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I am finishing my attic and have just completed laying the OSB subfloor. I plan to use this space as a yoga studio for my wife / guest room for family / playroom for my child. It is directly over my bedroom so I want to make it as quiet as possible so I am planning to put a thick rubber underlayment before I put down a laminate floor. Are there any things to consider when putting the underlayment down? For example, a vapor barrier, an extra underlayment for the laminate flooring, whether it needs to be secured down?

UPDATE: I ended up going with a much thinner underlayment which I installed first. I also purchased the laminate with an underlayment backing adhered to the underside of the laminate which seemed to be equal thickness of the extra underlayment I purchased. The result has been great. There is very very little sound transfer from the floor of the attic to rooms below. I have no practical experience to back this but I would be a little worried the thicker underlayment might make the floor too squishy.

  • What is your insulation above like? I'd be concerned about creating a vapour barrier between two heated spaces (with the rubber underlayment). Either you need two climate control zones to avoid moisture issues between, or else you must make sure you don't get condensation between. Without more details, not sure what else to say, but I bet other posts about basement finishing provide the same issues only inverted. – Andrew Sullivan Sep 6 '15 at 2:39
  • Hey @chris, do you mind giving us a quick update about the results of your install? Was it worth the effort, did it reduce the sound much? I'm thinking about using rubber in the kitchen floor underlay. Thanks! – Andrew Sep 20 '16 at 4:18
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Insulate the new subfloor in the joists below to thwart any condensation issues first, as much R-value as you can get in there. Other than that no, the "underlayment approved laminate" can just sit on the underlayment & you might be able to double or triple the planned underlayment, depending on the laminate manufacturer.

However, Carpet on top of carpet Padding would really be the overall best sound deadening solution & your underlayment doesn't care whether it's penetrated with fasteners.

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You could use Cork as a deadening agent or finished product, use it as a deadening agent and it would prevent any off gas issues.

As Iggy said, you would want to insulate the joist bay's, however I don't think you want to pack them tight. If you pack a bay tight with insulation the sound waves use it as a vehicle and transmit each wave easier, a loose pack with 2"-3" void on top allows the waves to travel then hit a stop much like offset common walls in a condominium. This is also a good time for another look at your bedroom- pipe in for speakers, additional ceiling fan, cans etc.

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For sound insulation, there are multiple layers where you can tackle the issue, and the more you do, the better the result:

  • Insulation between the joist bays. No vapor barrier needed since this is for sound, not thermal. This is going to eliminate the high pitched sounds like voices, more than the low thumping of walking that will transmit directly through the joists.
  • The underlayment, I've used homasote in the past, but your rubber product will likely do that job better. I personally wouldn't spend the money here if you're installing carpeting.
  • Finished flooring, carpeting that you can do from above is going to be best, and with a thick carpet pad, you don't really need the additional underlayment. Any kind of hard surface like wood or tile would be the worst option, but is where you'd see some benefit from the underlayment.
  • Sound attenuation channel, aka resilient channel, is probably second to the carpeting, but you'd need to remove the drywall ceiling in the bedroom below, or build a second lower ceiling, to install it. This works by separating the drywall from the joists which are going to transmit the low vibrations (walking, or kids jumping).

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