Underlayment Calculator

The Underlayment Calculator matches your flooring type and subfloor condition to the right underlayment product. Picking the wrong one causes noise, moisture damage, or voided flooring warranties. Follow this decision path.

1. Laminate flooring: Use foam underlayment rolls. Standard foam (2–3 mm thick) provides cushion and minor sound dampening. Over concrete subfloors, choose foam with an integrated polyethylene moisture barrier film — this is a single product that serves both functions. Most laminate manufacturers require underlayment and will void the warranty without it. Do not double up layers — two layers of foam create too much cushion, causing the click-lock joints to separate under foot traffic.

2. Luxury vinyl plank (LVP): Use thin foam (1–1.5 mm). LVP is already resilient and does not need the cushion that laminate requires. Thick underlayment under LVP makes the floor feel spongy and can cause plank edges to telegraph through the surface. Some LVP products come with pre-attached underlayment — adding a second layer voids the warranty. Check the box before buying separate underlayment.

3. Engineered hardwood: Cork underlayment provides the best combination of sound absorption, cushion, and thermal insulation. Cork sheets (typically 6 mm thick, 25 sq ft per sheet) are more expensive than foam but deliver higher STC and IIC ratings, which matter for upper floors in multi-storey homes. Foam rolls work as a budget alternative.

4. Ceramic or porcelain tile over plywood: Cement board underlayment is required. Tile cannot be set directly on plywood — the flex causes thinset bond failure and grout cracking. Cement board (¼-inch or 7/16-inch thick) provides the rigid, moisture-stable surface tile needs. Screw cement board to the subfloor every 8 inches with cement board screws, tape the seams with alkali-resistant mesh tape, and thin-set the tape joints.

5. Tile over concrete: A crack isolation membrane replaces cement board. Concrete slabs can develop hairline cracks from settling or curing shrinkage — these cracks telegraph through tile and grout without an isolation layer. Roll-on or sheet membranes decouple the tile from the slab, absorbing crack movement up to 1/8 inch.

6. Sheet vinyl over plywood: Use ¼-inch plywood or hardboard underlayment. Sheet vinyl conforms to every imperfection in the subfloor — seams, screw heads, grain texture — and telegraphs them through the surface. The smooth hardboard layer creates a flat, uniform substrate. Nail or staple every 4 inches along edges and 6 inches in the field.

Underlayment compensates for subfloor imperfections, but it cannot fix fundamental structural problems. Before laying any underlayment, check three things: flatness, moisture, and structural soundness.

Flatness matters because floating floors (laminate, LVP, engineered hardwood) amplify any bump or dip in the subfloor. Industry standard is 3/16 inch of variation over a 10-foot span. Use a straightedge to check — lay it across the floor and look for gaps. High spots get ground down with a floor sander or concrete grinder. Low spots get filled with floor-levelling compound. Foam underlayment absorbs minor imperfections (1/16 inch or less) but cannot bridge a ½-inch dip.

Moisture is the silent floor killer. Concrete slabs emit water vapour continuously, even years after pouring. This moisture migrates upward through the slab and into any floor material that is not protected. Over plywood subfloors, moisture comes from below (crawl space humidity, basement dampness) and from above (spills, pet accidents). For concrete slabs, perform an ASTM F1869 calcium chloride test or ASTM F2170 relative humidity probe test before installing any floor system. If emissions exceed the flooring manufacturer's limit (typically 3 lbs/1000 sq ft/24 hrs for laminate), apply a vapor barrier before the underlayment.

Structural soundness means the subfloor does not bounce, creak, or flex excessively under foot traffic. A plywood subfloor over joists should meet L/360 deflection limits for tile and L/180 for resilient flooring. If the floor bounces when you walk across it, the joists may be undersized, overspanned, or both — adding underlayment on top of a bouncing subfloor does not fix the underlying problem. The floor joist size calculator can help verify whether the existing joists support the intended floor system.

The table below shows underlayment product types, coverage per unit, and approximate costs as of March 2026 (US national averages). All figures assume 5% overage for seams and wall-edge trim.

