Underlayment Guide: What Goes Under Your Flooring and Why It Matters

Updated March 2026 · By the FlooringCalcs Team

Underlayment is the layer between your subfloor and your finished flooring material. It may be invisible once installed, but it dramatically affects how your floor feels, sounds, and performs over time. The right underlayment smooths minor subfloor imperfections, reduces noise, adds cushioning, and provides moisture protection. The wrong choice — or skipping it entirely — leads to hollow sounds, premature wear, and moisture problems that can ruin your flooring investment.

What Underlayment Does and Why You Need It

Underlayment serves four critical functions. First, it smooths minor subfloor irregularities, preventing bumps from telegraphing through thin flooring materials. Second, it provides a moisture barrier that protects floating floors from moisture migrating up through concrete slabs. Third, it dampens sound, both impact noise from walking (heard in rooms below) and ambient noise within the room. Fourth, it adds a layer of thermal insulation and cushioning.

Some flooring types absolutely require underlayment: laminate and vinyl plank floors that "float" without being glued or nailed down need underlayment for proper performance. Hardwood installed over plywood subfloors benefits from a vapor retarder. Tile installed over concrete needs an uncoupling membrane in some conditions. Carpet uses padding rather than traditional underlayment but serves the same functions.

Smooths minor subfloor imperfections (up to 1/8")
Provides moisture barrier (essential over concrete)
Reduces impact and airborne sound transmission
Adds thermal insulation and underfoot cushioning
Required for floating laminate and vinyl plank floors

Pro tip: Never double up underlayment layers. Some vinyl plank and laminate products come with padding pre-attached. Adding a second layer underneath makes the floor too soft and spongy, causing the locking mechanism to break down and joints to separate.

Underlayment Types for Laminate Flooring

Standard foam underlayment (polyethylene) is the most common and affordable option at $0.15 to $0.30 per square foot. It provides basic cushioning and sound reduction and is suitable for laminate installed on dry plywood subfloors on upper levels. It does not include a moisture barrier, so it is not appropriate for concrete slabs.

Foam with attached moisture barrier ($0.25 to $0.60 per square foot) adds a built-in polyethylene film that prevents moisture from reaching the laminate. This is required for any installation over concrete or on ground-level slabs. Premium options include cross-linked foam and combination products that offer superior sound reduction (measured in STC and IIC ratings) for multi-story homes and condominiums.

Standard foam: $0.15-$0.30/sq ft, basic cushion, no moisture barrier
Foam with moisture barrier: $0.25-$0.60/sq ft, concrete slab installations
Cross-linked foam: $0.30-$0.75/sq ft, superior sound and durability
Cork underlayment: $0.50-$1.50/sq ft, excellent sound and thermal properties
Rubber underlayment: $0.75-$2.00/sq ft, best sound reduction

Pro tip: For condos and apartments, check your HOA requirements. Many require a minimum STC (Sound Transmission Class) rating of 50 or higher for floor underlayment. Cork and rubber underlayments typically meet these requirements while standard foam may not.

Underlayment for Vinyl Plank and LVP

Vinyl plank flooring has specific underlayment requirements that differ from laminate. Many LVP and SPC (stone polymer composite) products come with padding already attached to the bottom of each plank. If your vinyl plank has pre-attached padding, do not add additional underlayment underneath. This is a common mistake that leads to a floor that feels unstable and develops joint failures.

If your vinyl plank does not have attached padding, use a thin underlayment specifically designed for vinyl: 1 to 2 millimeters of cork or LVP-specific foam. Do not use standard laminate underlayment, as it is often too thick and spongy for vinyl plank. Over concrete, ensure the underlayment includes a moisture barrier, or lay a separate 6-mil polyethylene sheet.

If LVP has attached pad: NO additional underlayment
If LVP has no pad: use 1-2mm cork or vinyl-specific foam
Do NOT use standard thick laminate underlayment under LVP
Over concrete: moisture barrier required
Check manufacturer specifications: wrong underlayment can void warranty

Pro tip: Read your vinyl plank manufacturer warranty carefully. Many manufacturers void the warranty if the wrong underlayment is used. The manufacturer recommended underlayment or their own branded product is always the safest choice.

