Floor Underlayment Calculator

Most floating floors need it; most glue-down and nail-down floors do not. Check the plank edge first — if the spec says IXPE, EVA, or "pad attached," the underlayment is already there, and adding a second layer voids the warranty on nearly every click-lock product. Solid hardwood nailed to a plywood subfloor uses 15 lb felt or rosin paper, not foam. Unpadded laminate, LVP, or SPC over plywood wants a 1 to 2 mm foam or fiber pad; over concrete it wants that pad plus a vapor retarder. Tile is different again — it needs an uncoupling or crack-isolation membrane.

Concrete releases moisture for its entire life, including slabs poured decades ago. Over any slab, on grade or below grade, use an underlayment with a built-in vapor retarder or lay 6 mil polyethylene first, lapping seams 6 to 8 inches and taping them. If you want numbers before committing, an ASTM F1869 calcium chloride test should read at or below about 3 lb per 1,000 ft² per 24 hours, or an ASTM F2170 in-slab relative humidity probe at or below 75 to 80 percent — whichever your flooring manufacturer specifies. Skipping this step is the most common cause of cupping, adhesive failure, and mold under a new floor.

Many condo and HOA bylaws set a minimum impact insulation class (IIC) and sound transmission class (STC) for hard flooring above another unit. IIC 50 and STC 50 are the common lab-tested minimums, and stricter buildings ask for IIC 55 or a field-tested FIIC. Ordinary 2 mm foam does very little; rated acoustic underlayments are typically 2 to 5 mm of cork, rubber, or dense fiber and cost two to four times as much. Ratings describe a whole assembly — underlayment, floor, subfloor, and the ceiling below — so the number on the package only holds for the tested build-up. Get your board requirement in writing before ordering.

No — stacking pads is the fastest way to wreck a floating floor. Click-lock joints are engineered over a firm base, and the extra deflection flexes the locking profile until tongues shear and seams peak or gap. Most laminate and LVP manufacturers cap total underlayment at roughly 2 to 3 mm and specifically exclude damage from doubled pads. If your plank already has pad attached, lay it straight onto the subfloor over a vapor retarder if the slab calls for one. Fix an uneven subfloor with leveling compound instead — the usual tolerance is 3/16 inch over 10 feet — not with extra foam.

Rolls are typically 3.3 feet (1 metre) wide, so a room takes one course every 3.3 feet and one taped seam per course. That is why the tape figure here is simply your coverage divided by 3.3 — a 400 ft² room with 10 percent waste needs about 133 feet. Tape the seams whenever the underlayment doubles as your vapor retarder over concrete; over a dry wood subfloor, butt-and-tape is good practice but not always required. Tape sells in 180 to 200 foot rolls, and rooms with many jogs, closets, and obstacles will use somewhat more than the math shows.