We're about to install solid hardwood floors (nailed down) into our home and I have some confusion / questions about the install. I live in the San Francisco Bay Area, ~ 25 miles from the coast.

The home was built in 1965 but the subfloor seems newer - 3/4" Weldwood Plyscord plywood. I have removed the old carpet and cleaned up. The hardwood has been acclimating for a week now. So here are my questions:

* I've been reading that I need a vapor barrier installed in my crawl space to reduce the chances of moisture damage to the floors. The home is on a hillside with a large crawl space and adequate ventilation. The lowest point in the crawl space is ~4-5ft with the highest being over 8ft. The dirt in the crawlspace is dry except around the perimeter foundation which gets wet but not muddy or flooded after rain. The joists and subfloor are fully exposed with no barrier or insulation and is in excellent condition. So, is the vapor barrier necessary?

* EDIT: going to get aquabar instead for the underlayment.

* I see some dipping between the joists but it is 1/8" at the lowest point. I plan to "fill" it with extra aquabar before putting down the underlayment. Do I need to staple it in place?

Aquabar is a good choice. Stapling is not necessary, but it will hold it still and make it a bit easier.