Solid Hardwood, Engineered and Laminate Flooring - How much foam to put under the hardwood?

Why am I laying this 3/4" thick, 2 1/4 wide prefinished red oak flooring on top of the existing second floor of this house? Because there is very little sound isolation and the first floor hears everything. Plus the hardwood that exists is at end of life in this old house.

What to lay under the floor to increase sound isolation? I could use red paper, that is cheapest and thinnest. Instead I bought that rolled foam. I am thinking, hoping that will increase sound isolation.

Question, could I use two layers of foam under the new hardwood flooring? Any unforseen danger there?

foam? do you mean you've bought the foam underlayment [meant for using under floating laminate floors] to use under your hardwoods? i'm no pro, but i think you should be using some sort of felt or paper instead.

as for 2 layers, more isn't better in this situation. you don't want something soft & cushy under a hard surface flooring material. you'll be creating a mushy surface that will cause up & down movement of your floor, which will just cause all sorts of problems.

but all that's just educated guessing from reading the pro's answers here.....

You do not put any foam under 3/4" solid wood flooring. I understand what you want to do but this is not a solution. It is actually a future problem for the longevity of the new floor.
You mention that you want to put this new floor over an existing floor, but you don't mention what that floor is. There are very few finished floors that you can just "go over". A call to the manufacturer will let you know if what you want to do is correct.
Ron

I have a 2nd floor hardwood floor, it is end of life hardwood that I will place the new floor over. A driver for doing this is to increase the sound muffling from this apt and what the 1st flr hears.

Since this I called the manufacturer, they recommend only red paper or roofing felt.

Thx for your responses!