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Here in Brazil I can only find insulation that is about 35-40 mm thick. I intend to use it inside steel studs that are 70mm thick and I'm worried it might crumple or sag due to the extra space.

What is the proper way to do it? Should I fix it to the drywall somehow?

Edit those are internal walls, and the insulation is being used for its acoustic properties.

  • Are you opposed to using two layers? – tahwos Mar 3 '16 at 23:10
  • Considering the cost would double, yes... – Luiz Borges Mar 3 '16 at 23:21
  • Is insulation expensive there? If you're nickel and diming it, over basically R3 vs R7, you might as well just throw some foam board in there, and call it day - because neither one of those, is going to have any "acoustic properties", to speak of anyway. – tahwos Mar 5 '16 at 0:11
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If paper-faced insulation is available, I'd consider using an adhesive caulk to hold it to the studs. The product we use for polyethylene vapor barriers here is called "acoustical sealant". It remains very flexible, so it's less likely to cause bulges in your drywall.

Alternatively, use a spray adhesive on the back side of the drywall that's already installed. Press the fiberglass onto it while it's tacky, then install the opposite drywall face.

  • Actually, this is being used in the internal walls as acoustic insulation. – Luiz Borges Mar 2 '16 at 16:57
  • Ok. I still think I'd use paper-faced batts as I described. – isherwood Mar 2 '16 at 17:04

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