STC and IIC are two different sound scores. STC (Sound Transmission Class) rates how well a wall or floor assembly blocks airborne noise such as voices, TV and music. IIC (Impact Insulation Class) rates how well a floor assembly blocks impact noise such as footsteps and dropped objects from reaching the room below. For flooring, IIC is usually the number that decides whether your downstairs neighbor, or your own family, hears you walking. Neither number belongs to the planks alone. Both describe a whole assembly.

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TL;DR

  • STC = airborne sound (voices, music). IIC = impact sound (footsteps, chairs, dropped items). Higher is better on both.
  • Building codes ask for a minimum of 50 in the lab, and 45 when tested in the field. Many owners and condo boards ask for much more.
  • The rating belongs to the full assembly: subfloor, underlayment, adhesive or fasteners, and the ceiling below. A flooring box alone cannot promise an IIC number.
  • Lab results usually fall in the field. Plan a margin, and ask for the full assembly test report, not just the underlayment’s marketing figure.
  • Delta IIC (ΔIIC) is the fair way to compare underlayments with each other.

Table of Contents

STC and IIC side by side

Intertek’s acoustics team describes STC as a single-number rating of how well an interior building element reduces airborne noise from one side to the other. IIC is its impact counterpart. A table keeps them straight:

STC IIC ΔIIC
Sound type Airborne: voices, TV, music Impact: footsteps, furniture, dropped objects Impact improvement from a covering or underlayment
Applies to Walls and floor/ceiling assemblies Floor/ceiling assemblies One product on a standard test slab
Lab test ASTM E90 ASTM E492 (tapping machine) ASTM E2179 (bare concrete slab)
Reads as One whole number One whole number Points gained

The two scores do not move together. The Floor Covering Reference Manual covers airborne and impact noise as separate issues. A floor with carpet and pad can score high on IIC and still do little against voices coming through, because impact and airborne sound travel differently. If you only remember one thing: footstep complaints are an IIC problem, and talking and TV complaints are an STC problem.

What counts as a good rating

For multifamily buildings, the International Building Code references both tests, setting IIC and STC at a minimum of 50 in the lab and 45 in the field. That is the legal floor, and local amendments or your building’s own rules can raise it.

Experts at Intertek stress that code is a minimum, not a goal for quiet living. Builders of high-end residential and hospitality projects often target IIC 55 or 60 and above, and an assembly below 50 is often treated as non-compliant for modern multifamily work.

A rough way to read the scale for your own project:

  • Below 50: You will hear normal footsteps. Not acceptable in an apartment or condo.
  • 50 to 54: Meets the minimum on paper. Heavy walking and dragged chairs may still come through.
  • 55 to 60: A noticeable step up and a sensible target for upstairs units and bedrooms below.
  • 60 and above: Premium assemblies, typically with a good underlayment plus a decoupled ceiling below.

These bands are a guide, not a guarantee. Your condo association may name a specific number, so check its rules before you buy anything.

Why lab ratings drop in real buildings

Why lab ratings drop in real buildings — overview diagram

Lab tests use ideal conditions. Real buildings have pipes, ducts, recessed lights and walls that carry sound around the floor. According to the Floor Covering Reference Manual, these flanking paths, penetrations and even adhesive quality can cost 5 to 10 points or more. An assembly that hits IIC 60 in a lab might land around 50 to 55 in the field. That is why the code distinguishes between a lab minimum of 50 and a field minimum of 45.

Two more details catch people out:

  1. Fasteners. Nails or screws that pass through the floor into the subfloor without isolation can act as a sound bridge and cancel much of the benefit of a costly underlayment. The same source says the isolation layer should be separated from vertical surfaces with perimeter strips.
  2. Low frequencies. The standard impact test does not capture sound below 100 Hz, and that deep thud is what bothers people most in wood-frame buildings. ASTM E989 defines the single-number impact metric, and the single number it produces is only part of the story. A floor can meet IIC 50 and still draw complaints about thumping.

Reading an underlayment’s number

The number printed on an underlayment box is usually ΔIIC, not the finished IIC. Commercial Acoustics explains that ΔIIC is tested on bare concrete under ASTM E2179 and shows the points a product adds, for example ΔIIC 23. That makes it handy for comparing two underlayments.

Two underlayments compared on concrete slabs

It does not tell you your final IIC. The ASTM E2179 standard itself notes that the impact rating comes from both the basic floor assembly and the floor covering on top, and the same covering performs differently over different structures. A 6-inch slab, an 8-inch slab and a wood-frame floor will give different results with identical flooring.

Before you rely on a number, ask:

  • Was it tested on concrete or on a wood-frame floor like mine?
  • Is there a ceiling below, and is it resilient or direct-mounted?
  • Was it tested with this exact floor covering, and was the floor glued, floated or nailed?
  • Is this the full assembly report, or just the underlayment sheet?

