Acoustic underlay: what the ratings mean

What acoustic underlay do I need under my flooring?

Most Canadian condos and stratas require an acoustic underlay rated IIC 55 to 65, and you should get the exact number in writing from your corporation before ordering. IIC (Impact Insulation Class) measures how well a complete floor assembly blocks footstep noise to the unit below, and higher numbers are better. Because IIC ratings assume a specific concrete slab thickness in the lab, many newer buildings now ask for delta-IIC instead, which measures the improvement the underlay itself provides regardless of slab. A typical good acoustic underlay costs $0.50 to $1.50 per square foot installed. Even in a house with no rules, an acoustic underlay is worth installing over a basement suite or under a bedroom above a living room, because retrofitting sound control later means lifting the entire floor.

IIC and STC explained

IIC measures impact noise — footsteps, dropped objects, chair legs. STC (Sound Transmission Class) measures airborne noise like voices and music. Flooring rules almost always concern IIC, because that is what a hard floor changes when it replaces carpet. STC is mostly a function of the building structure, which flooring cannot alter much.

Why delta-IIC is replacing IIC

A lab IIC rating tests the underlay on a standard concrete slab, usually 6 or 8 inches. Put the same underlay on a thinner slab in a real building and the result differs substantially. Delta-IIC reports how much the underlay improves the assembly, which transfers more honestly between buildings. Vancouver stratas increasingly specify it.

What makes an underlay work

Mass, resilience and decoupling. Dense rubber and cork underlays absorb impact energy; foam underlays are cheaper and generally less effective acoustically. Thickness helps but density matters more, which is why a thin dense rubber can outperform a thick soft foam.

Attached pads and double-padding

Many rigid-core vinyl products come with an attached acoustic pad, which is convenient but usually provides a modest rating. Adding a second underlay beneath a product that already has one is a mistake: the combined cushion stresses and can break the locking joints. If you need a specific rating, buy a product without an attached pad.

Beyond the rules

Even where nothing requires it, acoustic underlay is the cheapest sound improvement available in a multi-level home. Over a basement suite, under a child's bedroom, or in any home where footsteps travel, $0.50 to $1.50 per square foot buys a noticeably quieter house.

Common questions

Does underlay go under every floor type?

Floating floors — laminate and click vinyl without an attached pad — always need one. Nailed hardwood uses building paper or a rosin paper rather than acoustic underlay unless sound control is specifically required. Glue-down products bond directly to the substrate, so acoustic control has to be built into the assembly differently.

Can I use a thicker underlay for better sound?

Up to a point, and only with the right material. Excessive cushion under a click floor causes the locking joints to flex and eventually fail, so manufacturers specify maximum thickness. Choose a denser underlay rather than a thicker one.

Will underlay fix noise from my neighbours above?

No. Underlay under your floor reduces the noise you transmit downward, not what reaches you from above. Improving what you hear from upstairs requires work in their unit or ceiling treatment in yours.

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