Attached underlayment

The question underneath this one is usually simpler than it sounds: do you buy a plank that already has the cushion stuck to it, or do you buy a bare plank and a roll of something to go under it. It gets asked as a value question and answered as a value question, when the thing that should decide it is what your subfloor is made of and what is on the other side of it.

What that layer is there to do

A laminate floor floats, which means it is one large panel resting on the house rather than fastened to it, a point the laminate category page sets out in full. Whatever sits between that panel and the subfloor is doing up to four separate jobs, and they are worth separating because a product can do one and not the others.

• Sound. It absorbs the hollow click a hard floating panel makes and keeps some of it from carrying into the room below.
• Comfort. A little give under the panel takes the harshness out of footfall without making the floor feel soft.
• Moisture. Some products include a film that slows water vapor rising out of a slab. Many do not, and this is the job people wrongly assume is included.
• Minor surface forgiveness. A pad bridges the grain and the small texture of a subfloor. It does not correct a hump or a dip, and no pad ever has.

The three configurations you will actually be offered

First, a bare plank with a separate roll chosen to suit the room. Foam, felt or rubber, with or without an integral moisture film, and the choice is genuinely yours. Second, a plank with a cushion pad bonded to the back and nothing else, which covers sound and comfort only. Third, a plank with a bonded pad that also carries a moisture film, sold as a combined product. All three are legitimate. Only one of them is right in a given room, and the label on the box will not always make clear which one you are holding.

The practical appeal of a bonded pad is real. It removes a step from the install, it cannot shift or bunch as planks are laid over it, and it never ends up short of a doorway. The practical cost is that you have surrendered the choice. If the room needs a specific acoustic performance or a specific moisture strategy, you are stuck with whatever the factory bonded on.

If you go the separate route, there is a second choice waiting behind it. Foam is the common default, light and easy to work with, and the quality range inside it is wide. Felt is denser, tends to do more for impact sound, and sits flatter under a heavy panel. Rubber is the dense end, usually specified where sound is the driving requirement rather than comfort. None of them is the correct answer in the abstract. The room decides, the same way it decides everything else on a flooring job, and a store that hands you the same roll regardless of what is under the floor is not really answering the question.

The rule that settles most arguments: never stack two

If the plank arrives with a pad on the back, that is the pad. You do not roll a second one underneath it because the first seems thin, and this is the single most common way a well chosen laminate floor gets ruined by a well meaning install.

The reason is mechanical. A locking profile is milled into the edge of the plank and it holds by geometry, tongue into groove, under the assumption that the panel is supported on something close to firm. Give the panel too much cushion and it flexes at every step. Every flex works the joint slightly, and a walking lane repeats it endlessly until the joint opens, which is the beginning of a floor lifting at the seams. Manufacturers state a maximum for what may go under their product, and doubling up is usually outside it, which puts the warranty outside it too.

The conditions that pick between them

• Concrete. A slab moves vapor upward every day, dry or not, which is the argument the basement page makes at length. Over concrete you need a moisture retarder in the assembly, so either the bonded pad includes a film and says so, or you are laying a separate film first. This is not optional and it is not a regional quirk.
• A room over a finished space. If someone is sitting underneath, acoustic performance matters, and a separate underlayment lets you choose one rated for impact and airborne sound. Bonded pads vary widely here and rarely publish a rating.
• Radiant heat. A pad is insulation, and insulation is the opposite of what a heated floor wants. Thermal resistance becomes the deciding specification, which the radiant heat entry covers.
• A subfloor that is not flat. Neither answer helps. Flatness is fixed before anything is rolled out, with a leveling compound or by addressing the deck, and what it takes is assessed at the measure rather than guessed at. How that lands against everything else on a job is set out on the cost page.
• What the manufacturer permits. Their warranty language names what may go under the floor. Read it at the counter rather than after.

What to look at on the sample and the sheet

This is one you can settle with the board in your hand, which is the reason to bring one home rather than judge from a photograph. Turn the sample over. A bonded pad is obvious the moment you see the back, usually a gray or white foam skin, and you can press it with a thumbnail to feel how much give it has. Then find the spec sheet and look for the word vapor or moisture in the description of that pad, because a cushion pad and a cushion pad with a film look identical from the back and behave entirely differently over a slab.

Bring three things when you come in: the sample if you already have one, a note of what the subfloor is, and whether there is finished space below the room. That is enough to close this question at the counter. Vinyl planks handle the same problem differently, and the attached pad entry on the vinyl side is worth a look if you are weighing both categories. If you want it settled properly, call 636-332-0654 and we will look at the back of the boards with you.