Acoustic underlay: the layer that deserves a closer look

Underlay disappears when the floor is finished. Its specification should be settled long before that happens.

Begin with the finish you intend to use and how it will be fixed. Then check the evidence for the whole construction. That gives you a useful shortlist before you consider the feel of a sample or the thickness printed on a roll.

What does acoustic underlay do?

A resilient layer helps isolate vibration from footsteps and other impacts. It is one part of a floor and ceiling system, alongside the finish, slab or supporting structure, ceiling and junctions. The ABCB sound insulation handbook explains isolation as a way to control structure-borne sound.

There are three different questions to keep apart:

• How does walking sound to someone in the room?
• How much impact sound reaches the apartment below?
• How much airborne sound, such as speech, passes between the rooms?

A comfortable sample underfoot answers none of these on its own. If voices or music are the main problem, start with the soundproof flooring guide before choosing an underlay.

Which underlay guide matches your floor?

Read the detailed guides for hybrid, laminate, timber, tiles or carpet. If you are comparing materials, see rubber, foam and cork. If two quotes name different brands, the Regupol vs Damtec comparison checklist helps you compare the documents.

What do the acoustic numbers mean?

A laboratory report and a completed-floor test answer different questions. ISO 10140-3 describes laboratory impact testing and warns that applying its results to a building requires allowance for other sound paths and site conditions. ISO 717-2 covers the rating of those measurements and of field measurements.

When reading a report, distinguish an improvement figure such as ΔLw from a result for the tested construction such as Ln,w, or a field result such as L’nT,w. Copy the complete notation, including any adjustment term, rather than recording only “dB”.

Use the acoustic test report reader to organise the figures. Our ratings guide explains their direction and limits. Do not subtract a brochure improvement from an unrelated building result to predict a pass.

Does a 5 star or 6 star label settle the choice?

Ask which rating scheme the label refers to and what was assessed. The AAAC explanatory note makes clear that an isolated product cannot guarantee an AAAC rating.

For a useful answer, request the report identifying the floor finish, underlay, structure and ceiling. Establish whether it records a measurement, a prediction or a professional opinion. Ask an acoustic consultant to explain any differences between that construction and yours.

How much thickness can the floor accept?

Treat thickness as a dimension to check, not an acoustic ranking. The ATFA underlay guidance notes that equal-thickness products can have different properties and performance.

Prepare a section through the proposed floor. Include the finish, attached backing, separate underlay, adhesive, preparation and any supporting sheet or screed. Compare the finished height with the surface being removed. The floor-height calculator can organise the arithmetic; the installer still needs to check the actual doors, thresholds and junctions.

What needs attention during installation?

Ask for a written sequence covering preparation, moisture assessment, permitted layers and perimeter details. Underlay is not a reason to skip checks on the subfloor.

The ABCB handbook warns that fixings through an isolated floor into the slab can reduce the benefit of the resilient layer. Ask how trims, door stops and other fixed items will be detailed. For timber systems, obtain the agreed movement allowance rather than improvising a sealant repair.

Keep moisture control separate from acoustic performance. A sound-reduction claim does not identify the required moisture treatment. Request the moisture test results and the proposed vapour or waterproofing detail appropriate to the floor and room.

What should you keep for strata approval?

Obtain the registered by-laws or rules and the approval process before committing to a system. Use the strata flooring checker to organise questions for your building.

• Exact flooring and underlay names, thicknesses and installation method
• Complete acoustic report, including its date, test basis and construction
• Written compatibility of the flooring, underlay, adhesives and moisture provisions
• Drawing or description of perimeter and threshold details
• Written approval conditions and any testing requirement
• Photos and records of the layers before they are covered

If the underlay changes after approval, submit the revised system for assessment before installation. A similar-looking roll is not enough to show that the approved construction has been retained.