In a house, a noisy floor is your own problem. In a condo, apartment, or any other multi-family building, it's your neighbor's problem too — which is why hard-surface flooring in these buildings is so often governed by a sound-rating requirement written into the building's own rules, on top of whatever the local building code sets as a legal floor. Three ratings come up: IIC (impact sound), STC (airborne sound), and Delta IIC (what a specific underlayment adds on its own). None of them is a property of the plank sitting in a showroom — they describe the tested performance of the whole floor-ceiling assembly, or, for Delta IIC, of a specific underlayment product. Before you shop, the first real step is checking your building's actual bylaws or renovation policy — not assuming a number from this guide, a neighbor's install, or a salesperson's claim applies to your unit.
Check your condo, co-op, HOA, or strata's actual governing documents (bylaws, rules, or a renovation/alteration policy) for a specific required rating and any named underlayment or thickness spec — before you buy flooring. It's the one number that actually applies to you, and it can be higher than the code minimum and different from the building next door.
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IIC vs. STC vs. Delta IIC — what each one actually measures
All three ratings get used loosely in showrooms and listings, but they measure three different things, at two different levels (whole assembly vs. single product):
| Rating | What it measures | Sound type | Test standard | Where it shows up |
|---|---|---|---|---|
| IIC (Impact Insulation Class) | How well the floor-ceiling assembly blocks impact sound | Impact — footsteps, dropped objects, furniture | ASTM E492 (lab) | Assembly-level; the figure most condo/HOA rules quote |
| STC (Sound Transmission Class) | How well the assembly blocks airborne sound | Airborne — voices, TV, music | ASTM E90 (lab) | Assembly-level; the IBC pairs it with IIC |
| Delta IIC | The IIC points one underlayment product adds, on its own, over a reference bare concrete slab | Impact — isolates the pad's own contribution | ASTM E2179 (lab) | Product-level; the number for comparing underlayment options directly |
IIC (Impact Insulation Class) describes how well a floor-ceiling assembly blocks impact sound — footsteps, a dropped object, a dragged chair — tested under ASTM E492. STC (Sound Transmission Class) describes how well the same assembly blocks airborne sound — voices, television, music — tested under ASTM E90. Delta IIC is different in kind from the other two: it isolates how many IIC points a specific underlayment product adds, on its own, over a standardized reference bare concrete slab, tested under ASTM E2179. That makes Delta IIC the most useful figure for comparing underlayment products directly, since it strips out everything about the rest of the building and reports only the pad's own contribution. IIC and STC, by contrast, are always reported for a complete tested assembly — flooring, underlayment if any, the subfloor or slab, and (in a lab test) a standardized ceiling below — not for a product in isolation. This is exactly why "our vinyl has an IIC of 60" is a claim worth asking follow-up questions about: it usually means the vinyl was tested in one specific assembly, not that the plank itself carries a fixed number wherever it goes.
Code minimums vs. what your board likely requires
In the US, the International Building Code sets floor-ceiling assemblies between dwelling units — or between a dwelling unit and a public or service area — to a lab-tested minimum of IIC 50 and STC 50. Those lab figures are measured under ASTM E492 (impact) and ASTM E90 (airborne). The same code allows a lower figure — a Normalized Impact Sound Rating of not less than 45 — when an assembly is instead tested in the field in a completed building under ASTM E1007, the field counterpart to E492. Real construction tolerances and sound that flanks through adjoining walls and floor junctions typically measure a few points below a lab result, so the code's field allowance reflects that gap rather than a laxer standard.
Treat that pairing as a legal floor, not a target for comfort. How much higher a given building sets its own bar varies, and there is no single universal number: many condo, co-op, HOA, and strata bylaws — especially in newer or higher-end buildings — commonly reference IIC somewhere in the 55 to 60 range for hard-surface flooring, sometimes alongside a required or pre-approved underlayment product and a minimum thickness. We're citing that range as a commonly seen pattern in condo-flooring rules, not as a rule of our own or a universal requirement — the only number that actually governs your installation is whatever is written into your building's own governing documents, so get that in writing before you buy anything.
The assembly decides it, not the plank
A rating on a spec sheet describes a specific tested floor-ceiling assembly — the flooring, any underlayment, the subfloor or slab beneath it, and (in a lab test) a standardized ceiling below — not the flooring product sitting alone in a showroom. The same plank can perform very differently over a concrete slab than over a wood-frame structure with joists, subflooring, and a drywall ceiling, because the structure itself carries and radiates sound differently. Concrete-slab construction, common in many mid-rise and high-rise condo buildings, generally starts from more baseline mass and impact isolation than a lighter wood-frame assembly — though a well-designed wood-frame floor-ceiling system (added mass, insulation, resilient connections) can still reach a high rating. This is exactly why Delta IIC matters so much in practice: it's the number that isolates what a specific underlayment adds on top of whatever your structure already provides, which is the actual decision in front of you once the building's structure is fixed. See our underlayment guide for how IXPE, EVA, cork, felt, and rubber underlayment compare on acoustics, moisture, and thickness.
