Solid Hardwood, Engineered and Laminate Flooring - question about subfloors & moisture

Going to lay down Pergo in a couple rooms. Two of the rooms, after the current floor/carpet is removed, will be 3/4" plywood that will be easy enough to prep. The other room is an addition that has, I believe, a plywood subfloor with a small 'crawl space' of about 3 inches, followed by another layer of wood. This is sitting on the concrete slab.

So for moisture barrier, want would I need in these two scenarios? I'm thinking that for the floors that are part of the original house, I would need to make sure that there is a moisture barrier in the crawl space. For the 'addition', I'm not sure because there is no way to 1) rip up the subfloor without damaging the bottom wod, and 2) lay a moisture barrier in the 3" crawl space.

How important is the moisture barrier in these two cases?

Also, when a wood floor is not level and filling is required, is it acceptable to use a wood filler?

Thanks guys !

I just spoke to my carpenter and also another flooring company. They stated that moisture barriers are only necessary when installing directly onto a concrete slab, or directly onto something that is fastened to a concrete slab (existing floor). In my situation, with the existing crawl space, there is no concrete. The other situation with the addtion, the concrete is 'under' the addition, with wood, then small crawl space, then more wood (from the ground up).

Do you guys agree with just going with the standard underlayment?

Personally, I'd still cover the soil under the one crawl space, with 6-mil poly moisture barrier.

The other sounds OK, but a wood moisture meter will be the true verdict, in both cases. Test the subfloors for moisture content.

ok thanks. would the concern be that the moisture from the soil would somehow be transfered to the underside of the plywood, and then into the pergo? The pergo video tells you to do this also, but the only way that moisture could be transfered is either in a flood or thru vapor. And since the crawl space is vented, the air moves around and vaporized liquid would not be in only once space; i.e. shouldn't I cover the entire footprint if vapor is the issue? Otherwise, I'm not sure I understand how the moisture could be transfered from the soil to a surface that is 1-2 feet above it.

Without writing a book on the subject here. I think the www.NOFMA.org site has some good crawl space info.

Your crawl space is going to be cooler then the outside air, that in itself can be detrimental given the right conditions, but add moisture that is always emitting from the earths soil, you have a water factory, when a dew point is reached.

yep, just checked that site. Looks like the barrier is the safe way to go. Thanks for the help !!