about condensation on the foam, although you'd probably be OK either way.

        If you do that, I don't see a need for GPS or polyiso on the interior side. Nothing wrong with it, but with your thin web I-joist setup, you won't have much thermal bridging, if that's what you wanted to mitigate with continuous insulation.

        1. Expert Member
          Akos | | #7

          There are a number of ways of looking at this assembly. You can treat it as a hybrid insulated assembly with rigid rigid insulation for condensation control or as a vented assembly.

          If it is with rigid for condensation control, look at table 2(B) here:

          In zone 4 you have a couple of options. Without a warm side vapor retarder, you need close about the same R value rigid and fluffy in the roof assembly. With a class III (ie painted drywall) this drops to 20%. Provided you have the right warm side air barrier and vapor retarder and sufficient R value in the foam baffle the roof will be fine.

          If you want to build it as a vented assembly, than you need a very permeable baffle, 7/16 OSB is borderline, unfaced EPS, CDX or fiberboard are much better. You then need to install a warm side vapor retarder that is lower permeability than the baffle. For low cost this means 6mil poly after dense packing the roof. A layer of taped foam/drywall/OSB would work just as well and would save you the cost of netting.

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