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6 inch EPS in bonus room?

Bonusroom1 | Posted in Green Building Techniques on

I’m in the process of finishing a bonus room. I have rafters that are 11 inches deep.

I found a deal on 6 inch EPS foam. I planned to cut-and-cobble, and leave 1 1/2 inch ventilation gap between the roof deck and the EPS, then fill the remaining 3.5 inches with fiberglass batt insulation.

Is this plan solid? Should I use faced or unfaced insulation?

Thanks in advance.

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  1. Expert Member
    Dana Dorsett | | #1

    At 6" the EPS is good for R23- R25, depending on density 3.5" of fiberglass is good for R11-R15. So that's a total-R of R34- R40 at center cavity.

    From a dew point control point of view the worst-case is R23 EPS, R15 fiberglass, which would be a ratio of R23/R38= 61% of the total R exterior to the fiber. That's sufficient dew point control for US climate zone 7 or lower (all of the lower 48 of the US would be covered. There is no need for an interior side vapor retarder (such as a kraft facer) in zones 1-7 with that stackup- standard ceiling latex on gypsum board would be fine.

    R15 rock wool is far more fireproof than R15 fiberglass, and would be the better product to use here. EPS melts while burning, and fiberglass will melt under an intense flame, whereas rock wool does not. (The house would already be gone at the temperatures needed to melt rock wool.)

    It's important to air-seal the EPS to the rafters. Cut them ~3/8-1/2" narrower than the rafter bays to make it easy to fit them in, then use can-foam or a small 2-part foam kit to fill the gaps .

    Code minimum for US climate zones 4 and higher is R49, or U-0.026 (= R38.5 "whole assembly") not R34-R40. With 2" deep ( 1.75" is good enough) Bonfiglioli strips and R23 there is room for R23 rock wool batts, which would get you there on a U-factor basis, at the cost of a couple inches of headroom. See:

    That stackup would have sufficient dew point control for climate zones 6 or lower, but not zone 7.

    1. Bonusroom1 | | #4

      Ive got free kraft faced fiberglass, any problems using that in this setup?

  2. Bonusroom1 | | #2

    Thanks and ill definitely look into rockwool
    To fill remainder of cavity.

  3. Expert Member
    Dana Dorsett | | #3

    A 2x12 is actaully 11.25" deep, not 11". So with 1.5" of vent + 6" of EPS it leaves 3.75" , not 3.5". While it's possible to fluff an R15 by another 0.25" it's not a snug fit.

    Adding 1.75" edge strips (1" polyiso +0.75" furring) would bring that to 5.5", the perfect depth for R23 rock wool or R21 fiberglass.

  4. Peter Yost | | #5

    HI Eric -

    Never good to assume so make sure you read Martin Holladay's blog on this topic:

    If I was going to invest all the time and effort into using this approach on such an important assembly, I would somehow perform-test it: depressurize and infrared imaging.

    Given the insulation ratio you propose, the air-permeable batt insulation does not need to be faced with a vapor retarder, but it's ok to use, if for no other reason than the tabs it provides for stapling during installation.

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