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Friction Fit of Rockwool Batts

canada_deck | Posted in General Questions on

The situation:
For this energy efficient shed, I have a very small attic.  All of the trusses are now up.
To make life easier, the plan was to put in the ceiling first (vapor barrier and plywood ceiling) and then put in batts of Rockwool from above before sheathing the roof.
Due to a bad weather forecast, I am now thinking of sheathing the roof first and installing the batts from the bottom.  That will mean jamming them in from below between the studs and the friction-fit will need to hold them up until I can at least get some vapor barrier stapled up.  Should that work? Having never attempted this before, I am having a hard time knowing if this is as practical as it sounds.

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Replies

  1. Expert Member
    PETER G ENGLE PE | | #1

    This should work. Rockwool is slightly "stiffer" than FG, and it is designed to friction-fit in the cavities. It will generally hold itself up indefinitely, certainly long enough to install a VR and/or ceiling.

  2. jbracey | | #2

    From some recent experience with 24in R30 Rockwool batts, it works well if the bay is the correct size, or a little narrow. If it is a little too wide, it’s a pain, especially when your up on a ladder and about to boil from the heat…

    I tried strapping, and that was cumbersome. Best fix I found was to cut little 2x6 in blocks of 1in foam board, put to short nails in them, and have those available to tack up to support the edge for a moment, and then add a bit of extra loose rockwool to the edge to give the friction fit.

    May help, but hopefully your rafter/joist spacing is all spot on!

  3. AlexPoi | | #3

    You can buy Simpson Strong Tie insulation supports at your local HD to hold the insulation in place if you have problems.

  4. Expert Member
    BILL WICHERS | | #4

    It will work fine. Mineral wool is pretty stiff stuff, and should hold itself in place without much trouble.

    A trick I have used in the past is to run some nylon twine across the bottom of the trusses stapled up with a staple gun. The twin acts to support insulation until your ceiling is up, and it's cheap and can be left in place without worry. To do this, tie a knot in the end of the twine and staple it up at one end of the room so that the knot keeps the twine from sliding under the staple. Stretch the twine across the room and staple it up at the far end. Now zig-zag across to make your insulation supports across the ceiling. Once you have that done, add additional staples along each run of twine sufficient to hold things up "enough". You can drywall right over the twine, but the twine will support batts until the finished ceiling can be installed.

    Bill

  5. canada_deck | | #5

    Thanks all! This is very helpful. We're going to go ahead with throwing on the sheathing today and then doing the insulation on a later day. Our trusses are 16" on center exactly (except the two end ones which I will need to cut anyway so it sounds like this should be possible. I really like the twine idea. Amazing how many "order of operations" problems come up along the way.

    1. Expert Member
      MALCOLM TAYLOR | | #6

      C_D,

      There is an established sequence of construction. You work towards lock-up by:
      - Framing and sheathing the entire structure.
      - Putting the roof on.
      - Installing windows and doors.
      - Roughing in ducts, plumbing and electrical.

      Apart from making sense in terms of ensuring the more vulnerable elements like insulation and wiring stay dry and don't get stolen, it's also important to know on projects where you have taken out a construction loan, where draws are made at various benchmarks. The lender expects you to work systematically and complete all the required tasks at each stage before releasing the funds.

      Problems of sequencing typically only come up when you are using new technologies or innovative building assemblies.

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