Choosing Underlayment That Stops LVP Noise in Apartments: IIC, STC and What the Code Requires

Developers and owners like vinyl plank in apartments. It is durable, waterproof and easy to clean. The tenants below like it less. Footsteps, dropped objects and dragged chairs on a hard floor travel through the structure into the ceiling underneath. That impact noise is the most common complaint about hard floors in multifamily buildings, and it is a code issue as well as a comfort one. The underlayment is usually the part of the assembly that decides whether the floor passes.

What the code requires

The International Building Code sets minimum sound ratings between dwelling units in Section 1206. An explainer from the acoustical firm Commercial Acoustics summarizes it. Floor-ceiling assemblies separating dwelling or sleeping units, or separating them from public areas, need STC 50 for airborne sound and IIC 50 for impact sound in the lab.¹ In field tests on the finished building, the thresholds are 45.¹ The Los Angeles building department's bulletin on sound-rated assemblies states the same numbers for its local code. It requires an STC rating of 50 for walls and floors and an IIC rating of 50 for floors. It reserves the field rating of 45 for acceptance of individual jobs.²

The section covers corridors, stairwells and other public service areas as well as neighboring units.¹ It is written for residential occupancies, though Commercial Acoustics notes the same targets are often adopted for healthcare, hospitality and education projects.¹

IIC and STC

The two ratings measure different things. IIC, impact insulation class, measures how well a floor-ceiling assembly blocks impact noise, such as footsteps, dropped objects and dragged chairs.⁴ STC, sound transmission class, measures airborne sound, such as voices and television. Roberts' underlayment sheet describes IIC as covering moving furniture and walking, and STC as voices and television.³ For hard floors, IIC is usually the hard one to meet.

The lab tests are named. Commercial Acoustics lists ASTM E492 for lab impact testing and ASTM E1007 for field impact testing.¹ It describes the lab-to-field gap as allowing for flanking paths and workmanship, and says field results often test 2–5 points lower.⁴ For that reason it recommends designing to land about 5 points above the code target.⁴

What the structure does

The floor finish and underlayment are only part of the assembly. Commercial Acoustics' chart puts a bare 6″ concrete slab at about IIC 25–30, and a hard vinyl finish on that slab at IIC 28–32.⁴ Adding an acoustic underlayment under a hard surface on concrete brings typical assemblies to IIC 55–65.⁴ It notes that wood-framed assemblies almost always need a resilient channel ceiling as well as acoustic underlayment.⁴ So the same underlayment can pass on a concrete building and fail on a wood-framed one. Ask what the structure and ceiling are before choosing the product.

Reading an underlayment's ratings

Underlayment sheets list IIC and STC numbers, but those numbers belong to a tested assembly, not the pad alone. Roberts lists IIC 71, STC 65 and Delta IIC 24 for its AcoustiShield under LVT and LVP.³ It describes the delta rating as the measure of sound reduction from a product rather than a floor assembly.³ Delta IIC is the improvement the underlayment adds over the same assembly without it. When comparing products, compare delta ratings, or compare IIC numbers only when the tested assemblies match: the same slab, ceiling and flooring type.

Ask for the test report, not just the sheet. It names the slab thickness, the ceiling, the flooring tested and the lab. An impressive IIC on a thick slab with a suspended ceiling says little about a wood-framed floor with no ceiling treatment.

The flooring maker's rules come first

Underlayment choice is limited by the plank. Many rigid planks have an attached pad, and their makers restrict what can go underneath. Elite says its click end joints are designed to work with its attached pad. It does not warrant the product with an additional pad installed under it.⁵ Marquis likewise bars a second layer under its cushioned planks.⁶ Mohawk's warranty excludes problems caused by non-recommended underlayment.⁷ A soft extra pad can also let the plank flex at the joints; see hollow spots and voids.

If the building needs more acoustic performance than the plank and its pad provide, look elsewhere. Choose a plank whose maker approves a specific acoustic underlayment, or solve it in the ceiling. Do not add a pad the plank's maker has not approved.

Installation details that lose points

Field results fall short of lab results through the details. The Los Angeles bulletin requires an approved permanent, resilient acoustical sealant along the joint between the floor and the separation walls.² It requires penetrations in sound-rated floor-ceiling assemblies to be sealed, lined or insulated.² Commercial Acoustics names unsealed perimeter joints among common field failures, saying a 1/8-inch gap can cost 5 dB.¹ For the floor, that means continuous underlayment with taped seams and no hard contact between the floor and the walls. Trim must not bridge the floor to the wall.

Kitchens and baths in the same unit

Acoustic choices meet moisture choices in kitchens and bathrooms. The Los Angeles bulletin's standard assemblies allow sheet vinyl and linoleum with a resilient backing of 1/8" minimum thickness in kitchens and bathrooms, in place of carpet and pad, where ceilings are on resilient channels.² Many acoustic underlayments also act as vapor retarders, which matters over slabs. See underlayment for kitchens and baths.

A selection checklist

• Confirm the code target and whether the project will be field tested.
• Identify the structure and ceiling; get test data for a matching assembly.
• Compare underlayments by delta IIC, or by IIC on matching assemblies.
• Use only an underlayment the plank maker approves, or none if it has an attached pad.
• Detail the perimeter and penetrations to keep the floor isolated.

Sources

1. Commercial Acoustics, “IBC Section 1206 Sound Transmission Requirements Explained”. commercial-acoustics.com/sound-advice/ibc-1206-sound-transmission-requir
2. Los Angeles Department of Building and Safety, “Sound Rated Assemblies (Information Bulletin P/BC 2023-069)”. dbs.lacity.gov/sites/default/files/efs/forms/pc17/IB-P-BC-2023-069-Sound
3. Roberts Consolidated, “AcoustiShield Underlayment sample sheet”. robertsconsolidated.com/wp-content/uploads/2026/05/70-113_AcoustiShield-
4. Commercial Acoustics, “IIC Rating Chart: How Quiet Is the Floor Above?”. commercial-acoustics.com/sound-advice/iic-rating-chart/
5. Elite Flooring, “Välinge Installation Guide (Woodlands)”. eliteflooringusa.com/links/install-instructions/Elite-Installation-Guide
6. Marquis Industries, “Rigid Vinyl Plank Installation Instructions (Angle)”. marquisind.com/static/resources/Angle_Angle%20Installation%20Instruction
7. Mohawk (via Lowe's), “LVT/LVP Residential Limited Warranty”. pdf.lowes.com/warrantyguides/842374006031_warranty.pdf

General information for the flooring trade. This article was drafted with AI assistance and checked against the sources listed above; the manufacturer's current instructions and the project specification govern any real job. If something here is wrong or out of date, tell us.