If the seller can only answer the last question with a product sheet, treat the number as a rough comparison, not a promise.

How flooring type affects the score

Hard surfaces transmit impact sound readily, so every hard floor leans on its underlayment and structure. Here is how the main types we sell tend to behave in practice:

  • Solid and engineered hardwood. Nailed or glued down, the floor is tied to the subfloor, so sound control depends on a proper underlayment or acoustic adhesive system. Floating engineered installs give more flexibility for underlayment, which matters in condos. See our floating vs nail-down comparison.
  • Luxury vinyl plank and SPC. Thin and dense, so impact noise can be sharp without a good underlayment. Many products come with an attached pad, but an attached pad does not prove a building-level rating.
  • Laminate. Floats over underlayment by design, so the pad choice matters a great deal. Our guide to AC ratings for laminate covers durability, which is a separate question from sound.
  • Cork. Popular as an underlayment for its acoustic damping, but thickness has trade-offs. We cover them in cork underlayment benefits and the 6 mm risk.

On concrete you start with more mass and often a better baseline than on joists. If you are laying floor over a slab, our guide on the best flooring over a concrete slab in Michigan is a useful companion, and the moisture side matters too: see ASTM F2170 testing for vinyl plank vapor barriers.

A practical way to choose

Follow this order and you avoid most expensive mistakes.

  1. Find the requirement first. Ask your condo board, HOA or building manager for the written minimum. Many state it as IIC, some as ΔIIC, and some demand a specific tested assembly.
  2. Know your structure. Concrete slab or wood joists? Is there a finished ceiling below? Your answer shapes what is realistic.
  3. Choose the floor you want, then the underlayment. Start from the look and durability you need. Then match an underlayment that the manufacturer has tested with that floor type.
  4. Request the full test report. Match it to your assembly as closely as possible. If your structure differs from the tested one, treat the result as indicative only.
  5. Build in a margin. Given the field drop described above, aim above the minimum if you can.
  6. Have it installed with isolation details. Perimeter strips, no fasteners bridging the layers, and sealed gaps. Installation quality can undo a good product.

If you are choosing flooring for an upstairs bedroom in a house, you do not need a condo-grade report, but the same logic holds. Our guide to the quietest flooring for upstairs bedrooms walks through it. When you are comparing several samples, our room-first method for evaluating flooring samples helps you judge them in the actual space.

Our perspective

We have spent more than 40 years around flooring jobsites, and the sound questions we hear have changed little. Someone buys a beautiful floor for a second-floor unit, and then the building rejects it, or the neighbor below complains. Almost always, the buyer looked at the flooring’s own spec and not at the assembly.

Our advice is plain. Treat the rating as a property of the whole system. Do not let a single marketed IIC figure close the decision. Ask for the report that matches your subfloor and ceiling, and expect the field result to land below the lab number. Contractors and builders who have to pass an inspection know this already. Homeowners who simply want a quieter house deserve the same honesty.

What we can do is help you match materials: hardwood, engineered, vinyl plank, SPC or laminate, with the underlayment and install method your project needs. We sell materials and refer installers, and we do not perform installation ourselves, so we have no reason to steer you toward anything but the right fit.

— Mike Solomich

Get help matching a floor to your sound requirement

If your building has a stated IIC or STC minimum, bring it with you. At Quality Hardwoods of Michigan you can talk through your project with staff who have installation and builder experience, then browse hardwood, luxury vinyl plank and SPC or laminate options. You can visit the showroom, request a phone or online quote, and pick up locally or arrange delivery in Michigan. We can also point you to installers. Start at qualityhardwood.com.

FAQ

What is the difference between IIC and STC?

STC measures how well an assembly blocks airborne sound such as voices, music and TV. IIC measures how well a floor-ceiling assembly stops impact sound such as footsteps and dragged furniture from reaching the space below. A floor can do well on one and poorly on the other.

What is a good IIC rating for a floor?

The building code minimum is 50 in the lab and 45 in the field for multifamily housing, as summarized here. Many high-end projects aim for 55 to 60 or more. Check your building’s own rules, which can be stricter than code.

Does flooring have its own STC or IIC rating?

Not really. Both ratings describe the full assembly of floor covering, underlayment, subfloor and ceiling below. Manufacturers often publish a ΔIIC for an underlayment or a result for one tested assembly, which applies to your floor only if your construction is similar.

Why is my floor louder than its rating says?

Lab conditions rarely match real buildings. Pipes, flanking paths, fasteners that bridge layers and poor adhesive application can reduce performance by 5 to 10 points or more. Low-frequency thuds in wood-frame floors also fall outside what the standard test measures.

Sources