Do you need a moisture barrier under LVP?

[Ryan Normand]

I've got a flooring job coming up in about a week in Flagstaff, AZ. It's about 1600 sf of waterproof LVP with pre-attached underlayment. The subfloor is mostly wood (with a cawl space below it) with the exception of a couple rooms that are slab-on-grade. Other contractors - What are your thoughts on putting a moisture barrier between the subfloor and the LVP? It seems redundant to me, but I've read some forums where people recommend doing it. If so, would you just use 6 mil Visqueen?

I've done a number of smaller flooring jobs here in Phoenix and always put LVP (with pre-attached underlayment) directly over the subfloor. It's never been an issue, but then again we have a very dry climate here. I'm not sure if the climate up in Flagstaff should change the installation technique.