Does anybody have any suggestions for acoustical underlay for the floor? I need a dirt-cheap solution to somewhat de-couple the floor without putting another floor on top. Can strips of acoustical underlay mat be put between the floor and the joists? Currently, the plan is to just have Roxul safe n sound between the joists, but based on previous info that you guys have given me, I don’t think that will be enough.
I can’t say I have a dirt cheap solution. However, I am not sure I fully understand what you are asking. You said you are having floor joists on screw piles into the ground. so that means the floor will be suspended directly over the ground. So you are not pouring a concrete floor?
If what I said is correct your issue is going to be your floor acting like a drum. It is actually the worst possible way to deal with your floor as it will amplify any vibration. The best way to alleviate this drum effect is to fill the void between the floor joists with sand. It will add mass and dampen vibration significantly. But it doesn’t sound like this is a possibility based on the info above. If you are not using a full poured concrete floor this is going to compromise anything else you do as far as sound attenuation. Give me more details as to what you are doing, plans and maybe we can find a workable solution for you.