I am installing 4" wide x 3/4" thick prefinished maple tongue and groove hardwood onto a plywood subfloor. I have zero humidity or moisture issues.
I wanted to use Aquabar B as my underlay, but it is not available in my area right now.
Other options are waxed paper, which I have used before with zero issues:
https://www.homedepot.ca/product/roberts-750-sq-ft-3-ft-x-250-ft-x-009-inch-waxed-paper-underlayment-for-wood-flooring/1000496301
Or I go with 15 lb roofing felt.
Any reason not to go with the paper? Should I wait two weeks to get Aquabar? I don't think I really need it.