I'm installing a 3/8" engineered floor for a friend over Quiet Walk underlayment. Initially, they requested the floor to be a floating floor. However, we are now wondering if it is possible to nail the floor with the underlayment still in place.

The quite walk underlayment is about 1/8" thick. Can I do a nail down installation on top of this? If so, what type of nailer/stapler is best?

No, you do not use Quietwalk under nailed down floors. As for the stapler, look into the Bostich flooring stapler. Its basically an upside down 18 gauge staple gun with a shoe. I got one for a job and didn't need it for 2 years (lots of glue down in these parts), ended up selling it on EBay for damn near what I paid for it. Point being, sell it when you're done with it.

Thanks for the advice. Nail down simply won't work through an underlayment? Or it just isn't usually done? Just curious as to the reasoning here. Always wanting to learn more about flooring. Thanks again.

Like I said, I do a lot more glue down and floating than nail downs. But the way I look at it is that a nail down should be solid to the floor. Any flex or cusion could work the fasteners loose.

Quiet Walk is specified for floating floors, ( Hardwood or Laminate). It is not quite dense enough for naildown. The same company manufacturers Insulayment ( looks just like Quiet Walk ). This is a very dense underlayment for naildown or gluedown. I have done many installs with it. Insulayment qualifies for sound requirements in condos, also other benefits. Also have done many installs over cork underlayment.

Provided that the underlayment is sufficiently dense, you can nail through it without detrimental effects. We often use cork under nail-down floors since the customer perceives that that will reduce noise transfer from one level to another.