Where hardwood is in the mix, the same structure-first logic applies. A floating engineered hardwood installation sits on a separate underlayment the same way LVT or laminate does, so the Delta IIC of that underlayment is what you're actually shopping for. A nailed or glued solid-wood installation over a wood-frame structure depends far more on the structure below — joist spacing, subfloor, ceiling assembly — than on anything added at the surface, which is worth knowing before assuming a particular wood species or plank thickness will solve a sound complaint by itself.
Practical guidance: rugs, floating vs. glue-down, and board approval
- Hard surfaces plus area rugs. Even a well-tested assembly will transmit more everyday impact noise than the carpet it replaced — that's physics, not a paperwork failure. Area rugs and runners in high-traffic paths — hallways, and rooms that sit directly above another unit's living space — meaningfully reduce perceived impact noise for the people below, on top of whatever the assembly is rated for. Treat rugs as a genuine, low-cost mitigation, not a substitute for a documented, board-approved assembly where one is required.
- Floating vs. glue-down, for sound specifically. A floating installation's acoustic performance rests heavily on its separate underlayment's own tested Delta IIC contribution — which is also why many boards ask specifically what underlayment is going in, not just what flooring. A glue-down installation bonds the flooring directly to the subfloor across its full surface, so there's no separate cushioning layer in the usual sense; some glue-down systems instead pair with an acoustically rated adhesive or membrane. Confirm with the manufacturer whether a given glue-down system carries its own tested acoustic data before assuming it meets a board's requirement — don't assume one installation method is inherently quieter than the other without the numbers in hand.
- What a board typically wants to see. A manufacturer's or independent lab's test report showing the flooring-and-underlayment combination's IIC and/or Delta IIC (and STC, if the building's rules mention it) — ideally tested over a subfloor type that matches yours, since the same product can test differently over concrete than over a wood-frame structure; the specific underlayment product name and thickness, since some bylaws name an approved list or a thickness range; and, in some buildings, a signed statement from the installer confirming the approved assembly was actually what went down. Ask your board or management company for their exact documentation format before you buy — it varies by building, and it's far cheaper to ask first than to be asked to remove a floor after the fact.
A Canadian note
Sound isolation rules aren't a US-only concept, but Canada's framework is structured differently, and it's worth getting right rather than assuming a border-for-border match. The National Building Code of Canada's mandatory sound-isolation requirement between dwelling units has historically centered on airborne sound rather than impact sound — a Sound Transmission Class figure, with more recent editions moving toward an Apparent Sound Transmission Class (ASTC) metric that also accounts for flanking sound transmitted through adjoining walls and floor junctions. Unlike the US IBC's IIC/STC pairing, the NBC has not historically mandated a minimum impact (footstep) sound rating for floors; Canadian research bodies have recommended a voluntary IIC benchmark around 55 for bare floors, and later code editions have reportedly considered adding a mandatory impact-sound provision. Provinces and municipalities can also amend or exceed the model national code. We're deliberately not citing a specific clause number here, because it genuinely varies by jurisdiction and changes between code cycles — confirm the current figure with your province's building code and, just as importantly, with your own condo corporation's or strata council's bylaws, rather than assuming either a US number or a figure from this guide applies directly to your building.
Myths & mistakes
- Myth: "Thicker or softer underlayment is always quieter." Past the range a flooring manufacturer specifies, more thickness or softness doesn't reliably raise an assembly's IIC — it can instead stress the floor's locking joints, and it ignores that Delta IIC is a tested figure for a specific product, not a function of raw thickness.
- Myth: "IIC and STC are the same rating." They measure different things — IIC is impact sound (footsteps, dropped objects), STC is airborne sound (voices, music) — tested under different ASTM standards, and a condo board's requirement is almost always specifically about IIC.
- Myth: "If a flooring type is allowed in one condo, it's allowed in mine." Different buildings have different bylaws, different underlying structures (a concrete-slab tower and a wood-frame low-rise can need very different assemblies to hit the same board-required rating), and sometimes different requirements from one unit to the next depending on what's directly below.
- Myth: "Meeting the IBC's IIC/STC 50 minimum is automatically enough for my condo." That's a legal floor, not your building's actual rule — many boards set their own requirement above it, commonly cited in the 55–60 range for hard flooring.
- Myth: "A hard, dense floor can't meet a high sound rating." Acoustic performance is a property of the whole tested assembly — especially the underlayment's Delta IIC — not of the surface material's hardness. Vinyl, laminate, and engineered hardwood can all meet a high IIC rating when paired with an assembly tested to support it.
