Underlayment is a thin foam, felt, or rubber layer installed under some floors to cushion them and smooth out the subfloor. When it gets wet under a floating floor, it can stay damp for a long time.
What Underlayment Does
Underlayment sits between the slab or subfloor and the floor you walk on. Under a floating floor, the boards rest on a thin underlayment or foam padding instead of being glued down. It softens footsteps, quiets noise, and evens out small dips in the subfloor. Some types also include a plastic film meant to slow moisture coming up from concrete.
Think of it like a mattress pad. You don’t see it, but it changes how the floor feels. Carpet has its own version, called carpet pad.
Underlayment vs. Subfloor
People mix these up, but they’re different layers. The subfloor is the structural surface the floor is built on. In most basements, that’s the concrete slab itself. Underlayment is the thin cushioning layer that goes on top of the subfloor and under the finished floor.
The difference matters after a flood. A concrete slab can get wet and be cleaned and dried. A foam or felt underlayment is a separate, soft layer that can hold water against the slab.
Why Wet Underlayment Is a Problem After a Basement Flood
When water gets under a floating floor, the underlayment is right in its path. It’s trapped between the slab and the planks, so air can’t reach it. Wet areas need to be dried within 24 to 48 hours to prevent mold growth, and that’s hard to do with a layer you can’t see or get to.
Mold also likes hidden padding. It can grow on the underside of carpets and pads, and porous materials like these may have to be thrown away once they’re moldy. After a flood, wet underlayment usually gets pulled and replaced along with any carpet pad.
What to Ask About Underlayment in a Basement
- Does this floor need underlayment at all? Coated, stained, or polished concrete has no underlayment, so there’s nothing underneath to stay wet.
- What is it made of? Foam, felt, cork, and rubber handle moisture differently. Ask how the one you’re quoted holds up if it gets wet.
- Does it have a moisture film? A built-in film can help with everyday dampness from the slab, but it won’t stop a flood from running in at the seams and edges.
- Has the slab been checked? Ask how the installer will test the slab for moisture before anything goes down.
If your basement has flooded before, our guide to basement flooring after a flood covers floors that don’t need a padded layer at all.
Underlayment FAQ
What does an underlayment do?
Underlayment cushions the floor, smooths out small flaws in the subfloor, and reduces noise. Some types also add a film to slow moisture coming up from concrete. Under a floating floor, it’s the layer the planks actually rest on.
What’s the difference between underlayment and subfloor?
The subfloor is the structural base, such as a basement’s concrete slab or a wood deck upstairs. Underlayment is a thin cushioning layer on top of the subfloor, under the finished floor. The subfloor stays put; underlayment can be replaced.
What floors need underlayment?
Floating floors like laminate and many click-together vinyl planks are usually installed over underlayment, and carpet goes over a pad. Floors bonded straight to the slab, along with coated or polished concrete, don’t use one.
What happens if you don’t put underlayment under flooring?
For a floor that’s designed to use it, skipping underlayment can make it louder, less comfortable, and more likely to show bumps in the subfloor. It may also affect the warranty. Follow the floor maker’s instructions.