How to Calculate Floor Underlayment
Underlayment is the thin layer between your subfloor and finish flooring. It absorbs minor imperfections in the subfloor, reduces impact and airborne noise, blocks moisture from concrete substrates, and adds comfort underfoot. Choosing the right type - and the right quantity - depends on your finish floor material, your subfloor, and whether sound reduction is a priority.
Roll Coverage Formula
For roll underlayment (foam, cork, felt), divide the area by the coverage per roll:
Rolls=⌈Area×(1 + Waste)Roll W×Roll L⌉
Sheet Coverage Formula
For sheet products like cement backer board, divide area by the sheet area and round up:
Sheets=⌈AreaSheet W×Sheet L⌉
Underlayment Types Comparison
| Type | Best For | Coverage / roll | Sound (IIC) |
|---|---|---|---|
| Foam (2-3 mm) | Laminate, LVP without attached pad | 100 sq ft | 50-55 |
| Cork (3-6 mm) | Sound reduction, high-rise condos | 100-200 sq ft | 60-70 |
| Felt / Rubber (3 mm) | Engineered hardwood, premium LVP | 100 sq ft | 55-65 |
| Vapor Barrier (6 mil) | Concrete subfloor moisture block | 500+ sq ft | - |
| Cement Backer (1/4 - 1/2 in) | Tile floors over wood subfloor | 15-32 sq ft / sheet | - |
Sound Ratings Explained
Condo and apartment HOAs often require minimum sound ratings before allowing hard surface floors. Two ratings matter:
- IIC (Impact Insulation Class) - blocks footfall noise; code minimum 50, HOA often 60+
- STC (Sound Transmission Class) - blocks airborne sound (TVs, voices); code minimum 50
- Delta IIC - the specific improvement from underlayment alone; 20+ is premium
Concrete Subfloor Rule
Concrete continuously releases moisture vapor, even years after curing. Always use underlayment with a built-in moisture barrier, or install 6 mil polyethylene sheeting before laying foam underlayment on any concrete subfloor. Overlap seams 8 inches and tape them to form a continuous barrier.Installation by Floor Type
| Finish Floor | Recommended Underlayment |
|---|---|
| Laminate | 2-3 mm foam (with vapor barrier on concrete) |
| LVP / LVT rigid core | Usually attached - skip additional; vapor barrier only |
| Engineered hardwood (floating) | 3 mm felt or cork |
| Solid hardwood (nail-down) | 15 lb roofing felt or rosin paper |
| Ceramic / porcelain tile | 1/4 in cement backer board on wood subfloor |
| Carpet | Dedicated carpet padding (not foam underlayment) |
Installation Tips
- Clean subfloor - sweep then vacuum before unrolling
- Unroll perpendicular to the direction of finish flooring planks
- Butt seams tight - no gaps, no overlaps (unless vapor-barrier flaps)
- Tape seams with manufacturer-specified underlayment tape
- Trim at walls - leave 1/4 inch gap for expansion
- Do not install over damp subfloor - use a moisture meter to verify under 12% MC for wood or under 4 lb/1000 sq ft per 24 hr for concrete
Do Not Stack Underlayment
Most modern rigid-core LVP and laminate has attached underlayment. Adding extra underlayment creates excessive flex that stresses click joints. Check your floor manufacturer spec - using extra underlayment can void the warranty.Cost Estimate (US, 2026)
Standard foam: $0.25 - $0.50 per sq ft. Vapor barrier foam: $0.50 - $0.80 per sq ft. Cork: $0.80 - $1.50 per sq ft. Premium sound reduction: $1.20 - $2.50 per sq ft. Cement backer board: $0.35 - $0.60 per sq ft. A typical 200 sq ft room costs $50 - $300 in underlayment materials.