The single fastest way to ruin a $3,000 LVP installation is to put the wrong thing underneath it. What do you put under vinyl flooring depends on three factors most homeowners never check: subfloor material, moisture conditions, and whether your planks already have an attached pad. After 15 years of installs across slab basements and second-story condos, I have learned that the layer beneath the floor matters more than the planks themselves. This guide breaks down every option so you choose right the first time.
Read Your Plank Specs First
Before buying any underlayment, flip a plank over and inspect the bottom. If you see a gray, beige, or cork-colored pad already attached, that is your underlayment. Most premium SPC and LVP products from Shaw, COREtec, Mohawk, and LifeProof include 1-1.5mm of IXPE foam or cork bonded to the core during manufacturing.
Adding a second underlayment under planks that already have one is the most common warranty-voiding mistake I see. The combined cushion exceeds what the click-lock joints can handle, and the seams pop apart within a few months under normal foot traffic.
Over Concrete: Always Start With a Vapor Barrier
Concrete slabs continuously release moisture vapor, even ten years after the pour. That vapor will lift, warp, and discolor vinyl planks from below. Test the slab with a calcium chloride kit (about $15 at any flooring supplier) and check the result against your plank manufacturer’s spec.
For most installs, a 6-mil polyethylene sheet does the job. Lay it across the entire floor, overlap seams by 8 inches, and tape them with vapor-rated tape. For chronically damp basements or below-grade installs, upgrade to a dimpled membrane like DMX 1-Step at around $1.40 per square foot, which adds an air gap and lets minor moisture migrate to a perimeter drain.
Over Plywood or OSB Subfloor
Wood subfloors do not need a vapor barrier in most US climates. If your planks have an attached pad, you can install directly on a clean, flat plywood surface. If your planks are bare on the bottom (typical of cheaper LVP under 5mm thick), add a thin underlayment for sound and minor leveling.
I prefer 3mm cork or 2mm IXPE foam over wood subfloors. Cork at $0.80 per square foot offers excellent acoustic dampening, while IXPE at $0.35 per square foot is the budget choice. Both bridge subfloor imperfections up to 3/16 inch over 10 feet.
The Best Underlayment Options Compared
Not every underlayment performs the same. Here is what I have used and how each one stacks up in real homes:
- Cork (3-6mm): Best acoustic dampening (IIC 51+), antimicrobial, $0.70-$1.20 per sq ft.
- IXPE foam (1.5-2mm): Lightweight, moisture-resistant, $0.25-$0.45 per sq ft.
- Felt (recycled fiber): Decent R-value, good cushion, $0.40-$0.55 per sq ft.
- Rubber (3-5mm): Top-tier sound and impact protection, $1.40-$2.00 per sq ft.
- Combo (foam + vapor barrier): All-in-one for concrete, $0.50-$0.80 per sq ft.
Subfloor Prep Before Anything Goes Down
No underlayment fixes a bad subfloor. Sweep, vacuum, and scrape away any drywall mud, paint splatter, or old adhesive. Pull or set every protruding nail. Run a 6-foot straightedge across the floor and mark dips deeper than 3/16 inch.
Fill low spots with self-leveling compound such as Henry 555 ($35-$45 per 50-pound bag, covers about 25 square feet at 1/8 inch). Grind down high spots with a belt sander or concrete grinder. This prep stage takes a full day on a typical 400-square-foot room but adds 10+ years to your floor’s life.
Bathrooms and Wet Areas
For bathrooms, laundry rooms, and mudrooms, skip cushion underlayment entirely and use a waterproof barrier instead. Schluter DITRA at $4 per square foot is overkill for vinyl, but a basic 6-mil poly sheet under attached-pad planks handles incidental splashes from showers and sinks.
Run a bead of 100% silicone caulk around the perimeter where the plank meets the wall. This stops water from migrating under the floor during accidental overflows.
Radiant Heated Floors
If you have electric or hydronic radiant heat, your underlayment must allow heat to transfer up through the floor. Use a thin (1-1.5mm) IXPE foam rated for radiant systems, such as MP Global Quietwalk Plus. Avoid cork over 3mm and any rubber underlayment, both of which trap heat and can cause vinyl to soften or expand.
Confirm the planks themselves are radiant-rated and never let surface temperature exceed 85°F. Most newer SPC cores handle radiant heat better than older WPC products.
Common Mistakes I See on Job Sites
Three mistakes show up again and again. First, installing over residual carpet padding or thick old vinyl, which creates uneven compression and eventual joint failure. Second, skipping the expansion gap at walls (you need 1/4 inch around the perimeter), which causes buckling in summer humidity. Third, using red rosin paper as a vapor barrier; it absorbs moisture and creates the exact problem you wanted to prevent.
Frequently Asked Questions
Do you need underlayment under vinyl plank flooring?
Not if your planks have an attached pad. Most premium LVP includes 1-1.5mm of IXPE or cork on the bottom. A vapor barrier over concrete is still required either way.
Can you put vinyl directly on concrete?
Yes, with a vapor barrier. Concrete must be tested for moisture, fully cured (60+ days), and flat to within 3/16 inch over 10 feet. Skip those checks and the floor will fail.
What is the best underlayment for vinyl plank flooring?
For most homes, 3mm cork over plywood and a combo foam-plus-vapor-barrier over concrete. Both balance cost, comfort, and acoustic performance.
Can I use carpet padding under vinyl plank flooring?
No. Carpet pad compresses unevenly under hard planks, causing joint separation and warranty voids. Use only underlayment specifically rated for LVP.
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