Underlayment for Laminate Flooring on Plywood

Skip the underlayment under a laminate floor and you will hear about it every time you cross the room: a hollow, clacking echo that never goes away. Choosing the right underlayment for laminate flooring on plywood is not an upsell; it is what makes the floor quiet, comfortable, and quietly forgiving of a subfloor that is never perfectly flat. Over a plywood subfloor specifically, you get to skip the pricey moisture barriers that concrete demands and focus on cushioning and sound. Here is exactly what to buy and why.

Why Underlayment Matters Over Plywood

Laminate is a floating floor, meaning it is not fastened to the subfloor; it rests on top and expands and contracts as a unit. Underlayment is the thin resilient layer between the laminate and the plywood, and it does three jobs. It smooths out minor subfloor irregularities so the planks do not rock or click at the joints. It cushions each footstep so the floor feels solid rather than hard and hollow. And it dampens sound, both the noise you make walking and the noise that travels to rooms below.

The Good News About Plywood Subfloors

Unlike concrete, a plywood subfloor is a dry, above-grade wood surface, so you usually do not need a vapor barrier. Concrete slabs wick ground moisture and require a 6 mil poly film or an underlayment with a built-in moisture barrier. Plywood over a ventilated crawlspace or a conditioned basement is typically dry enough that a standard foam underlayment is all you need. The exception is a plywood subfloor over an unconditioned crawlspace or any area prone to humidity, where a moisture-barrier underlayment is cheap insurance.

Underlayment Types Compared

For laminate over plywood, three materials cover almost every situation:

• Standard foam (polyethylene): the basic 2 to 3 mm closed-cell foam. Inexpensive at $0.20 to $0.40 per sq ft, it handles cushioning and minor smoothing. Fine for bedrooms and low-traffic rooms over dry plywood.
• Premium/combination foam: 3 mm foam with better density plus an attached film or acoustic layer. At $0.40 to $0.75 per sq ft, it adds real sound reduction and extra durability. The best all-around choice for most homes.
• Cork: 3 to 6 mm natural cork, $0.50 to $1.25 per sq ft. Superb sound absorption and a firm, quality feel underfoot, though it costs more and is less forgiving of moisture.

Felt underlayment exists too and offers excellent sound dampening, but it is denser and pricier; foam and cork cover the vast majority of residential jobs.

Thickness: Why 3 mm Is the Sweet Spot

Underlayment thickness for laminate is measured in millimeters, and 3 mm is the standard for good reason. Thinner 2 mm foam saves money but does less to muffle sound and smooth the subfloor. Going thicker than 3 mm is a mistake many DIYers make: too much cushion lets the floating floor flex, which stresses the click joints and can cause them to separate or fail over time. Manufacturers specify a maximum underlayment thickness, and exceeding it can void the warranty. Stick with 2 to 3 mm unless your product sheet says otherwise.

Reading the IIC and STC Ratings

If noise matters, especially in a second-floor room or a condo, check the acoustic ratings printed on the underlayment:

• IIC (Impact Insulation Class): measures impact noise like footsteps traveling to the room below. Higher is better; look for 60 or above for good performance.
• STC (Sound Transmission Class): measures airborne sound like voices and TV. Again, higher is better, with 60-plus being solid.

A premium foam or cork underlayment with an IIC in the mid-60s makes a noticeable difference underfoot and to the neighbors below. For a ground-floor room with nothing beneath it, these ratings matter far less.

Do You Need Underlayment if the Laminate Has It Attached?

Some laminate planks come with a thin pad pre-attached to the back. If yours does, do not add a second layer of underlayment; doubling up creates excess cushion, over-flexes the joints, and can void the warranty. Attached-pad laminate goes directly onto the plywood (with a moisture barrier only if the location demands one). Reserve separate underlayment for laminate that ships bare-backed.

Installation Tips Over Plywood

Get the subfloor right first. The plywood should be flat within 3/16 inch over a 10-foot span; sand down high spots and fill low spots with a leveling compound. Sweep it spotless. Roll out the underlayment perpendicular to the direction the planks will run, butt the seams without overlapping, and tape them with the manufacturer’s seam tape so nothing shifts during installation. Trim it flush at the walls and leave the laminate’s expansion gap. Then lay your first row and the floor will feel solid and quiet from the first step.

When You Do Need a Moisture Barrier on Plywood

The general rule that plywood does not need a vapor barrier has real exceptions worth knowing. A plywood subfloor over an unconditioned or vented crawlspace can pick up humidity from below, especially in humid climates or during wet seasons, and that moisture can migrate up into the laminate’s wood-fiber core. Same story for a plywood subfloor in a bathroom, kitchen, or laundry area where surface spills are likely. In those cases, choose a combination underlayment with an integrated moisture barrier, or lay a 6 mil polyethylene film under a standard foam. When in doubt, run a simple plastic-sheet test: tape a two-foot square of poly to the plywood overnight and check for condensation underneath in the morning. If it beads up, add the barrier.

Estimating How Much Underlayment to Buy

Underlayment is sold by the roll, typically covering 100 to 200 square feet each, so match the coverage to your room’s square footage and add about 5 percent for trimming and seam overlaps. Buy the seam tape at the same time, since most rolls need their edges taped to stay put during installation. If your laminate already has an attached pad, you buy no separate underlayment at all, only a moisture barrier if the location demands one. Getting the quantity right in one trip saves a mid-project run to the store, which stalls the whole floor.

Signs You Chose the Wrong Underlayment

A mismatched underlayment announces itself after installation. If the floor feels bouncy or spongy underfoot, the cushion is too thick and is over-flexing the joints, a setup for premature seam failure. A hollow, loud clack with each step usually means the underlayment is too thin or too hard for the space, common when a bargain 2 mm foam goes under a floor in an upstairs room. And clicking or separating seams over the first few months often trace back to either excess thickness or an uneven subfloor the underlayment could not compensate for. Matching a 3 mm quality foam or cork to a well-prepped plywood subfloor avoids all three.

Frequently Asked Questions

Do I need a moisture barrier under laminate on plywood?

Usually no. Plywood over a dry, conditioned space does not require a vapor barrier. Add a moisture-barrier underlayment or 6 mil poly film only if the plywood sits over an unconditioned crawlspace, a damp basement, or any humidity-prone area.

What thickness underlayment is best for laminate on plywood?

Stick with 3 mm for the best balance of cushioning, sound reduction, and subfloor smoothing. Avoid going thicker than the manufacturer specifies, because too much cushion over-flexes the click joints and can cause them to separate.

Can I use foam underlayment under laminate?

Yes, closed-cell polyethylene foam is the most common and cost-effective underlayment for laminate over plywood. A premium combination foam with an acoustic layer gives better sound control for a small extra cost.

Do I need underlayment if my laminate has a pad attached?

No. If the laminate has an attached pad, install it directly on the plywood without additional underlayment. Adding a second layer creates too much cushion, stresses the joints, and can void the warranty.

Is cork or foam better under laminate on plywood?

Cork offers superior sound absorption and a firmer, higher-quality feel, making it worth the extra cost in upstairs rooms and condos. Foam is cheaper, easier to install, and perfectly adequate for ground-floor and low-traffic rooms over dry plywood.