What Goes Under Vinyl Flooring: A Contractor’s Complete Layering Guide

Skip the wrong layer beneath your planks and you will hear every footstep echo across the room within six months. What goes under vinyl flooring is more important than the planks themselves, because the substrate controls noise, moisture, comfort, and warranty validity. After installing thousands of square feet of LVP across basements, kitchens, and second-story bedrooms, I can tell you most failures trace back to the layer no one sees. This guide walks through every option from bare concrete to premium acoustic pads.

Start With the Subfloor: The Layer Beneath Everything

Before any underlayment, your subfloor must be clean, dry, and flat to within 3/16 inch over a 10-foot span. Plywood and OSB are the most common substrates in residential homes, while concrete dominates basements and slab-on-grade construction. Sweep the surface, vacuum the corners, and check for protruding fasteners with a putty knife.

Use a 6-foot straightedge to spot dips and humps. Fill low spots with a self-leveling compound such as Henry 555 (around $40 per 50-pound bag) and grind down high points with a concrete grinder or belt sander. Skipping this step is the number one reason vinyl planks separate at the seams.

Moisture Barriers: Non-Negotiable Over Concrete

Concrete slabs release water vapor for the life of the structure, and that vapor will warp click-lock vinyl from below. Run a calcium chloride test (about $15 per kit) before installation. If readings exceed 3 pounds per 1,000 square feet over 24 hours, you need a vapor barrier.

A 6-mil polyethylene sheet works for most homes, with seams overlapped 8 inches and taped with vapor-rated tape. For finished basements with chronic moisture, step up to a dimpled membrane like DMX 1-Step ($1.40 per square foot), which also adds a small thermal break.

Attached Underlayment vs. Separate Pad

Most premium LVP and SPC products today ship with an attached IXPE or cork pad on the bottom. Brands like COREtec and Shaw Floorte include 1mm to 1.5mm of cushion that handles minor subfloor imperfections and dampens hollow sound.

If your planks already have an attached pad, do not add another underlayment. Doubling up exceeds the 1.5mm vertical compression most click-lock systems can tolerate, and the joints will fail. The exception is a vapor barrier over concrete, which is still required regardless of attached padding.

Best Standalone Underlayment Options

For unattached vinyl planks under 5mm thick or sheet vinyl, a separate underlayment improves comfort and acoustics. Here are the materials I recommend after testing dozens of brands:

• Cork (3mm to 6mm): Excellent sound dampening, naturally antimicrobial, around $0.80 per square foot. Best for upstairs bedrooms.
• Foam (IXPE or EVA): Inexpensive at $0.30 per square foot, decent cushion, but compresses over time under heavy furniture.
• Felt (recycled fiber): Roberts Super Felt at $0.45 per square foot offers good R-value and works well over plywood.
• Rubber: The best acoustic performance, around $1.50 per square foot, ideal for condos with HOA noise rules.

Plywood Overlay: When You Need It

Old subfloors with deep gaps, knot holes, or board-to-board height variation often need a 1/4-inch luan or 3/8-inch BC plywood overlay before vinyl goes down. Fasten the panels every 6 inches along the edges and 8 inches in the field using ring-shank nails or screws. Stagger the seams and leave a 1/8-inch gap between sheets to allow seasonal movement.

This overlay adds about $1.20 per square foot in materials and labor, but it prevents the telegraphing effect where every flaw in the original subfloor shows through your new vinyl within a year.

Radiant Heat Compatibility

If you have hydronic or electric radiant floor heat, check the underlayment’s tog rating before buying. Most LVP manufacturers cap the surface temperature at 85°F, and a thick cork or rubber pad can trap heat and trigger expansion damage. Use a thin (1mm) IXPE underlayment specifically rated for radiant systems, such as MP Global Quietwalk Plus.

Also confirm the vinyl planks themselves are radiant-compatible. SPC cores handle heat better than older WPC formulations, which can soften and dent at sustained temperatures above 80°F.

Common Mistakes to Avoid

I see the same errors on job site after job site. First, never install vinyl over carpet or carpet pad, even temporarily. The compression is wildly uneven and the planks will flex until the locking edges shear off within the first six months of use.

Second, do not skip the perimeter expansion gap. Vinyl expands and contracts 1/4 inch per 30 linear feet with temperature swings, and a tight wall installation buckles by summer. Finally, never use red rosin paper as a moisture barrier under vinyl; it absorbs water and creates the exact problem you are trying to prevent. Standard kraft paper has the same flaw and should never appear in a vinyl install.

Sound Transmission and Building Codes

If you live in a condo, townhouse, or apartment, your HOA or building code likely specifies a minimum impact insulation class (IIC) and sound transmission class (STC) rating. Most multifamily buildings require IIC 50 or higher to keep footstep noise from disturbing neighbors below.

Standard 1mm IXPE under most LVP yields IIC 50-55 in lab testing. Upgrade to a 5mm cork or 3mm rubber underlayment to hit IIC 65-70 if your building requires it. Document the lab test certificate from the underlayment manufacturer and keep it on file for HOA approval.

Frequently Asked Questions

Do I need underlayment under vinyl plank flooring?

Only if your planks do not have an attached pad. If they do, adding underlayment voids most warranties and causes joint failure. A vapor barrier over concrete is still required either way.

Can I install vinyl over old vinyl?

You can if the existing floor is fully bonded, flat, and free of cushion backing. Sheet vinyl with a thick cushion must come up first because it acts like an oversized underlayment that compresses unevenly.

What is the cheapest underlayment for vinyl flooring?

Standard 6-mil poly sheeting at about $0.10 per square foot serves as a moisture barrier. For cushion, 3mm IXPE foam runs $0.25 to $0.35 per square foot at most home centers.

How long does vinyl flooring last with proper underlayment?

Quality LVP with the correct subfloor prep and underlayment lasts 20 to 25 years residentially. Skipping the underlayment or moisture barrier can cut lifespan in half.