Two different problems get mashed into the word “moisture”:
- Vapor drive through concrete or soil — slow, silent, seasonal
- Liquid water — leaks, floods, shower failures
Poly film under a floating living-room floor is mostly about vapor. Kerdi in a shower is about liquid. Treating them as the same SKU is how people waterproof a hallway and still rot a curb.
Vapor drive is the floating-floor villain
Concrete on grade is a moisture battery. Even when the surface feels dry, vapor can move into a floating assembly. Flooring brands publish RH and MVER limits for a reason. Your options usually look like:
- Underlay with an attached vapor film (seams sealed)
- Separate polyethylene film under a breathable underlay
- Brand-specific membranes or ratings you can’t freestyle past
Floating floors: one moisture story, sealed seams
If you run poly, overlap seams the way the sheet says (often 6–8"+) and tape them. Wrinkles become telegraph lines. Tears under a doorway become a moisture highway. Walk gently until the floor is down — film punches are free if you’re careless with tools.
Don’t double-bag without a reason
Two vapor barriers with a sponge layer between them can trap water that got in from above (spills, wet mopping, a plant tray). Integrated underlay + extra poly “just in case” is a common overbuild. Prefer one designed stack.
Vapor-open underlays are not broken — they’re different
Wood fiber and some natural pads advertise breathability. That’s useful when the assembly is designed to dry, and risky when the brand wanted a sealed class under slab. Pair open underlays with the vapor component the flooring manufacturer specifies — or pick a different underlay.
Tile is a different planet
Uncoupling membranes (Ditra and peers) and bonded waterproofing sheets (Kerdi class) are for tile assemblies set in thinset. They are not LVP underlay, not carpet pad, and not a reason to buy peel-and-stick adhesive merchandise if your site policy is pad-and-membrane only. Standard Ditra still wants proper thinset technique; waterproofing wants seams detailed like a roof, not a gift wrap job.
Crawlspaces and liners
Heavy poly liners and dimple mats show up under homes and in damp storage. That’s building envelope work adjacent to flooring — great to understand, easy to under-spec. If you’re encapsulating a crawl, talk to local code and a moisture-aware contractor; Amazon rolls alone don’t replace drainage logic.
When moisture testing is not optional
Big-box impulse installs often skip testing. Commercial and warranty-heavy residential jobs do not. If the flooring brand cites RH (relative humidity in the slab) or MVER (moisture vapor emission rate), those are lab-ish procedures with kits and wait times — not a palm on the concrete. Skipping them to save a day is how you donate a full material package to the dumpster.
Basements that “never had a problem under carpet” can still be hostile under a vapor-impermeable floating floor. Carpet was breathable. Vinyl often is not. The building did not change character because you bought prettier planks.
Seam discipline
Film only works as a system. Overlaps, tape type, and wall turns matter. A perfect field with open seams at doorways is a fake barrier. Same for underlay lip-and-tape systems: if the tape is dusty or cold and never bonds, you have a decorative flap, not a seal.
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Straight answers
Is 6 mil enough?
For many floating floor specs, yes — when seams are sealed and the brand accepts it. Some jobs want thicker film or a rated underlay. Thicker is not automatically smarter if it doesn’t match the system.
Can underlay replace a shower membrane?
No. Foam underlayment is not a wet-area waterproofing system. Showers need products designed and detailed for continuous liquid protection.
Basement LVP always needs poly?
Often yes, but follow the specific flooring brand. Some rigid cores with attached pad still demand a moisture control layer on slab.
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