My plank has an attached pad. Do I still need underlayment?

You do not need a second pad, and adding one usually voids the warranty — two layers of cushion let the click joints flex more than they were designed to and the seams work loose. What you may still need is a vapour retarder, which is a different product doing a different job. Check whether the attached pad includes one; many do not.

What happens if I skip the vapour barrier over a slab?

Nothing visible for months, which is what makes it dangerous. Concrete transmits moisture vapour continuously. Sealed under a floor with nowhere to go, it condenses on the underside, and you get mould at the seams, cupped or lifting planks, and a musty smell with no spill to explain it. By then the fix is lifting the floor.

Is a 6 mil plastic sheet good enough?

It is the minimum many manufacturers specify and it works if it is detailed properly — lapped at the seams by the stated amount, taped, and turned up at the perimeter behind the trim. Most failures are not the material, they are an unlapped seam or a sheet that stops at the wall. A purpose-made underlayment with an integral retarder removes most of the chances to get it wrong.

Do I need a barrier on a second floor?

Over a wood subfloor upstairs, no — there is no vapour source below, and a barrier there can actually trap moisture from a spill that would otherwise dry. Vapour retarders belong over concrete, over a crawl space, and on grade. Putting one everywhere out of caution is a real mistake rather than harmless over-engineering.

Climate & Local

Moisture Barrier Underlayment for Vinyl Plank Flooring

September 1, 2026 8 min read By Leander Flooring

Two products, one word

“Underlayment” gets used for two completely different things, and confusing them is behind most of the mistakes on this subject.

A cushion underlayment is a thin foam, felt or rubber layer. Its job is comfort, sound and bridging very minor substrate variation.

A vapour retarder is a film — often 6 mil polyethylene or a purpose-made equivalent. Its job is to stop moisture vapour from a concrete slab reaching the floor above it.

Some products are one. Some are both. Knowing which you have is the whole question.

Why concrete needs one

A concrete slab on grade is in continuous contact with the ground and it transmits moisture vapour upward, permanently. That is normal and it is not a defect.

While the slab is open, that vapour disperses into the room and nobody notices. Put a relatively impermeable floor over it and the vapour has nowhere to go. It accumulates at the underside of the floor, and over months you get condensation, mould at the seams, adhesive breakdown if the floor is glued, and planks that cup or lift.

The characteristic tell is a musty smell in a room with no spill to explain it, six to eighteen months after a new floor went down.

A vapour retarder does not stop the vapour. It slows it enough that it disperses laterally and at the perimeter instead of concentrating under the floor.

When you need one, and when you do not

Over a slab on grade — yes. Every time, unless the plank’s attached pad already includes one.

Over a crawl space — usually yes, and the crawl space’s own ground cover matters as much or more.

Over a wood subfloor upstairs — no. There is no vapour source below. A retarder there can actually trap moisture from an upstairs spill that would otherwise dry downward, which makes a small problem into a floor replacement. This is the most common piece of over-engineering we see and it does harm rather than nothing.

The doubling-up mistake

Most rigid-core vinyl plank now ships with an attached IXPE or cork pad, and the manufacturer’s instructions generally say not to add a second cushion under it.

That is not upselling in reverse — it is structural. Click joints are designed to flex within a tolerance. Two layers of cushion let the floor move more than the joint geometry allows, and the seams work loose and eventually separate. Doing it usually voids the warranty, and the failure is real rather than theoretical.

But — and this is the bit people get wrong — an attached pad is not automatically a vapour retarder. Many are not. If the plank has an attached pad and you are on a slab, you still need to check.

The correct combination in that case is a thin vapour retarder film directly on the slab, with the attached-pad plank floating on top. Film, not another cushion.

Detailing, which is where it fails

Almost every retarder failure is installation rather than material.

Lap the seams by the manufacturer’s stated overlap and tape them. An unlapped seam is a hole.

Turn it up at the perimeter so it goes behind the trim rather than stopping at the wall. Vapour finds the edge.

Detail the penetrations — floor vents, plumbing, the gap around a fireplace hearth.

Do not puncture it. Trim gets fastened to the wall, never through the floor into the slab.

A purpose-made underlayment with an integral retarder removes several of those chances to go wrong, which is usually why the extra cost is worth it.

Why this matters more on this side of town

Around Leander and Cedar Park, up on the Edwards limestone, slabs sit on rock that drains well and vapour drive is often modest.

Out east on the Taylor Marl and Blackland clay — Pflugerville, Hutto, Manor, eastern Round Rock — the ground under the slab holds water. The slab cannot dry downward the way a slab on free-draining limestone does, and it can carry a genuine vapour load through a wet season regardless of how old it is.

That is why the same plank, the same brand and the same instructions produce different outcomes in two houses eighteen miles apart. On the clay side, the retarder is not belt and braces. It is the thing keeping the floor dry.

For a glue-down installation the question goes further still, because there the reading decides the adhesive and possibly requires a full epoxy moisture mitigation coating. A retarder film is for floating floors; a glue-down floor needs the slab tested and treated instead.