Do You Need Underlayment for Vinyl Plank Flooring? A Yes/No Guide by Subfloor

Do you need underlayment for vinyl plank flooring? Usually not a second foam pad, but often a plastic vapor barrier.

If the planks already have a pad attached, as all three SPC vinyl lines we stock do, an extra pad is rarely needed. Over a concrete slab at or below ground level, a 6-mil polyethylene sheet goes underneath. And glued-down planks take no underlayment at all.

Every rule below comes from the installation manuals and warranties published by the makers of the floors we sell, linked at the end. Those documents, not a forum thread, decide whether the warranty holds.

Start here: a three-question decision tree

Answer these in order and stop at the first one that settles it.

1. Will the planks be glued down? Then nothing goes between the plank and the prepared subfloor. In PermShield’s glue-down method the plank is set into wet adhesive and rolled with a 100-lb roller. The same manual calls for glue-down wherever the floor will carry rolling loads (office chairs on casters, carts, wheelchairs) or very heavy objects such as a safe.
2. Is a pad already bonded to the back of the plank? If yes, don’t add a second one unless the manufacturer allows it, and then only inside its thickness limit (next sections).
3. What is under the floor? Concrete on or below grade: lay a 6-mil polyethylene vapor barrier first. Plywood or OSB: no vapor barrier is required by PermShield; flatness and dryness matter more.

The pads already on the SPC planks we stock

Each of our SPC vinyl lines arrives with 1 to 1.5 mm of high-density IXPE foam bonded to the back of every plank, so the cushion and the sound layer are already there. The numbers below are the ones listed on our product pages.

Every SPC line we stock has its pad attached. If you are installing a vinyl plank that has none, use only the underlayment type and maximum thickness named in that plank’s installation guide; the rule differs by brand.

The 3 mm ceiling is PermShield’s; the subtraction in the last column is ours. If you are looking at another brand, the same logic applies: find the attached-pad thickness on the spec sheet, then find the manufacturer’s rule for a second layer. You can compare all of the SPC vinyl plank lines we carry on one page.

Adding a second pad: the 3 mm ceiling and the sound test

PermShield allows a second underlayment under its attached-pad floors in residential installations, provided the two layers together measure 3 mm or less. The same manual says it does not recommend 4 mm or 6 mm cork underneath.

The detail most guides miss is acoustic. PermShield reports that lab testing of impact and airborne sound (IIC under ASTM E492, STC under ASTM E90) on its floors, with and without a second layer, has not shown an added benefit from the extra pad.

If the goal is a quieter floor for the neighbor below, a thicker sandwich is not the lever. In a condo or HOA building with a noise rule, ask the association for the rating it requires, then ask the flooring manufacturer which tested assembly meets it, before you buy the roll.

Why a ceiling at all? The manual pairs it with a strict flatness rule (3/16 inch over 10 feet). A rigid-core click floor is built to sit on a firm, flat base, and a soft, thick layer works against that.

Concrete slabs: the layer you can’t skip is plastic, not foam

Over concrete that is on or below grade, PermShield requires a 6-mil polyethylene vapor barrier under its floating floors, even though the planks carry their own pad. Overlap the sheets at least 4 inches, tape the seams with clear tape, run the film 2 to 3 inches up the wall and trim it after the floor is down.

The foam on the back of the plank is not a moisture barrier, and neither is the plank: PermShield’s warranty states that vinyl plank is not a moisture barrier and excludes damage caused by moisture under the flooring.

That is also why a waterproof plank still needs this sheet; our guide to waterproof vs. water-resistant flooring explains what each label actually covers.

Before the plastic goes down, the slab has to pass a moisture test. PermShield sets the limits at a moisture vapor emission rate of 8 lb per 1,000 sq ft (ASTM F1869) and an internal relative humidity of 90% (ASTM F2170), with a pH between 5 and 9.

It also asks that test locations and results be written down. Keep that page with your receipt.

Plywood, OSB and old tile underneath

Yes, SPC can go straight onto a sound plywood or OSB subfloor. PermShield does not require a vapor barrier on wood; poly is optional there, and if you use it the seams are left untaped.

What it does require is wood moisture content no higher than 12%, a subfloor flat within 3/16 inch over 10 feet, and no bounce or squeaks between joists.

• Existing ceramic or porcelain tile: acceptable with grout joints up to 1/4 inch wide; wider joints get filled with a Portland cement-based patch so they don’t telegraph through.
• Not acceptable underneath: carpet or carpet pad, cushion-backed vinyl, self-adhering plank or tile, laminate or any other floating floor, and wood subfloors laid directly over concrete.
• Need height to meet an adjacent floor? Rigid plywood or OSB under the planks is allowed; a thicker foam is not the way to gain it.

Laminate plays by different rules

If you are weighing vinyl against laminate, don’t carry the rule across. Chesapeake, maker of the two laminate lines we carry (Fusion Design XL and All American Premium, both 10 mm with a 2 mm pre-attached pad), says no additional underlayment pad should be used when the pad is already attached.

Laminate without a pad goes over a single layer of laminate underlayment no thicker than 1.5 mm.

On concrete, Chesapeake requires a 6-mil poly sheet between the laminate and the slab when a concrete moisture meter reads above 4.0, and limits the slab to 3 lb per 1,000 sq ft (ASTM F1869) or 75% relative humidity (ASTM F2170).

Two manufacturers, two different sets of numbers: read the one printed for the floor you are buying.

What decides the cost of what goes under the floor

Because our SPC lines already carry a pad, the extra material is often just a roll of 6-mil poly and tape. What moves the total is everything around it:

• Subfloor type and level: slab on or below grade (barrier required) or wood (barrier optional).
• Slab or subfloor prep: filling low spots and grinding high spots to reach 3/16 inch over 10 feet.
• Moisture results: a slab outside the manufacturer’s limits needs to be addressed before any floor goes down.
• Install method: floating, or glue-down where rolling loads or heavy objects require it.
• Square footage plus waste: PermShield suggests ordering 5% extra, and 10 to 15% in rooms with many angles, plus at least one spare carton for future repairs.

Bring those details to our North Charleston or Richmond showroom (concrete or wood, on or below grade, any moisture readings) and we will help you match them to a line and a quote, in English, Spanish or Portuguese. Start by comparing our SPC vinyl plank flooring options.

Sources: PermShield Rigid Core Floating Luxury Vinyl Flooring Installation Instructions (rev. 09/01/2023); PermShield Luxury Vinyl Flooring Warranty (rev. 09/01/2023); Chesapeake Installation Guidelines for Laminate Flooring (rev. 6/1/2025).