Davoud wrote:
Is it necessary or desirable to use a soft underlayment when installing
3/4" nail-down hardwood flooring on 3/4" plywood sub-flooring? The
floor is above ground level.

A Google search turns up a bewildering array of underlayments, but
details on applications are often hazy. If you have used an
underlayment such as I described I would be obliged if you could name
the brand and/or type (material, thickness, means of attaching to the
sub-floor, e.g.)

Do any of them describe the purpose of the soft underlayment?

I have read that years ago heavy roofing felt (tarpaper) was
sometimes used between hardwood flooring and solid timber
underlayment. The result was a non-squeaking floor. The
felt acted as a cussion, glue and vapor barrier. It is not
clear which of the three was the original purpose.

If you understand _why_ something is done the way it is,
then probably you can decide what and how to do it for
yourself. If you _misunderstand_ why something is done
the way it is, you can produce a real disaster.

--

FF