Acoustic underlay for apartments — IIC, IIC ratings, and meeting NCC requirements
Specifying acoustic underlay to meet Australian apartment standards. Lab vs. field IIC, integrated vs. separate underlay, and the spec language that survives certifier review.
An acoustic underlay is the layer the building surveyor cares about most and the homeowner notices least. Choose it well and the flooring above sounds like a thoughtful renovation; choose it badly and the floor below sounds like a corridor of footsteps and dropped cutlery. For apartments and mixed-use buildings in Australian cities, the underlay is rarely optional — it is the basis on which the floor is approved and the basis on which it remains compliant.
This article is written for specifiers, builders and end clients who need to navigate the AAAC star system, the relevant strata by-laws, and the practical realities of getting an apartment floor signed off the first time. It is technical, but the decisions are not as opaque as they first appear when the language is unpacked.
AAAC stars and what they really mean
The Association of Australasian Acoustical Consultants publishes a voluntary star rating for apartments and dwellings. Four stars is widely treated as the entry point for hard flooring in apartments and corresponds to L'nT,w of 50 dB or below — a noticeable but acceptable level of impact transmission. Five stars (45 dB or below) is the standard premium developments target. Six stars (40 dB or below) belongs to acoustically engineered hotels and recording-studio-adjacent projects.
The star rating is a system measurement, not a product measurement. It applies to the finished floor/ceiling assembly tested in place. An underlay alone cannot earn a star rating — it can only contribute to one when paired with a specified plank, slab and ceiling configuration. Reading a star rating off an underlay datasheet is the most common specification error in the category.
What the strata by-law usually says
Most apartment by-laws written in the past two decades require either a specific star rating or a maximum L'nT,w. Many also require that any hard flooring installed after handover be approved by the owners' corporation before work begins. A signed acoustic report from a registered acoustic consultant is the usual evidence.
If the by-law nominates a star rating, the underlay datasheet must demonstrate that the proposed system meets that rating on the building's actual slab type. A 4-star result on a 200 mm post-tensioned slab will not necessarily be a 4-star result on a 150 mm precast slab. The smartest moment to read the by-law is before the underlay is ordered, not after the install has started.
Choosing the right underlay for the floor
Acoustic underlays for hybrid and laminate are typically high-density closed-cell foam, rubber, or composite cork-rubber. For SPC hybrid, the underlay is rolled out separately and the planks installed over it; pre-attached pads are convenient but rarely achieve more than 3 stars. For engineered timber installed floating, the underlay also performs a moisture-control role and must be compatible with the plank's locking system.
For glue-down installations of engineered timber, the adhesive itself becomes part of the acoustic system. Specialist elastic adhesives are designed to provide both bond and impact dampening, and several major brands publish tested star ratings for their products. Our floating vs glue-down guide covers the trade-offs in detail, and the engineered timber landing page outlines our range positioning.
Detailing decides the result
An acoustic underlay is only as good as its edges. Continuous coverage, sealed seams, and a properly isolated perimeter are non-negotiable. Skirtings should not pinch the underlay against the wall, and door thresholds should be detailed so the underlay continues — not stops — under the join. Plumbing penetrations, downlight cut-outs in the ceiling below, and recessed services all create flanking paths that degrade the field result.
It is often worth involving an acoustic consultant before the underlay is selected, particularly on a project with a borderline by-law requirement. The fee is small compared to the cost of failing the post-installation test and remediating after the floor is laid.
Testing, sign-off and handover
Field testing under ISO 16283 is performed on the completed floor with a tapping machine and microphone in the room below. The result is compared against the by-law threshold. If the floor passes, the report is lodged with the owners' corporation; if it fails, the most common remediation is to lift sections of plank, replace the underlay with a higher-rated product, and retest.
Specifiers wanting to understand the wider regulatory context can refer to the Standards Australia catalogue (AS/ISO 717.2 for impact ratings) and the Australian Resilient Flooring Association technical notes for current best practice. The technical data hub includes our system pairings for hybrid in apartments, and the contact page reaches the studio team for project-specific advice.
