STC vs IIC: Code Rules for Prefab Units
If I’m reviewing a prefab unit, I treat STC and IIC as two separate code checks: STC for airborne noise and IIC for impact noise. In most multifamily cases, I’m looking for STC 50 / IIC 50 in lab tests or 45 / 45 in field tests for unit separations. And I don’t rely on a product sheet. I check the full tested assembly.
Here’s the short version:
- STC covers sound like voices, TV, and music.
- IIC covers sound like footsteps, dropped items, and chair movement.
- STC applies to walls and floor-ceiling assemblies between units.
- IIC applies to floor-ceiling assemblies only, usually when there’s occupied space below.
- Field results are often 5 to 10 points lower than lab results.
- If the tested assembly changes – like the flooring, ceiling, or module joint detail – the rating may no longer apply.
- Buyers should ask for the full acoustic test report, including the ASTM test method and whether the result is lab-tested or field-tested.
The main rule I follow: code looks at the whole assembly, not one part of it.
STC vs IIC: Code Requirements for Prefab Multifamily Units
STC & IIC – Code Compliance vs Acoustic Performance
Quick Comparison
| Item | STC | IIC |
|---|---|---|
| What it measures | Airborne sound | Impact sound |
| Common noise type | Voices, music, TV | Footsteps, dropped objects |
| Where it applies | Walls and floor-ceiling assemblies | Floor-ceiling assemblies |
| Common code target | 50 lab / 45 field | 50 lab / 45 field |
| Main things that affect it | Mass, air gaps, insulation, sealed joints | Underlayment, floor finish, ceiling below |
| Common review mistake | Using a panel rating alone | Using flooring claims without assembly data |
If I’m approving a prefab submittal, I want assembly-level proof that matches what will be built in the field.
STC: Airborne Noise Rules for Walls and Floor-Ceiling Assemblies
In prefab units, STC applies to party walls, corridor walls, and floor-ceiling assemblies between dwelling units. The IBC sets the baseline at STC 50 for lab-tested assemblies and STC 45 for field-tested assemblies in multifamily housing. That point matters more than it may seem: plan review looks at the tested assembly as a whole, not just the panel on its own.
What STC Measures and How It Is Tested
Lab ratings are set using ASTM E90 and ASTM E413. Field STC is measured under ASTM E336, and that split matters because lab tests happen in controlled settings with fewer flanking paths. In actual buildings, sound can slip through joints, gaps, and penetrations and move around the assembly.
That’s why field-measured STC is often 5 to 10 points lower than the lab rating. A wall can look strong on paper and still underperform once it’s installed. So if you’re reviewing a prefab system, ask for field test data, not just lab-only numbers.
How Prefab Wall Design Affects STC
The main drivers of STC are mass, decoupling, and sealed penetrations. Mass helps block sound. Decoupling keeps vibration from moving straight through the wall. Cavity insulation helps absorb sound that makes it into the assembly.
Put those parts together, and you get two dense layers with an air gap between them. That setup usually performs better than one solid layer with the same total weight.
In prefab units, STC problems often show up at seams, penetrations, and service openings. That’s where sound finds the easy route. Sealing joints with acoustic caulking and adding solid blocking around utility penetrations helps keep installed performance closer to the rated assembly.
| Common Flanking Path | Mitigation Strategy |
|---|---|
| Module joints | Acoustic sealant at module joints |
| Wiring, plumbing, and sprinkler penetrations | Solid blocking and acoustic baffles at penetrations |
| Ductwork | Sound-attenuating ducts or silencers |
| Gaps at the floor track | Level subfloors and close track gaps |
STC covers airborne sound. IIC covers footsteps and other impact noise.
IIC: Impact Noise Rules for Multi-Story Prefab Units
If STC deals with wall sound, IIC deals with what happens through the floor-ceiling assembly in stacked prefab units. IIC – Impact Insulation Class – measures impact noise from footsteps, dropped items, and furniture being dragged across the floor. It applies to floor-ceiling assemblies in stacked prefab units. If there’s no occupied space below, IIC usually doesn’t come into play.
What IIC Measures and How It Is Tested
IIC is tested with a tapping machine – a standard device that hits the floor at set intervals – while a microphone in the room below records how much sound gets through. Lab testing follows ASTM E492, and field testing follows ASTM E1007.
In practice, field values are often 5 to 10 points lower than lab ratings. Why the gap? Real buildings have flanking paths that lab setups don’t.
A lot of multi-story projects call for IIC 55, 60, or even 65. So a submittal that only clears the code minimum may still miss the project’s actual target.
Why Flooring and Underlayment Matter More for IIC
The IIC rating reflects the entire assembly – the flooring, underlayment, subfloor, and ceiling treatment working together. Swap out just the floor finish, and the IIC number often won’t move much.
Underlayment does most of the heavy lifting here. A 2 mm to 3 mm cork underlayment can improve a baseline assembly’s IIC by 20 to 25 points.
Floating (click-lock) installations tend to beat glue-down systems for IIC because they decouple the finish from the subfloor. Glue-down and nail-down methods create direct contact, which lets impact noise travel into the structure. In prefab units where IIC compliance matters, floating floors are usually the safer bet.