Frequently asked questions
What sound rating does my condo actually need?
There's no single answer — it depends entirely on your building. The legal floor in the US is the International Building Code's lab-tested minimum of IIC 50 and STC 50 (or a somewhat lower figure, commonly cited around 45, when tested in the field in a completed building). Many HOAs, condo boards, and strata councils set their own requirement above that code floor — often cited in the IIC 55–60 range for hard-surface flooring — written into the building's own bylaws or renovation policy. Get your specific building's actual required number in writing before you shop for flooring or underlayment.
Is IIC the same thing as STC?
No. IIC (Impact Insulation Class, tested under ASTM E492) measures how well a floor-ceiling assembly blocks impact sound — footsteps, dropped objects, furniture moving. STC (Sound Transmission Class, tested under ASTM E90) measures how well it blocks airborne sound — voices, television, music. A floor-ceiling assembly can test well on one and poorly on the other, and a condo board's flooring requirement is almost always specifically about IIC, since impact noise transmitted downward from the unit above is the more common source of neighbor complaints.
What is Delta IIC, and why does it matter more than an underlayment's thickness?
Delta IIC (tested under ASTM E2179) is the number of IIC points a specific underlayment product adds on its own, over a standardized reference bare concrete slab. It's the number that actually lets you compare underlayment products head-to-head, rather than assuming a thicker or softer pad automatically performs better — it often doesn't, and exceeding a flooring manufacturer's specified thickness range can cause its own problems. See our underlayment guide for how the common types compare.
Does a floating floor or a glue-down floor perform better acoustically?
Neither wins automatically — it depends on the specific tested numbers, not the installation method by itself. A floating floor's acoustic performance rests heavily on its separate underlayment's own Delta IIC contribution. A glue-down floor bonds directly to the subfloor with no separate cushioning layer in the usual sense, though some glue-down systems pair with an acoustically rated adhesive or membrane instead. Ask for tested IIC/Delta IIC data for the specific system either way, rather than assuming one method is inherently quieter.
What should I actually bring to my condo board or HOA for flooring approval?
Typically: a manufacturer's or independent lab's test report showing the flooring-and-underlayment combination's IIC and/or Delta IIC (and STC, if your building's rules mention it) — ideally tested over a subfloor type that matches yours, since the same product can test differently over concrete versus a wood-frame structure; the exact underlayment product name and thickness, since some bylaws name an approved list or a thickness range; and sometimes a signed statement from the installer confirming the approved assembly was actually installed as specified. Ask your board or management company for their exact documentation format before you buy anything — requirements vary by building.
Do Canadian condos follow the same IIC 50 rule as US buildings?
Not necessarily, and the frameworks aren't a direct match. The National Building Code of Canada's mandatory sound-isolation requirement between dwelling units has historically centered on airborne sound (a Sound Transmission Class figure, with newer editions moving toward an Apparent Sound Transmission Class metric that also accounts for flanking sound) — it has not historically mandated a minimum impact (footstep) sound rating for floors the way the US IBC pairs an IIC figure with STC, though a voluntary IIC benchmark around 55 has been recommended by Canadian research bodies and future code editions have reportedly considered adding a mandatory impact provision. Provinces and municipalities can also amend the model code. Confirm the current requirement with your province's building code and your own condo corporation's or strata council's bylaws rather than assuming a US figure applies.
Sources
This guide describes standard test methods and code provisions as defined by the bodies below; we are not affiliated with any of them. Individual condo, co-op, HOA, and strata requirements are set by each building's own governing documents, not by any of these bodies — confirm your building's actual requirement directly.
- ASTM International — ASTM E492 (laboratory impact sound transmission / IIC), ASTM E90 (laboratory airborne sound transmission loss / STC), and ASTM E2179 (effectiveness of floor coverings in reducing impact sound transmission through concrete floors / Delta IIC).
- International Code Council (ICC) — International Building Code (IBC) sound-isolation provisions, which set the lab-tested IIC 50 / STC 50 minimum between dwelling units (ASTM E492 / E90), with a lower field-tested allowance of 45 under ASTM E1007.
- National Research Council Canada (NRC) — research on the National Building Code of Canada's acoustic provisions, including its historical focus on airborne sound isolation and recommendations around impact-sound insulation for floors.
Keep going: compare underlayment types and their acoustic performance in our underlayment guide; see how sound and thickness fit into luxury vinyl (LVP/LVT) and laminate flooring; check floating-install specifics in engineered hardwood explained; or start from room needs with the how-to-choose framework.
Educational information — not professional advice. Confirm your building's actual sound-rating requirement with your board, HOA, or property manager, and check any test data against the specific assembly you plan to install, before you buy. Last verified: 23 July 2026. See our Disclaimer & Terms.