Underlayment and carpet pad

There is a single word doing three different jobs in most flooring conversations, and it is worth pulling apart before you are standing in the showroom trying to remember which one your salesperson meant. Underlayment can mean a layer already bonded to the plank in your hand. It can mean a roll of foam or felt you choose separately. And it very often gets used, loosely, for something that is not underlayment at all: carpet pad, a different product with its own name and its own rules.

The real decision, and why it is not about which underlayment is best

The question worth asking is not which underlayment performs best in the abstract. It is which of three situations your project actually falls into, because that answer is mostly decided already by the floor you picked and what is currently underneath it, not by browsing options in a display. Which family applies gets confirmed at the same visit covered on our installation process page, alongside everything else about the job. Sort the family out first and the rest of the decision gets a lot smaller.

When it is already decided for you

A growing share of luxury vinyl plank and some laminate lines arrive with a thin pad bonded to the back of every plank at the factory. If that is what you bought, there is nothing separate to add, and adding a second layer under it anyway is the one genuine mistake to avoid, since two cushioned layers stacked on top of each other let the floor flex more than the locking system was built to handle. What an attached pad does and does not include, and the narrow cases where it still needs a moisture film underneath it on a concrete slab, is covered in full on our laminate underlayment page rather than repeated here.

When you are choosing a separate layer

Whether there is a separate layer at all, and what that layer is made of, is decided by how the floor is installed more than by what the floor is made of. Sorting the install method out first saves comparing two products that were never in the same category to begin with.

A floating floor, meaning laminate or a rigid-core plank that locks together and rests loose over the subfloor, is the case that takes a rolled underlayment. Foam gives basic cushioning and a little sound control, a denser felt does more of the sound work under a hard surface, and a film or membrane is what goes down when the concern is vapor coming up out of a slab rather than noise going down through a ceiling.

A nail-down floor is a different specification. Solid hardwood fastened to a wood subfloor traditionally gets asphalt-saturated felt or building paper rolled out first, with the fasteners driven straight through it, which slows vapor without sealing the assembly shut. A glue-down floor gets neither of those, because the adhesive is the layer, and a good many wood and vinyl adhesives are sold as the moisture control as well. Anything compressible between the plank and the substrate defeats the bond the adhesive exists to make, so a foam roll under a glue-down is not a cautious extra step, it is the mistake. The wood-specific version of that split, method by method, is on our vapor barrier page.

Over a concrete slab, moisture is the question that matters most, for the reasons our basement page covers in full: a slab can look and feel dry and still be moving vapor upward every day. On a floating install over a plywood or OSB subfloor upstairs, sound is usually the bigger concern, particularly in a bedroom sitting above a living space, where the underlayment is doing real work to keep footsteps from carrying downstairs.

When pad means something else entirely

Carpet uses a cushion underneath it too, and people call it underlayment out of habit, but it is a different product built for a different job. Carpet is a textile with almost no cushioning of its own, so the pad underneath is what actually absorbs a footstep and protects the backing, and it is chosen by density rather than by how soft it feels under a hand. That whole decision, and why a thicker pad is not automatically a better one, is its own subject, covered fully on our carpet pad page. If you are recarpeting a room and wondering whether the pad already down can stay, that is a separate and almost always disappointing answer, covered on its own page as well.

The one rule that holds across all three

Never stack two cushioned layers, whichever family you are in. An attached pad under a new roll of foam, a felt underlayment under a plank that already has its own pad, or a fresh cushion laid over cushion that is still down from the last carpet: all three create the same problem, a floor with more give than its locking system, its seams or its backing were built to handle. The fix is not adding a layer for insurance. It is reading the spec sheet for the specific product you chose and following it exactly.

What to bring to the showroom

The fastest way through this decision is the flooring sample itself, or the box it came in. Manufacturers print what underlayment a product calls for directly on the packaging or the spec sheet, and that is a better guide than any general rule on this page, because it is written for the exact plank or board in your hand. Bring it in, along with what the new floor is going down onto, slab or plywood, upstairs or on grade, which is exactly the kind of detail settled at the free in-home measure, one stop on the installation process that runs the same way on every job. Call 636-332-0654 or stop by the showroom on Veterans Memorial Pkwy with the sample in hand.