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How to get to R-60 roof/attic

Lesley_Farm_Girl | Posted in Green Building Techniques on

I live in Central Montana and would like to get to an R60 roof for my new construction. 

I get confused on venting the attic or not… Want to prevent ice damming, etc… Do you put exterior insulation on the roof and then the Attic. What is the easiest, best value way to do this correctly?

I’m interested in Rockwool, spray foam, etc…

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  1. Expert Member
    Peter Engle | | #1

    The easiest, best value way to get to R60 is to put 18" of blown-in cellulose on an airtight, flat attic floor.

  2. creativedestruction | | #2

    What Peter said. Most cost-effective approach.

    Definitely vent the roof deck. Soffit to ridge, every bay and every attic area whether subdivided or vaulted. Avoid recessed can lights. Can't stress the airtight ceiling part enough.


    1. Lesley_Farm_Girl | | #3

      Thanks! We don't have to put exterior insulation on the roof, right??

      My husband was thinking of just using spray foam in our 4x6 walls. Would that. Be sufficient or would it be better to put Rockwool on the outside and inside or a combo of Rockwall exterior and spray inside. Cost is important... Best value.

      1. Expert Member
        Dana Dorsett | | #4

        >"My husband was thinking of just using spray foam in our 4x6 walls. Would that. Be sufficient or would it be better to put Rockwool on the outside and inside or a combo of Rockwall exterior and spray inside. Cost is important... Best value."

        The answer to the first part is "No"- 5.5-6" of foam between framing isn't going to even make code-minimum, even if using closed cell foam (which is bad idea on several fronts.) The thermal bridging of the 4x6 framing robs closed cell foam of it's potential thermal performance pretty badly, and dramatically reduces the ability of the sheathing to dry. Filling the cavities with cellulose and applying the high R/inch foam budget to continuous exterior insulating sheathing is a much better bang/buck.

        Are those true 4" x 6", or is it a milled 4x6 (3.5" x 5.5")?

        What is the spacing of those 4x6 framing elements?

        In US climate zone 6B (=most of Montana below 10,000' of elevation) it takes a minimum of R11.25 of exterior insulation to have dew point control at the sheathing layer with R20 cavity fill (5.5" of cellulose or half pound open cell foam.) That would take 3.5" of rigid rock wool, 3" of Type-II EPS, or 2" of 1lb polyisocyanurate. IRC 2018 code minimum from a thermal point of view would be R2o cavity + R5 continuous insulation, but that would require an interior side vapor barrier or "smart" vapor retarder for moisture control at the structural sheathing. The more exterior-R, the safer/better it is for the wood sheathing.

        Open cell cavity fill does a pretty good job of air sealing the cavities, but it still needs the proper sealants for any double-up framing (jack studs, top plates, etc) and between the bottom plate and the sub floor. With cellulose cavity fill extend the caulking to "picture frame" seal the cavity to the framing inside each stud bay works well (polyurethane caulk, or purpose-made low-rise foam sealants). Cellulose has a useful amount of thermal mass to work with as well if dense-packed to 3.5-4 lbs per cubic foot density, enough to make a measurable performance difference, but not something you're likely to feel in a 4x6 framed wall. Cellulose also has a negative carbon footprint- it's sequestered carbon, and the borate fire retardents help resist wood boring insect infestation. (The borates kill off the gut flora needed by the insect to digest wood.)

      2. Expert Member
        BILL WICHERS | | #5

        Dana already went into the details, but I want to also mention spray foam being a waste in walls. Spray foam should be thought of as a niche product — it is the best option for certain special situations, but in most of the regular locations you’d want to insulate spray foam is rarely the best option.

        I’m assuming you'll have your walls open so you have the ability to insulate them in multiple ways. If you have the walls open from the outside, then I’d add exterior rigid foam — at least 2” of polyiso as Dana suggested. More exterior foam is better in terms of insulation performance but remember that as you get really thick (over maybe 3-4” or so), all the exterior details become more complicated.

        I’d use mineral wool batts inside the walls because I like that product. Fiberglass would work too, but try to get the high density version for a bit more R value per inch. With sufficient rigid foam (the amounts Dana mentioned for your climate zone), you don’t need to worry about an interior side vapor barrier/retarder. If you will have your walls open from the inside, a smart vapor retarder (MemBrain, etc) is extra insurance against moisture problems.


  3. STEPHEN SHEEHY | | #6

    We used raised heel trusses and dumped about 20" of cellulose on top of the floor of the vented attic. From bottom up:
    Drywall, 1x4 strapping, air barrier membrane, truss bottom chord/ cellulose. Simple and pretty cheap.

  4. user-723121 | | #7

    I agree with the above posters, note Stephen's comment on the energy heel trusses, 12" minimum and full depth (of insulation) is better. The attic is an area you can superinsulate quite cost effectively. Now, about those walls.

  5. Lesley_Farm_Girl | | #8

    Thanks everyone!

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