Underlayment is the thin layer that goes between your subfloor and your laminate planks, and choosing the right one comes down to two questions: what's under the floor, and does the laminate already have a pad attached? Get those right and the underlayment does its three jobs quietly for the life of the floor, managing moisture, softening sound, and giving the planks a smooth, supportive base. Get them wrong and you either trap moisture under the floor or over-cushion it until the seams fail. This guide walks the decision by subfloor and by laminate type, and it flags the two mistakes that send people back to the showroom.
What Does Laminate Underlayment Actually Do?
Laminate underlayment does three jobs, and knowing which one matters most in your room is how you pick it. It slows moisture coming up from the subfloor, which is the whole ballgame over a concrete slab. It deadens sound, both the hollow click of a floating floor underfoot and the noise that travels to the room below. And it gives the planks a flat, slightly forgiving base so the locking joints aren't stressed by small dips in the subfloor. A floating laminate floor isn't glued down, so it leans on that even base more than a nailed or glued floor does. The room decides which job leads: moisture over concrete, sound in a condo, support over an older wood subfloor.
First, Does Your Laminate Already Have a Pad Attached?
Many laminate planks now ship with a foam or cork pad already bonded to the back, and if yours does, you should not add a separate foam underlayment on top of it. Stacking a second pad under an attached-pad plank over-cushions the floor, and that extra bounce flexes the locking joints with every step until the seams loosen or peak into ridges. Most manufacturers' installation instructions say this outright, and it's one of the most common install mistakes our crews get called to fix. There's one exception worth knowing: over concrete, an attached-pad laminate still needs a separate moisture barrier below it, because the attached pad only handles cushioning and sound while the slab's moisture still has to be blocked. Check the back of the plank or the box before you buy any underlayment, because the laminate we carry ranges from bare-backed planks to attached-pad systems and the answer changes what you need.
Underlayment Over Concrete: The Vapor Barrier Comes First
Over a concrete slab, moisture control is the underlayment's first job, because a slab is never truly dry. Concrete gives off water vapor from the ground below for its entire life, and once that vapor collects under a laminate floor it swells the plank cores and makes the floor buckle or lift. The standard defense is a 6-mil polyethylene vapor barrier laid over the slab, with its edges turned 2 to 3 inches up the wall and its seams spliced with a 3-mil tape so moisture can't find a gap. On a floating floor you can use a combination underlayment with the vapor barrier built in, which saves a step, and the one thing you can't do is leave it out. Before we install over any slab, our subfloor prep and installation starts by testing it, and a real test is more than one reading. The industry method is the ASTM F2170 relative-humidity test, and it calls for three probes in the first 1,000 square feet plus one more for every additional 1,000 square feet, each set into the slab to read how wet it is inside. Flooring manufacturers then set the ceiling, commonly around 75% for wood and glue-down floors and 80 to 85% for resilient ones. A slab that reads over the limit gets handled before a plank goes down, because the cheapest time to fix slab moisture is before the floor is on top of it.
Underlayment Over a Wood Subfloor
Over a plywood or OSB (oriented strand board) subfloor upstairs, sound and support matter more than moisture, so a standard foam or felt underlayment usually does the job. A wood subfloor above a heated, dry space doesn't push moisture the way a slab does, so a full vapor barrier is often unnecessary and can even trap the little moisture the wood needs to release. The exceptions are wood subfloors over a crawl space or an unconditioned basement, which behave more like a slab and want a moisture-rated underlayment. Foam adds cushion and a little sound control, felt adds more sound deadening and a firmer feel, and either one smooths the minor unevenness that makes a floating floor click. Match the underlayment to what the room below is doing, and the floor stays quiet and flat.
