Flooring Underlayment: What It Does and When You Can Skip It

Underlayment sits between the subfloor and the finished flooring and does up to three separate jobs: moisture protection, sound dampening, and minor levelling. Not every flooring type needs all three, and using the wrong underlayment causes problems that show up months later, not on installation day.

The three jobs, separated

By flooring type

• Laminate: almost always needs underlayment unless it is a pre-attached-pad product; provides both sound dampening and slight cushioning.
• Engineered wood floating floor: same as laminate; glued-down installations instead need a moisture-blocking adhesive rather than a separate sheet.
• Vinyl plank (LVP): rigid-core products often include an attached pad already; adding a second layer can void the warranty by making the click-lock joints flex too much.
• Solid hardwood nailed to a wood subfloor: typically uses rosin paper, not foam, mainly to reduce squeaks rather than for cushioning.
• Tile: uses an uncoupling or crack-isolation membrane, not foam, since tile needs a rigid substrate, not cushioning.

When it is genuinely fine to skip it

Rigid-core vinyl plank with a factory-attached pad, installed over a dry, flat subfloor, generally does not need a separate underlayment, and the manufacturer's installation guide should be followed exactly since adding one can breach warranty terms.

Choosing underlayment, step by step

1. Identify the subfloor: concrete slab, plywood, or existing resilient flooring being covered over.
2. Test for moisture on a concrete slab, especially below grade or in a newly poured basement floor.
3. Check the flooring manufacturer's installation instructions for any required or prohibited underlayment types.
4. Match the underlayment to the job's priority — moisture, sound, or minor levelling — using the table above.
5. Calculate area needed the same way as the flooring itself, adding roughly 5-10% for seams and cutting waste.
6. Confirm any building rules on impact sound ratings if installing in a shared-wall or multi-unit building.

• Common mistake: doubling up underlayment under click-lock flooring, which lets the joints flex too much and eventually click apart or gap.
• Common mistake: skipping a moisture test on a concrete slab because the surface feels dry to the touch.
• Common mistake: using foam underlayment under tile, where a rigid substrate is required instead of cushioning.
• Common mistake: ignoring a building's minimum sound rating requirement, which can require the whole floor to be pulled up and redone.

Worked example: costing underlayment for a room

A 12-foot by 15-foot room is 180 square feet. Standard foam underlayment is typically sold in rolls covering 100 square feet, so two rolls covers the area with a small amount left for waste, without needing to round up to a third roll. Adding a moisture-rated product for a slab installation instead of standard foam usually adds a modest per-square-foot premium, which is inexpensive relative to the cost of a flooring failure from ignored moisture.

Sound rating requirements in shared buildings

Rubber and cork underlayments generally test higher for impact sound than standard foam, at a higher material cost per square foot, which is often still worth paying once a building rule requires it.

Condominiums and apartment buildings often specify a minimum Impact Insulation Class (IIC) rating for flooring installed over a shared ceiling, and this is usually stated in the building's rules rather than left to preference. Confirm the required rating with the building management before buying underlayment, since a product that looks similar on the shelf can fall well short of a required rating.

Underlayment cost by type

On a 180 square foot room, the difference between basic foam and a sound-rated rubber product is roughly $100-$200 total — modest next to the cost of pulling up flooring later because a building's noise rules were not met.