Moisture Barriers and Vapor Retarders

Any flooring installed over a concrete slab, whether on or below grade, needs a moisture barrier. Concrete is porous and continuously transmits moisture vapor upward. Without a barrier, this moisture can cause laminate to swell, hardwood to cup, and mold to grow beneath the flooring.

The simplest moisture barrier is a 6-mil polyethylene sheet laid over the concrete with seams overlapped by 6 inches and taped. Many combination underlayments include a built-in moisture barrier, eliminating the need for a separate sheet. For serious moisture concerns, perform a calcium chloride test or relative humidity test on the slab. If moisture levels exceed the flooring manufacturer acceptable range, address the moisture issue before installing any flooring.

6-mil polyethylene sheet: $0.05-$0.15/sq ft
Overlap seams by 6 inches and tape with moisture-barrier tape
Required over all concrete slabs, especially below-grade
Test slab moisture before installation
Calcium chloride test: should be under 3 lbs per 1,000 sq ft per 24 hours

Pro tip: Perform a simple moisture test before installing over concrete. Tape a 2x2 foot piece of plastic sheeting to the slab and check after 24-48 hours. If moisture droplets appear under the plastic, the slab has excessive moisture that needs to be addressed first.

Installation Tips and Common Mistakes

Always run underlayment in the same direction as your flooring planks. Butt edges together without overlapping (unless instructed otherwise by the manufacturer) and tape seams to prevent shifting. Leave a gap at walls consistent with your flooring expansion gap, typically 1/4 to 3/8 inch. Trim excess underlayment at walls after the flooring is installed.

Common mistakes include using too thick an underlayment, which causes a soft, unstable surface that damages floating floor joints. Another error is using the wrong type of underlayment for the flooring: cork under laminate is fine, but thick rubber under thin vinyl plank is problematic. Failing to tape seams allows moisture to pass through and underlayment sheets to shift over time.

Run underlayment in same direction as flooring planks
Butt edges together, do not overlap (unless specified)
Tape all seams with underlayment tape
Leave expansion gap at walls (1/4"-3/8")
Never use more thickness than manufacturer recommends

Pro tip: Buy the underlayment recommended by your flooring manufacturer. Using a different product can void your warranty and may cause performance issues. The cost difference between recommended and generic underlayment is minimal compared to the risk.

Frequently Asked Questions

Do I need underlayment for vinyl plank flooring?

It depends. If your LVP has pre-attached padding (check the bottom of a plank), do NOT add underlayment. If it has no attached pad, use a thin 1-2mm cork or vinyl-specific foam. Using the wrong underlayment can void your warranty and damage the floor.

What is the best underlayment for laminate flooring?

Over plywood subfloors: standard foam or cross-linked foam ($0.15-$0.75/sq ft). Over concrete: foam with attached moisture barrier ($0.25-$0.60/sq ft). For sound reduction in condos: cork ($0.50-$1.50/sq ft) or rubber ($0.75-$2/sq ft). Check your manufacturer recommendation for warranty compliance.

Can I use carpet padding under laminate?

No. Carpet padding is too thick and soft for laminate flooring. It would cause the laminate planks to flex excessively, breaking the locking mechanism and causing gaps between planks. Always use underlayment specifically designed for hard-surface floating floors.

How much does underlayment cost for a room?

For a 200-square-foot room, standard foam underlayment costs $30-$60. Foam with moisture barrier costs $50-$120. Premium cork underlayment costs $100-$300. Rubber underlayment costs $150-$400. These costs are minor compared to the flooring itself but significantly impact performance.

Do I need a moisture barrier on the second floor?

Over a plywood subfloor on an upper floor, a moisture barrier is generally not required unless you are installing in a bathroom or laundry room. Standard foam without a moisture barrier is typically sufficient for dry, above-grade locations. Always check your specific flooring manufacturer requirements.