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Flooring Design Dilemma Basements

Vapor barrier is UNDER concrete; do I still need one on top for LVP?

5 years ago

Hello all,

I've searched the forum and web, but can't get a definitive answer. Hoping others can chime in.

Our contractors are going to install CoreTec Plus vinyl plank flooring in our walk-out basement next week. The basement has always been dry (we don't even have a sump pump). There also is a vapor barrier under the concrete slab. I keep reading that a 6 mil vapor barrier is recommended/required when LVP is installed on below-grade slabs. But what I can't determine, is if that barrier is still necessary when a vapor barrier is already present under the slab?

I asked U.S. Floors about it on the extensive CoreTec thread; but they only reiterated the statement that a 6 mill barrier is required to lay flooring on below-grade slabs. We have the barrier - it's just under the slab.

Any feedback is appreciated!

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Comments (3)

chiflipper
5 years ago
Contact the manufacturer directly @ 1-844-873-8123 and ask for tech support. Request that the answer be confirmed in an email.
SJ McCarthy
5 years ago
It is a requirement. You still need it and here is the reason why:

We ALWAYS ASSUME that the "underslab" vapour barrier HAS BEEN or WILL BE compromised at SOME POINT in its life. We don't know when, we don't know how badly, but it will happen. Heck, it can happen WHILE they are POURING the concrete. Or when they are spreading concrete. Or when tree roots grow underneath the concrete, etc.

So, the moral of the story is: always use vapour barrier (6mil poly sheeting = $0.11/sf at HD) over slab on or below grade when installing rigid flooring.

It is the CHEAPEST FORM of "warranty saver" you will ever have. I promise you, that vapour barrier will come in REALLY handy at some point in the house's life span.
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R H
Original Author
5 years ago
SJ, thank you for explaining the "why" behind your answer. That actually makes sense!

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