Once you’re aiming for IIC 65 or higher, surface changes alone usually won’t get you there. That level often needs a suspended or resilient-mounted ceiling below the structural assembly. And here’s the part people miss: if the tested assembly includes a suspended ceiling but the prefab unit in the field does not, actual performance can land 15 to 25 points lower.
| Floor System Component | Typical IIC Range | Notes |
|---|---|---|
| Carpet with quality pad | IIC 70+ | Quietest typical option |
| WPC vinyl, floating | IIC 58–66 | Foamed core adds a small decoupling effect |
| SPC vinyl, floating | IIC 50–58 | Denser core passes more vibration than WPC does |
| Glue-down vinyl or tile | IIC 25–35 | Direct bond removes decoupling |
| Bare concrete slab, no treatment | Below IIC 35 | Usually fails multifamily minimums |
Those assembly details are what shape the STC-versus-IIC comparison during plan review.
STC vs IIC: Side-by-Side Code Comparison for Prefab Units
Once the basics are in place, plan review usually comes down to a simple question: which assembly has to meet which rating? Reviewers check STC and IIC as two separate items because they cover different kinds of sound and different parts of the building.
| Feature | STC | IIC |
|---|---|---|
| Noise Type | Airborne (voices, TV, music) | Impact (footsteps, dropped objects) |
| Transmission Path | Walls and floor-ceiling assemblies | Floor-ceiling assemblies only |
| Key Design Factors | Mass, decoupling, insulation, airtight seals | Flooring material, underlayment, ceiling suspension |
| Plan Review Focus | Unit-separating walls and floor-ceiling assemblies | Floor-ceiling assemblies with occupied space below |
Where Each Rating Applies During Plan Review
STC is reviewed for unit-separating walls and rated floor-ceiling assemblies. IIC is reviewed only when there’s an occupied unit below.
For plan review, submittals should include assembly-level test data for each condition, not just a rating for one material on its own. That distinction matters. A unit can pass STC and still miss IIC, so reviewers treat them as separate checks.
Common Code Review Mistakes with Prefab Assemblies
The most common mistake is using a material claim as if it proves the whole assembly passes. It doesn’t. Code compliance is based on the full wall or floor-ceiling system, not just the finish material.
Another mistake is mixing lab and field ratings without making the difference clear. Lab ratings should not be used as proof of field compliance. A lab STC 50 result does not guarantee field STC 50. Flanking paths through joints, pipe penetrations, slab edges, and module connections can drag performance down. In prefab work, module joints, penetrations, and edge conditions are often where things slip.
A third mistake shows up after design changes. Teams assume a tested assembly still passes even after the details shift. But if carpet is swapped for LVP, a suspended ceiling is removed, or module connection details change, the original acoustic test report may no longer apply. The tested assembly needs to match the installed assembly.
That’s why buyers should ask for the full acoustic test report, not just a product sheet.
What Buyers Should Request Before Approving a Prefab Unit
Once a provider makes the assembly claim, buyers should ask for the full assembly test report. That report needs to show the complete assembly build-up, the ASTM method used, and whether the result came from a lab test or a field test. That’s what tells you if the rating matches what will actually be installed.
If you use Ameribuilds to find local providers, add this step to your submittal review. Ask for an acoustic test report from an accredited lab before any unit is approved for dwelling or sleeping units.
How to Read an Acoustic Test Report
The report should spell out every part of the tested assembly: subfloor type and thickness, insulation density, ceiling construction, whether resilient channels were used, underlayment material and thickness, plus fastener spacing and sealant details. If even one of those items is different in the installed build, the rating may not carry over.
You should also check who issued the report. Look for accreditation from NVLAP (National Voluntary Laboratory Accreditation Program) or Intertek.
Questions to Ask Providers During Submittal Review
Use these checks during submittal review:
- Match the tested assembly to the installed assembly, including the subfloor and ceiling type.
- Confirm that module joints and penetrations were part of the tested assembly.
- State whether the rating is lab-tested or field-tested. Field results are often 5 to 10 points lower than lab results.
- Confirm that the assembly meets the locally adopted code for the intended use – often STC 50 and IIC 50 under the IBC for dwelling unit separations.
If a provider can’t answer all four, the submittal is not ready for approval.
The Short Rule for STC and IIC
STC covers airborne noise. IIC covers impact noise. That’s the simple rule.
In prefab residential or sleeping units, U.S. code can require both. And each one has to be checked on its own. Approval should be based on complete-assembly test reports tied to the locally adopted code, not broad sales claims or single material specs.
FAQs
When do I need both STC and IIC?
You need both STC and IIC when your local building code or governing authority, such as an HOA, requires both for your project.
The International Building Code commonly sets a minimum of 50 for both in floor-ceiling assemblies between dwelling units, but some jurisdictions or luxury buildings ask for higher ratings. Get the requirement in writing from your building management or property manager.
Why are field ratings lower than lab ratings?
Field ratings are often lower because lab tests happen in tightly controlled spaces, with fewer leaks and fewer side paths for sound to slip through.
In actual buildings, sound can get around the main assembly through gaps, joints, and pipe penetrations. Field tests also reflect the room’s size, layout, and sound absorption. That’s why performance is often 5 to 10 points lower than lab results.
What changes can invalidate an acoustic test report?
An acoustic test report can become invalid – or simply misleading – when the installed build doesn’t match the assembly that was tested.
That matters because STC and IIC are tied to the entire floor-ceiling or wall assembly, not just one part of it. Change the subfloor, adjust the thickness, swap the ceiling build, or use different insulation, and the sound performance can shift. Sometimes a small change on paper turns into a big gap in the field.
Lab results can also fall short once the system is installed in a building. One big reason is flanking transmission. Sound doesn’t always travel straight through the tested surface; it can move around it through adjacent parts of the structure. That’s where a clean lab number can start to look less reliable in practice.
In prefab or modular units, this gap matters even more. A component-only report may describe how one piece performed in testing, but it cannot prove the acoustic performance of the fully assembled unit.