I'm replacing the carpeting in the second floor of our house with a hardwood floor. It will be 3/4' thick 5' wide nailed down solid maple.

I've finally finished prepping the subfloor and I'm close to putting the underlayment down (I've already removed the carpet/pad/staples, screwed the subfloor to the joists, leveled the subfloor, sanded the seams a little, am letting the maple acclimate, etc).

I'm not worried about moisture but I am worried about sound transmission. We rent out the second floor and have two toddlers on the first. Some tenants have gotten frustrated hearing our children scream and my wife doesn't like hearing upstairs stomping footseps.

I'm not in a position to tear up the subfloor or significantly add to it, so I was hoping to get a little help from an acoustical underlayment. Even a tiny reduction would be fantastic. Unfortunately almost all of acoustical underlayments I've found are for floating floors. And they seem a little squishy too, so the underlayment may gradually compress over time allowing the boards to flex and squeak.

But Insulayment is supposedly designed specifically for nail down floors and is at least somewhat effective in reducing noise, although MpGlobal specifically references the compressing possibility and says it may cause "floor noise."

So this sounds like a tradeoff in risk/reward.

I can't decide between insulayment or a straightforward safe 15 lb asphalt paper underlayment like I did on the first floor years ago.

1) Does anyone have any experience with insulayment over time?
2) How helpful is it in reducing sound transmission?
3) How bad does the floor noise get with insulayment?
4) Would 30 lb paper do any better to reduce sound than 15 lb?
5) FloorMuffler ultraseal has been recommended a few times to me too as the "top of the line" but it's made of foam which gives me pause. And marketing it for nail down applications sounds like an afterthought.

Any help is much appreciated!!

Well the best way is to use something between the underlayment (ply or OSB) and the subfloor as the fasteners them selves transmit noise through the floor.

I've talked to a few more folks since I posted that. Everyone was in agreement that a pad underlayment like insulayment for nail down is a crap shoot. It may help with noise, may not, may cause some flex in the hardwood, may not.

But it won't help with noise through the nails as you've mentioned.

I've decided to go with Aqubar. It's the safe option I have experience with. It's what the manufacturer recommends anyway.

But I had already used a jambsaw and cut the trim and doorjambs in anticipation of a 3 mm underlayment. Any ideas on what I could put under the hardwood to take up that space? I will use quarter round for the trim which should help, but can't really use that for the doorjambs.