- IIC is impact noise; STC is airborne noise. Higher numbers are better.
- Both rate the whole assembly: flooring, underlayment, slab and ceiling, never a product alone.
- Lab tests (ASTM E492, E90) and field tests (ASTM E1007, E336) give different results.
- We match the rating your association requires, but no installer can promise a field result.
Two kinds of noise, two ratings
Condo flooring rules usually mention two numbers, IIC and STC. They measure different kinds of sound, and a floor can do well on one and only average on the other.
- IIC, Impact Insulation Class, is about impact noise: footsteps, a dropped pan, a chair pushed back. That’s the sound that travels through the structure into the unit below, and it’s the one hard floors are most often blamed for.
- STC, Sound Transmission Class, is about airborne noise: voices, a television, music. It measures how well the floor-ceiling assembly blocks sound traveling through the air.
For both ratings, a higher number means better sound control. Both describe a whole floor-ceiling assembly, meaning the flooring, the underlayment, the slab and whatever ceiling sits below it. No plank or pad has an IIC or STC rating on its own.
How the lab tests work
IIC comes from ASTM E492, the standard for laboratory measurement of impact sound through floor-ceiling assemblies. A standard tapping machine, a row of small hammers, strikes the floor in a test chamber while microphones in the room below record the sound. The results are turned into a single IIC number using ASTM E989.
STC comes from ASTM E90, the laboratory test for airborne sound transmission loss. Sound is played in one room and measured in the next, across a range of frequencies, and ASTM E413 turns the results into an STC number.
A lab report names the exact assembly that was tested. One published Intertek report for an underlayment maker, for example, describes luxury vinyl plank over that company’s underlayment on a 152 mm (6 inch) concrete slab. The assembly sits in a steel frame isolated from the building with a neoprene gasket. The report also states that its results are relevant only to the sample tested. That line is worth remembering when you compare products.
Field ratings: AIIC, FIIC and ASTC
A lab chamber is built so the sound goes through the floor sample and nowhere else. A real building is different. Sound also travels sideways through walls, columns, pipes and ductwork. Acousticians call those flanking paths.
That is why there are separate field tests, run in the finished building:
- ASTM E1007 measures impact sound in the field with the same kind of tapping machine. Intertek, a testing lab, describes it as producing the apparent impact insulation class, AIIC, along with ISR and NISR ratings, calculated using ASTM E989. Many association documents and codes also use FIIC, field impact insulation class, for an in-place impact result.
- ASTM E336 measures airborne sound between rooms in a building. It produces ratings such as ASTC, apparent sound transmission class, and NIC, noise isolation class, using ASTM E413. Intertek’s summary of the standard notes that the result is shaped by indirect flanking paths as well as by the floor itself.
Because a field result includes everything the building adds, it’s often lower than the lab number for a similar floor. It’s common for rules to set a lower minimum for a field test than for a lab test. Read your association’s rule closely to see which one it asks for. If the rule gives a number without saying lab or field, ask the manager in writing before you choose an underlayment.
What your building adds or takes away
Two units with the same floor and underlayment can test differently. The factors are mostly things no flooring installer controls:
- Slab thickness. A thicker concrete slab blocks more sound than a thinner one.
- The ceiling below. A suspended or isolated ceiling in the unit below adds a lot. A plaster or drywall ceiling fixed straight to the slab adds much less. Lab reports usually say whether a ceiling was included.
- Flanking paths. Plumbing chases, shared walls and ductwork carry sound around the floor.
- Room size and furnishings. A bare, echoing room below can measure louder than a furnished one.
This is the honest reason we don’t promise that a given floor will pass a field test. We can control the product, the underlayment and the quality of the install. We can’t change the slab, the ceiling below or the building’s walls.
How to read a spec sheet
When you or your board look at an underlayment’s published numbers, ask these questions:
- Lab or field? Is the number IIC and STC (lab) or AIIC, FIIC or ASTC (field)?
- What assembly? What flooring was on top, how thick was the slab, and was there a ceiling below? A result with a suspended ceiling isn’t comparable to one without.
- Which floor covering? A pad tested under ceramic tile may perform differently under vinyl plank.
- Is there a delta rating? Some reports include a ΔIIC from ASTM E2179, which measures how much a floor covering improves impact sound over a bare concrete slab.
- Is the full report available? A one-line number on a box is less useful to a board than the lab’s report.
One more point from a manufacturer. Shaw’s SPC installation guidelines allow a second underlayment no thicker than 3 mm under planks with an attached pad, but note that its lab testing so far does not show a second layer adds acoustic benefit. More padding is not automatically quieter, and it can go against the plank maker’s rules. See our guide to choosing a sound-rated underlayment.
What we do
We read your association’s rule, choose an underlayment with published test results that match the rating your association requires, and include the report in the approval package. Our condo approval guide walks through the rest of the paperwork. Then we install it the way the test assembly and the maker’s instructions call for. If your building requires a field test after the install, that is arranged through a qualified acoustical tester, and we’ll work around the schedule. Our condo flooring installation page covers the full service.
What it costs
As an estimate, vinyl plank runs $5 to $12 per square foot installed, before your discount. A 200 square foot room is roughly $1,200 to $1,900 before the discount. Most jobs land in a working range of $6.00 to $9.50 per square foot before the discount, and about $5.00 to $7.75 after it, because the 50% off applies to installation labor. None of these numbers is a quote. The real figure comes from the free in-home measurement.
The sound-rated underlayment is included in the estimate we give at the measurement. The current offer is 50% off your flooring installation, $1,200 Off + 5% Senior & Military Savings, and financing with low monthly payments, 0% interest plans, and same-as-cash offers (for qualified applicants). Try the cost calculator or book the free measurement.
Sources: Intertek: ASTM E492 laboratory impact sound testing · Intertek: ASTM E1007 field impact sound testing · Intertek: ASTM E90 airborne sound transmission loss · Intertek: ASTM E336 field airborne sound attenuation · Intertek acoustical test report N2031.01 for MP Global underlayment