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I have an attic that originally had blown-in fiberglass insulation, with an additional layer of loosefill wool insulation added on top. This totaled up to a depth of 16 inches, which has an R-value of about 60.

Basically, I want to complete remove this insulation and instead find an alternative that would live within the rafters. Yes, this is a major undertaking, completely impractical, and costly, but I do enough work in the attic that I'm fatigued with dealing with blown-in insulation. There is no ceiling to the budget needed to make this happen.

The home is 2600sqf - below are a few images of the attic space to give an idea of the complexity regarding this project:

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I'm looking for material suggestions. Since there are ridge vents, I'd want to keep about 2" between the sheathing and material, so the goal would be as much R-value in as little depth as possible.

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  • I think your money might be better on placing a floor down. Those rafters look like 2x4s which are 3.5 inches deep. Even with only an inch for roof deck ventilation that gives you 2.5 inches for insulation. You might find some very expensive insulation that might do some of the job of keeping the living space comfortable.
    – crip659
    Commented Feb 20, 2023 at 14:35
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    With no regard to cost and apparently no bottom to your wallet....will you adopt me?
    – RMDman
    Commented Feb 20, 2023 at 14:37
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    I'd recommend floor runners made of 3/4" plywood/OSB to get you to the areas you need to service. This is what they did in our church attic, and we recently had 4 guys up there rerunning networking & audio cabling. Where we didn't have runners, we simply walked through the insulation on the top of the joists, then cleaned up the mess we made afterwards in those limited areas.
    – FreeMan
    Commented Feb 20, 2023 at 14:48

3 Answers 3

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This is, indeed, completely impractical and a waste of money unless you are using the attic as workshop or living space, and I see no evidence of that, as your reference to "doing enough work there" appears to be nothing that makes the visible parts of the attic look like anything but unused attic space, without even storage.

Feel free to waste your money however it makes you happy to do so.

You have trusses, not rafters, and any remotely equivalent amount of insulation will not fit within the top chords of the trusses.

The simplest solution, "cost no object" is to build an additional roof on top of the existing roof, and insulate (and leave space to vent above) between the old roof and that. Sheet foam with 2x4s to make a vent space fastened to the trusses below (and lumber around the edges) is a typical approach to this sort of "double-roof" - if your trusses can stand the additional weight. Cost no object, you have an engineer design a retrofit for your trusses to make them take the additional weight.

I think someone has mentioned using metal roofing and letting the raised channels in that serve as the vent space, without the additional 2x4s, though fastening seems like an issue for that approach. Perhaps they used 1x4's to fasten to, I can't recall. As for the insulation in place, rather than removing it all, simply getting it to the point you can floor over it with plywood should suffice (people with cost as an object might use OSB.)

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  • The roof is being replaced as well; that's an interesting idea with metal roofing, though. I was looking into using DECRA in place of asphalt shingles.
    – tripleblep
    Commented Feb 20, 2023 at 15:38
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    Before you settle on metal roofing, @tripleblep, I'd look into the noise factor. There have been several questions here about the metal rattling in the wind over long vertical spans. You may decide that's undesirable.
    – FreeMan
    Commented Feb 20, 2023 at 15:52
  • If (big if) you can find a contractor, sprayed on closed-cell foam roofing that does both insulation and waterproofing is an option. I think it gets an additional UV-protection coating over the top. But if you can't find a contractor in a reasonable distance, it's one more technology Good in Theory, but Not Available Here.
    – Ecnerwal
    Commented Feb 20, 2023 at 16:05
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A quick peek at my local big-box website shows that standard fiberglass insulation is R13 for a 3.5" depth. To get back to your R60, you'll have to put 4 layers on, supported under the ceiling, and that'll only get you R52, so make it 5 layers. That's supporting a lot of weight and depth from the ceiling (5 layers x 3.5" is 17.5" of roof head height you're loosing). As a bonus, though, you're getting R65 out of 5 layers, so you're upgrading. (I don't know how you'll suspend all of this and whether how much it will crush and loose insulating ability in the lower layers over time.)

On the bright side, XPS (I believe) foam insulation is R5 per inch, so you'll only need 12 inches of the stuff to get you back to R60. The good news is that you could cut the first layer into 14.5" wide strips (assuming 16" OC rafter spacing) and put the first layer between the rafters. This would maintain 1.5" of air gap under the roof for soffit vent air flow and would reduce the encroachment to only 10" of headroom lost. According to the website I looked at, it is fire resistant, but once it does start burning, it's going to put off some nasty chemicals.

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  • PolyIsoCyanurate foam does a bit better than XPS on R-per-inch. And definitely puts out toxic smoke. Above the living space, that aspect is less likely to be a major factor.
    – Ecnerwal
    Commented Feb 20, 2023 at 15:15
  • The one I looked at, @Ecnerwal, is green foam and unfaced with anything. I'm not sure exactly what type of foam it is, and couldn't find it specified in the listing.
    – FreeMan
    Commented Feb 20, 2023 at 15:54
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Unless you can find some kind of vacuum bottle made to lay between the joists, I don't think there is a lot you can do to get the desired degree of insulation in that space.

You might want to consider putting a plywood floor into that attic, with insulation both below and above it. You would still need to shovel your way through the blown-in, and shovel it back into place afterwards, but it would make working in that space a lot more convenient. If I spent more time working on wiring in my own attic I'd consider that.

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