Do you need a moisture barrier under LVP?

Question

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.

Ryan Normand · Answer

Quick update -

I finished the job. The instructions on the box did say to install a 6 mil polyethylene moisture barrier when applying over slab-on-grade (oddly enough the online instructions didn't mention this, which is why I was so confused in the first place).

Long story short, I installed a 6 mil poly barrier in all areas - both over the wooden and concrete subfloors. It might've been overkill, but the crawlspace did not have any sort of vapor barrier (yikes) so I figured I should play it safe and protect the flooring. It was a royal PITA but we got it done. Crumbs and dust kept getting trapped under the poly which was super annoying when we went to lay down the planks. I also had to redo part of the large bonus room because the poly would not lay flat and tended to get more and more wrinkled the further I got in the room. If anyone has any tips for making this easier in the future, let me know!

Bruce Woodruff · Answer

Better safe than sorry is my thinking. What about rolling on some DragonSkin instead of using Visqueen? I think that would make a better surface for the install.....

Bruce Woodruff · Answer

Yes. I think that would give a nice tacky surface, make the flooring solid and less likely to slide. A lot of tile guys use Dragonskin on the hardiboard in addition to a vapor barrier....

Bruce Woodruff · Answer

I just don't like the look of vinyl....it looks like...well, vinyl. I only ever installed it in an apartment building I was redoing.

Yeah that engineered wood on slab can be a beeatch. I remember the glue as well, but there are tricks to stay out in front of it. And laying it on second floor over plywood is easy using a toe nailer. It looks great and is water 'resistant' at least.....

Bill Brown · Answer

I think it depends on the conditions of the house as to whether or not you should put a VB under the flooring. If moisture can permeate up into the flooring, then probably yes. If a concrete slab already has moisture control under it, then you shouldn't need another layer under the flooring. I would say it seems like a good idea to put it over a crawl space without any moisture control. Sandwiching materials between moisture barriers would be a bad idea generally. And like @Mike Dmuchoski said, the manufacturer has the final word if you want protection with their warranty.I've had different experiences with scratch resistance from different manufacturers. The stuff we used in our last project (Lifeproof) seems pretty nice so far. The finishes seem to vary quite a bit between brands. We bought some Lumber Liquidators flooring for the same project and sent it back because it felt like fine sandpaper.

The worst thing to me is subfloor prep. If you're following manufacturer specs, you really need to get it FLAT. That can be a lot of work/ material/ time.

Ryan Normand · Answer

@Bruce Woodruff I hope you don't mean glue-down engineered wood :) That stuff is a sticky mess to install. I've done it once so far and came home covered in flooring adhesive every day (maybe that's my fault for wearing shorts and short sleeves haha)

But floating wood/laminate is starting to grow on me. I used to be a huge fan of LVP because of its waterproof qualities and low thermal expansion. I still think it was a good choice for this particular job since the client wanted it in the kitchen and bathrooms (needed to be waterproof) and the crawlspace was uninsulated (so I wanted something with low thermal expansion/contraction)... BUT...

I'm starting to realize LVP has its weaknesses. I personally like the look of the light gray-ish/brown LVPs, but I know a lot of people don't. The biggest issue I have with it is scratch-resistance. It scratches super easily if you're not careful with furniture and appliances. I'll definitely think twice before using it again.

Bruce Woodruff · Answer

Do not buy LVP again! :-)

Seriously, I greatly prefer engineered wood. Much more beautiful, easier to lay, more forgiving on uneven floors, better resale value, etc....

Mike Dmuchoski · Answer

I'd say default to the manufacturer's install guidelines.  They're typically conservative so that should give you peace of mind.  IMO - if there's a vapor barrier in the crawl space, that should be sufficient but "better safe than sorry" is always a good, safe approach.

Ryan Normand · Answer

@Bruce Woodruff Using a liquid applied sealer crossed my mind too. I've never heard of DragonSkin. Is that similar to Redgard and AquaDefense?

Do you need a moisture barrier under LVP?

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