Product	Packaging	Coverage	Cost / Unit	Best For
Standard foam roll (2-3 mm)	Roll	100 sq ft	$20–$30	Laminate, LVP over wood subfloor
Foam + moisture barrier	Roll	100 sq ft	$28–$38	Laminate, LVP over concrete
Cork sheet (6 mm)	Sheet pack	25 sq ft	$18–$28	Engineered hardwood, sound reduction
Cement board (¼")	3×5 ft sheet	15 sq ft	$10–$14	Tile over plywood
Crack isolation membrane	Roll	50 sq ft	$30–$45	Tile over concrete
Hardboard (¼")	4×8 ft sheet	32 sq ft	$14–$22	Sheet vinyl over plywood

Seam tape adds $6–$10 per roll (180 linear feet). Most rooms under 300 sq ft need only 1 roll of tape. Larger rooms or rooms with many seams (L-shaped layouts, closets) may need 2 rolls.

For tile underlayment (cement board), you also need cement board screws ($8–$12 per box of 200) and alkali-resistant mesh tape ($8–$12 per roll of 150 ft). Standard drywall tape and screws are not suitable — they corrode and fail in the alkaline environment of thinset mortar.

In single-storey homes over crawl spaces, sound performance is a minor consideration — the floor assembly is not transmitting noise to occupied space below. In multi-storey homes, apartments, and condominiums, underlayment sound ratings directly affect livability and may be code-required.

Two ratings define floor sound performance. STC (Sound Transmission Class) measures airborne sound — voices, music, TV — passing through the assembly. IIC (Impact Insulation Class) measures impact sound — footsteps, dropped objects, chair legs — transmitted through the structure. Most building codes require STC 50 and IIC 50 for floor-ceiling assemblies between dwelling units.

Standard foam (2-3 mm) provides STC 58–65 and IIC 58–65 over most subfloors. It meets code minimums for most residential construction and costs the least. Adequate for ground-floor and single-storey applications where impact noise transmission is not a concern.

Premium foam (5-6 mm) improves to STC 65–72 and IIC 65–72. The thicker profile absorbs more impact energy, noticeably reducing the footstep sound transmitted to the floor below. Use premium foam in upper floors of multi-storey homes and anywhere noise reduction matters.

Cork (6 mm) delivers the highest ratings of the common underlayment types: STC 65–73 and IIC 68–76. Cork's cellular structure absorbs both airborne and impact sound more effectively per millimetre than foam. The trade-off is cost — cork underlayment runs 40–60% more than standard foam per square foot. For condominiums with strict HOA sound requirements or upper-floor bedrooms over living spaces, cork often pays for itself in neighbour relations.

Cement board and hardboard provide no meaningful sound improvement — they are rigid and transmit impact noise freely. If you need sound reduction under tile, specify a rubber or cork anti-fracture membrane instead of cement board alone. Some products (e.g., Schluter DITRA-SOUND) combine crack isolation with IIC improvement.

If your drywall installation includes ceiling work below the floor in question, adding resilient channel to the ceiling joists provides another 8–12 points of IIC improvement on top of whatever the underlayment contributes.

Underlayment installation seems simple — roll it out, tape the seams, lay the floor. But several common mistakes void flooring warranties and create problems that are expensive to fix after the flooring is installed.

Overlapping seams instead of butting them. Foam and cork underlayment seams should butt edge-to-edge, not overlap. Overlapping creates a ridge that telegraphs through floating floors as a visible line. Tape the butt seam with the manufacturer's specified tape to prevent the edges from separating.

Skipping the moisture test on concrete. Most flooring warranties require a documented moisture test (ASTM F1869 or F2170) on concrete subfloors. If the flooring fails due to moisture and you cannot produce test results, the warranty claim is denied. The test kit costs $20–$30 and takes 24–72 hours. Cheap insurance.

Using the wrong underlayment thickness. More is not better. Laminate click-lock joints are engineered for a specific deflection range. Too-thick underlayment causes the joints to flex excessively, creating gaps and squeaks. Too-thin underlayment provides inadequate sound and cushion. Follow the flooring manufacturer's specification — it is printed on the box or in the installation guide.

Running underlayment up walls. Underlayment should stop at the wall edge. Running it up the wall creates a ramp that lifts the first row of flooring and prevents the baseboard from sitting flat. Cut the underlayment flush with the wall line, leaving the recommended expansion gap (typically ¼ to ⅜ inch) between the flooring and the wall.

Nailing through underlayment. Foam and cork underlayment should never have nails or staples driven through it. The underlayment must float freely to absorb floor movement. Cement board and hardboard underlayment are different — they must be mechanically fastened to the subfloor to prevent movement.