IIC stands for Impact Insulation Class, a single-number rating that measures how well a floor-ceiling assembly blocks impact noise like footsteps, dropped objects, or children jumping. The higher the IIC number, the better the assembly is at reducing that noise for the person in the room below. Most building codes require a minimum IIC rating of 50 for multifamily housing, though many condo associations and higher-end buildings aim for 55 or above.
What IIC Actually Measures
Impact noise is structurally different from the sound of a TV or a conversation. When someone walks across a floor, the vibration travels directly through the building materials rather than floating through the air. IIC captures how effectively a floor-ceiling assembly absorbs and blocks that vibration before it reaches the space below.
Testing follows a standardized method defined by ASTM E492. A mechanical tapping machine is placed on the floor in specific positions. The machine produces a continuous series of uniform impacts at a uniform rate, generating broadband sound pressure levels in the room below. Microphones in the receiving room measure how much of that impact noise makes it through. The results across a range of frequencies (from low thumps to higher-pitched taps) get distilled into one number: the IIC rating.
Because the test uses a standardized machine rather than, say, an actual person walking, every floor assembly is evaluated under the same conditions. That makes IIC ratings directly comparable across products and construction types.
How IIC Differs From STC
You’ll often see IIC and STC (Sound Transmission Class) listed side by side on product specs or in building codes. They measure fundamentally different types of noise. STC rates how well a wall, floor, or ceiling blocks airborne sound: voices, music, television audio. IIC rates how well a floor-ceiling assembly blocks structure-borne impact sound: footsteps, furniture being dragged, objects dropped on the floor.
A floor assembly can score well on one and poorly on the other. A thick concrete slab, for instance, naturally blocks airborne sound effectively (high STC) but transmits footstep vibrations easily without added treatment (lower IIC). That’s why both ratings matter in multifamily construction, and why building codes typically set minimum thresholds for each one independently.
What the Numbers Mean in Practice
IIC ratings typically range from the low 20s to the high 70s or beyond, depending on the full assembly. Here’s a rough guide to what you’d actually hear living below different ratings:
- Below 45: Footsteps and impact noise are clearly audible and likely disruptive. Common in older buildings with thin floors and no acoustic treatment.
- 50: The baseline code requirement in most jurisdictions for new multifamily construction. Impact noise is noticeable but reduced. You’ll hear someone walking in hard-soled shoes but may not notice socked feet.
- 55 to 60: A noticeable improvement. Casual walking is faint, though heavy impacts like jumping or dropped objects may still be audible.
- 65 and above: Impact noise is minimal. This is the range associated with luxury condominiums and buildings designed for acoustic comfort.
Every point on the IIC scale matters. The difference between a 50 and a 55 is perceptible to most people, and the jump from 50 to 60 represents a significant reduction in transmitted noise.
Lab Ratings vs. Field Ratings
There’s an important distinction between the IIC rating you see on a product label and what you get in an actual building. Lab testing (ASTM E492) happens in a controlled facility with no flanking paths, meaning sound can only travel through the floor-ceiling assembly being tested. Real buildings have additional routes for sound: structural connections, ductwork, gaps at wall edges.
Field testing follows a different standard (ASTM E1007) and produces what’s called an AIIC, or Apparent Impact Insulation Class. Field ratings are almost always lower than lab ratings for the same assembly, often by 3 to 5 points or more, because they capture those real-world sound paths. If your building code requires a field-tested IIC of 50, you’ll generally want to specify materials that test at 55 or higher in the lab to give yourself a margin.
What Delta IIC Means
When you’re shopping for underlayment or floor coverings, you’ll often see a “Delta IIC” rating instead of a straight IIC number. Delta IIC measures how many IIC points a specific material adds to a bare concrete reference floor. If an underlayment has a Delta IIC of 22, it means the underlayment improved the concrete slab’s IIC rating by 22 points during testing.
This metric is tested under ASTM E2179, which isolates the contribution of the floor covering or underlayment itself rather than the entire assembly. It’s useful for comparing products head to head, but it doesn’t tell you the final IIC rating of your specific floor system. Your actual IIC depends on the combination of your subfloor, underlayment, flooring material, ceiling treatment, and any airspace or insulation between them.
What Affects IIC Performance
No single layer determines an assembly’s IIC rating. It’s the full stack from finished floor to finished ceiling that matters. The main variables include:
- Subfloor type: Concrete slabs generally perform better than wood-framed subfloors because of their mass. Thicker concrete helps more.
- Underlayment: Resilient materials like rubber, cork, or specialized foam pads are the most common way to boost IIC. They decouple the flooring surface from the structure, absorbing vibration before it transfers downward.
- Flooring material: Hard surfaces like tile, hardwood, and laminate transmit more impact energy than carpet. Carpet with a thick pad can add 20 or more IIC points compared to a hard surface on the same subfloor.
- Ceiling treatment: Adding a resilient channel or sound isolation clips to the ceiling below, along with insulation in the cavity, can significantly improve the rating from the receiving end.
- Assembly depth: Deeper airspaces between the subfloor and the ceiling below generally allow for more insulation and decoupling, both of which improve impact isolation.
If you’re renovating a condo or apartment and your HOA requires a minimum IIC of 50 or 55, the most practical approach is pairing your chosen flooring with an underlayment that has a tested Delta IIC high enough to meet the requirement on your building’s floor type. Keep in mind that manufacturer claims based on lab testing will be optimistic compared to real-world performance, so building in a buffer of 5 points or more is a practical safeguard.