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Community and Q&A

Getting More Space for Roof Insulation Above the Top Plate

Joseph Dziedzic | Posted in General Questions on

When using solid-sawn rafters for a roof, what’s the approved method for getting more vertical space for insulation above the top plate on which the rafter rests?  A 2×12 with a site-built vent baffle might only have about 9-1/2″ for insulation, which only allows about R-35 for batts or blown-in insulation.

I’ve seen some examples on the Web of adding a rim joist to the joist ends and a new top plate above the joists, and resting the rafters on the new top plate, but I’ve also heard (not confirmed) that approach isn’t favored by inspectors in our area (New Hampshire).

Is going with raised-heel trusses instead of rafters the correct approach?

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Replies

  1. Expert Member
    Michael Maines | | #1

    Joseph, two options: you can frame the attic floor first, then set the rafters on top of the joists. I thought that was a new approach but I just saw it on a 1950s ranch house. Or use gusseted rafters--hang 2x4 sub-rafters below the main rafters, to create as much insulation space as you want.

    1. Joseph Dziedzic | | #5

      Thanks Michael, appreciate the suggestion on the gusseted rafters.

  2. Expert Member
    Akos | | #2

    You can also look at using I-joists. Labor is about the same and you can get them in much larger sizes.

    Since the web of the I-joist is a narrow piece of OSB, you get very little thermal bridging, a 11 7/8" I-joist assembly R value would be 10% to 15% higher than comparable dimensional lumber roof. Since they can be had up to 24" tall, you can get a very high R value roof without having to do any extra fancy framing. In most cases you can also go 19.2" or 24" OC which saves a bit of cost.

    1. Joseph Dziedzic | | #6

      Thanks, very interesting idea. Is there a need for special handling of the I-joist at the ridge? i.e., is a beam needed at the ridge to support the I-joists with this approach?

      1. Expert Member
        Malcolm Taylor | | #7

        Joseph,

        I joists act just like rafters made from dimensional lumber. Both of them need a ridge beam, or some way (like rafter ties) of dealing with the resultant horizontal loads.

        1. Expert Member
          Akos | | #9

          I don't think you can put rafter ties on I-joists, the flanges are not meant to take that type of tension load. An engineer might be able to spec a detail that will work.

          I-joists are really the best for something like a low slope or shed roof. A gabled roof is possible but probably needs a ridge beam.

          Make sure you read the guidelines for working with I-joist. Most are pretty straight forward, the big one is never to notch the flange. Since you can't do a birds mouth cut, for setting above a top plate, you need either a tapered shim or Simpson VPA connector.

          Another great part about I-joists is you cut some really large holes in them. Makes running any plumbing or HVAC very easy.

          1. Daniel Allen | | #11

            For cutting I-joists, be sure to carefully follow the manufacturer's limits/instructions. Fixing mistakes can be difficult.

        2. Expert Member
          Armando Cobo | | #10

          If TJIs is your choice, look at Truss Joist framing page details. They have several details in PDF and DWG.

  3. Expert Member
    Armando Cobo | | #3

    Here is a detail you described, and I've used for years here in the south. We now install 1" iso on the outside.

    1. Joseph Dziedzic | | #4

      Thanks for the illustration, very helpful!

  4. Joseph Dziedzic | | #8

    There's an example of the "rafter on raised top plate" approach over on the Department of Energy's "Building America Solution Center" Web site: https://basc.pnnl.gov/resource-guides/attic-eave-minimum-insulation#edit-group-description

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