I want to fix floorboards to supporting joists but isolate them acoustically?

reduction of impact sound

Answers:
You can lay sound insulation over the tops of the joists, not sure how it works while remaining structurally rigid though. I've seen it in a diagram in a text. Have a look on the B.R.E. website, or do a Google search "acoustic joist isolation" or something.

What will also help - Sand between the joists, but could be messy; polystyrene is less messy but less safe in fire; other types of insulation you'll find in a DIY store, easily.

Other answers:
you can use r-11 insulation or 90 pound roofing paper. the advantage of the paper is it sounds proof and is moisture resistant where insulation isn't.
you can use r-11 insulation or 90 pound roofing paper. the advantage of the paper is it sounds proof and is moisture resistant where insulation isn't.
Place rubber mounts and/or semi-compessed springs between the joists and the subflooring.

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