what is the purpose of the vapor barrier under the wood floor?Is it always nessary to insall vapor barrier?
Not much if you are going to install a nail down floor and punch a thousand holes through it. Some time ago we used flooring paper to speed up the installation, and it serves no other purpose. And as long as you have a heated ( in winter ) and dehumidified ( in summer ) basement under the floor you should have no moisture problems coming from under the floor. And it is always the moisture difference between one side of the board and the other that cause warping. But there are exceptions to this.
When you live in a damp climate with a crawl space under the floor, a lot of things have to be done to prevent moisture from the ground from entering the hardwood floor. The first is a thick 1″ plywood subfloor ( several layers is OK). This will prevent the 2″ flooring nails ( never use floor staples ) from penetrating through the subfloor. They will only go in 7/8″ and leave a moisture barrier under the floor. Also lay a 15 pound roofing felt on this subfloor, before you lay the floor.
Then there is the crawl space itself. It must be well ventilated in the summer, and at least 2 feet above grade, and the ground covered with plastic and dry sand. And possibly heat this space in the winter.
Oh, and finish these floor on site, with Oil Modified Polyurethane, that will prevent inside moisture from passing into the wood in the first place. Prefinished floor have many open seams, that won’t stop the indoor moisture.
These are all the real moisture barriers.
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As always your Most humble servant, Joseph, the Wood Floor Doctor.