We had Brazilian Koa hardwood installed on our 3/4 inch plywood subfloor that was level and solid. The installer used Bellwood's best underlayment and 16gauge nails. Everywhere you step the floor squeaks, every step, almost every board. We're having the installation redone. What shall I be looking for to avoid this problem reoccurring? Should the floor be glued and nailed to avoid the squeaking?
The underlayment being nailed down was the first mistake. Nails pull, wood slides against the metal, and it squeaks. Subflooring (all of it) should be screwed down (and glued, preferably).
The bounce is probably a problem with the span of your floor compared to the thickness of your joists. Was the floor bouncy before the hardwood install?
With the hardwood floor up just walk over the underflooring. If it bounces or sqeaks the underflooring needs to come up. Repeat the walk about. Still squeaking? The floorboards need to come up and the joists checked with possible cross braces nailed in place and the ends firmly fixed...not a quick job. If the former, then the post above is fine, screw the plywood firmly on no less than 12" centres across the level floor. Assuming your hardwood floor is T&G: The solution is to lay it so that it floats on the sub-floor, there are many makes of proprietary 'batts' these look like compressed soft board. Remove all the skirting boards. Lay the batts as a well fitted mosaic loose! They are soft so any movement will be taken up within the material. Lay the hardwood floor on top blind nailing through the tongues into adjacent boards NOT into the sub floor leaving appx 1/4"...6mm gap between the edges and ends by each wall. Replace the skirting boards leaving a tiny gap between the bottom and new floor. The new floor will 'float' on the surface and move as one. No squeaks!
Well your 3/4 inch plywood needs to be screwed in to the joists and it also needs to be glued down. Probably nothing to do with your nails or underlayment.