How Much Sound Does Underlayment Really Block? IIC and STC Explained
Sound is where underlayment earns its keep in a condo or a two-story home, and two ratings tell you how a floor assembly performs. Impact Insulation Class (IIC) measures how well the floor blocks impact sound, the footsteps and dropped-toy thuds that travel to the unit below. Sound Transmission Class (STC) measures airborne sound, the voices and TV noise that pass through. Higher is better on both. The International Building Code sets a minimum IIC of 50 (or 45 when a floor is tested in place) for floors between dwelling units, along with a matching floor for airborne sound, and many condo boards write stricter numbers into their bylaws. This is where a premium acoustic underlayment earns its price: one tested assembly reached an IIC of 71 and an STC of 66 that clear the 50 minimum with room to spare. If you're in a condo or over a finished room, that's the conversation to have first: ask the board or the code what your assembly has to hit, then pick an acoustic underlayment rated to get you there.
Foam, Felt, Cork, or a Combination?
Foam, felt, cork and combination rolls are the four common laminate underlayments, and each one trades off cushion, sound and moisture control. No single type wins outright. The right one matches your subfloor and your room, and the table sorts them out.
Type | Best for | Notes |
|---|---|---|
Foam | Wood subfloors, general use | Cheapest; light cushion and sound; needs a vapor barrier over concrete |
Combination foam with film | Concrete slabs | Foam plus a built-in vapor barrier in one roll |
Felt | Sound-sensitive and upstairs rooms | Denser; better sound and a firmer underfoot feel |
Cork | Condos and quiet rooms | Best sound control; naturally resists mold; costs more |
One Thing Underlayment Can't Fix: The Plank's Own Rating
Underlayment supports a laminate floor, but it can't make a cheap plank tougher, so read the plank's own durability rating separately. The AC number on the box is the plank's surface wear rating from the European EN 13329 abrasion standard, a scale that runs from AC1 up to AC6, and it says nothing about the underlayment below. In North America the plank is also certified by the North American Laminate Flooring Association (NALFA) under its ANSI-accredited LF-01 standard, which tests laminate across 14 categories and sorts it into four use levels from Residential to Heavy Commercial. Buy the plank for its rating and the underlayment for your subfloor, and treat them as two decisions rather than one.
Questions We Get About Laminate Underlayment
Do I always need underlayment under laminate?
You almost always need something under laminate, but not always a separate pad. If the plank has an attached pad, that layer already handles cushioning and sound, and over a dry wood subfloor you may be ready to install. Over concrete, you still need a moisture barrier even with an attached pad. The only floors that skip underlayment entirely are glue-down laminates, which are uncommon in homes.
Can you put laminate directly on concrete?
You can put laminate on concrete, but never straight onto bare slab. A floating laminate floor over concrete needs a 6-mil polyethylene vapor barrier, or a combination underlayment with the barrier built in, between the slab and the planks. Test the slab's moisture first with an ASTM F2170 probe, because a slab that reads too wet will damage the floor no matter how good the barrier is. Skipping either step is how a laminate floor over a basement slab ends up cupped within a year.
What underlayment is best for soundproofing a condo?
For a condo, cork or a premium acoustic underlayment gives you the most sound control, and the real target is whatever your building's code or bylaws call for. Start by asking the association or checking the code for the IIC and STC minimums your floor assembly has to meet, since the building sets the bar rather than the flooring store. Then choose an underlayment rated to reach it and keep the paperwork, because boards often ask for proof before they approve a floor.
Does thicker underlayment mean a better floor?
Thicker underlayment is not automatically better, and past a point it works against you. Too much cushion lets the planks flex and stresses the locking joints, which is exactly why you don't stack a pad under an attached-pad laminate. Match the underlayment's thickness to what the plank's manufacturer specifies, usually a couple of millimeters, and skip the urge to add more for comfort. The right base is firm and even.
Getting the Pairing Right
The right underlayment is the one matched to your subfloor and your plank, so the honest first step is to look at what you actually have: a slab or a wood subfloor, an attached pad or a bare plank, a quiet room or one over a neighbor. Bring those details and we'll tell you exactly what goes under your floor and why, with no upsell on padding you don't need. When you're ready to plan the whole job, request a free estimate and we'll check the subfloor, test for moisture where it matters, and pair the laminate with the underlayment that fits.
