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Insulating a strapped attic floor

user-901114 | Posted in Energy Efficiency and Durability on

Need to insulate an attic floor with 24oc/12in high truss joists that have been strapped with 1×3 strapping, 16 oc across the underneath of the joists. 1/2 blue board is attached under the strapping and veneer plaster makes up the ceiling. 2 coats of Behr ceiling paint is on the plaster.

My concern. The strapping creates a space under the joists, so it’ll make the joist bays “connected”. Air could communicate from bay to bay if I were to use unfaced roxul batts which is my preference because I have plenty. I also have some cellulose that I could spread as my first layer to fill the voids that the strapping made.

What are my options?


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  1. davidmeiland | | #1

    How wide are the I-joist flanges?

    If you really want to go for it, you could foam all the gaps under those flanges. I would seriously consider blown cellulose for the whole job. It's faster and better, possibly a LOT faster depending on what the headroom is and what you have to cut around. Maybe you could sell the batts to the insulator for a few bucks off on the cellulose bill.

  2. user-901114 | | #2

    Thanks. They're KD joists. This is all DIY so speed isn't a factor. Can't get under the attic deck to do spray foam but I could blow a layer of cellulose in there.

  3. davidmeiland | | #3

    Not sure I have a picture yet... you said "truss joists" so I figured you meant I joists, which sometimes have flanges as wide as 3-1/2". There's no way that cellulose is going to slump into a space that's 3/4 x 3-1/2, so I would prefill it with something if you want to be fastidious, otherwise you end up with air pockets (not the end of the world, really).

    Also not sure what you mean by attic deck. You're going to have to get your insulation right on top of the drywall, so you basically need full access, especially with batts.

    My attic has a catwalk over a large area, with a plywood floor. The cellulose had to go in first, then we put the plywood in.

  4. GBA Editor
    Martin Holladay | | #4

    Even if cellulose doesn't fill the 3/4-inch-high gaps left by the strapping, it will still stop the airflow problem in a way that the Roxul batts won't. I vote that you use a deep layer of cellulose for your attic floor, and save the Roxul insulation for a different job.

  5. user-901114 | | #5

    My plywood catwalk is already in because I thought I was going to insulate the rafters... The attic floor joists are not I joists. They're just 2x10. +2in spacer under the catwalk. Bringing a total space under the catwalk to 11in.

  6. Expert Member
    Dana Dorsett | | #6

    It's hard to beat cellulose in an application like this, and any batt solution will have gap & void performance issues that are even more problematic in cold-side up attic applications than they are in walls. If you MUST use batts, a 3"(min) overblow of cellulose is warranted to tighten up and fill in gaps. For the amount of space you're talking I don't see the benefit of using Roxul for any part of it. I'm singing with the choir on this one- to achieve the rated performance it's an all-cellulose show. But open blow attics are not a very difficult DIY job with a rental blower either.

    Blowing it to 3" or so over the tops of the joists wherever you can gives it an R10 thermal break over the framing too, which is a worthwhile performance enhancement.

  7. davidmeiland | | #7

    It sounds like 11" is the max before it starts covering the catwalk.

  8. user-901114 | | #8

    Thanks guys. I can use the roxul in the basement and go with cellulose in the attic.

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