"Underlayment" means two different things in flooring, which causes plenty of confusion. It can mean a thin pad or film laid under a floating floor, or a panel underlayment (like plywood) or leveling compound installed to create a flat, smooth surface. This guide covers mostly the first kind: the pads and barriers under floating floors.
What underlayment does
Depending on the product, an underlayment can:
- Block moisture vapor coming up from concrete.
- Reduce sound, both impact noise to the room below and the hollow "click-clack" in the room itself.
- Smooth over tiny imperfections, although it doesn't fix an uneven subfloor.
- Add a little comfort and warmth underfoot.
- Support the locking joints of click floors, if it's the right density.
Glue-down floors
Glue-down LVP is bonded directly to the prepared subfloor, so it doesn't use a pad. Some commercial systems use specialized sound-control membranes that are also glued, but only as part of the manufacturer's system.
Click LVP and laminate
Attached pads
Many click LVP products come with a pad attached to the bottom of each plank. Most manufacturers say not to add another pad under these. Doubling up makes the floor too soft, letting the planks flex at the joints, which can damage the locking system and void the warranty. Some allow a thin vapor barrier film over concrete even with an attached pad. Check the instructions.
Separate underlayment
Products without an attached pad usually call for a specific type or thickness of underlayment. Rigid core LVP generally needs a thin, dense underlayment; thick, soft foam made for laminate may be too soft. Laminate typically needs an underlayment, and over concrete it needs a vapor barrier.
Vapor barriers over concrete
Concrete slabs release moisture vapor. For floating floors over concrete, manufacturers commonly require a vapor barrier, such as a polyethylene film of a specified thickness with overlapped and taped seams, or an underlayment with a built-in barrier. Don't put a polyethylene vapor barrier over a wood subfloor unless the manufacturer specifically calls for it, because it can trap moisture in the wood. Test concrete moisture first; see moisture testing.
Sound ratings
Two ratings often appear on underlayment and flooring specs:
- IIC (Impact Insulation Class): how well the floor-ceiling assembly reduces impact noise, like footsteps, to the room below. Higher is better.
- STC (Sound Transmission Class): how well the assembly reduces airborne sound, like voices. Higher is better.
These ratings depend on the whole assembly, including the joists, subfloor and ceiling below, not just the pad. Condos and townhomes often have minimum requirements from the association; check your bylaws before choosing a floor.
Choosing the right underlayment
- Read the flooring manufacturer's instructions first; they often specify exactly what's allowed.
- Check whether the product has an attached pad, and don't double up if it does.
- Over concrete, plan for the required vapor barrier.
- In condos and upper floors, check sound requirements.
- Choose a density suited to rigid core click floors if you're using one.
Underlayment won't fix a bad subfloor
A pad isn't a leveling product. A dip in the subfloor will still be a dip with underlayment on top, and the planks will still flex there. Get the subfloor flat first; see subfloor prep.