Acoustic underlay for timber: a complete floor specification

Timber’s character is visible in the grain. The way it feels and sounds underfoot also depends on how the floor is supported and fixed.

Floating engineered boards, bonded timber and solid boards on a supporting system need different details. Settle the installation method first. Then check the complete construction for acoustic evidence, moisture provisions and the height available at doors.

Which timber floor system are you laying?

The Australasian Timber Flooring Association (ATFA) information sheet on acoustic underlays describes three systems for apartments. Foam underlay is “used with floating engineered, laminate and bamboo floors”. Adhesive-fixed underlay suits floating floors and “adhesive-fixed engineered and parquetry floors”. And “For solid strip timber flooring, plywood is usually adhesive-fixed to the underlay and the timber flooring fixed to the plywood.”

How much do rubber and cork underlays reduce impact noise?

Here are the published laboratory figures we could trace to a data sheet or test report for underlays under timber. All are ΔLw (improvement, higher is better) and, where given, Ln,w (the tested floor’s level, lower is better).

Sources: REGUPOL tests RG110 and RG114, the sonus eco 5 data sheet, test RG018, the Amorim T66 data sheet, the DAMTEC brochure and CSIRO report INR289-16-1. DAMTEC says its brochure figures are its “maximum tested values”.

Three things stand out. Most results sit between 16 and 20 dB. The big number, 27 dB, came from a much deeper build-up with a particleboard layer. And the bare slabs in these labs measured around Ln,w 80 dB, far worse than a real apartment with a ceiling, so these are comparisons between products, not predictions for your unit.

Is glue-down or floating timber quieter?

In the one matched pair we found, floating was slightly quieter. REGUPOL had CSIRO test the same 14 mm engineered board on the same 5 mm sonus eco 5 underlay both ways. Floating gave Ln,w 59 dB (ΔLw 17 dB). Glued to the underlay and the underlay glued to the slab gave Ln,w 62 dB (ΔLw 16 dB). That is one product and one board, so treat it as a hint, not a rule.

Glue-down is the ATFA’s system for parquetry and direct-stick engineered boards. The catch is warranty. The ATFA says “When fixing directly to the acoustic underlay material, flooring manufacturer warranties and performance warranties need to be considered as this often falls outside of published floor installation practices.”

What about solid floorboards on battens or plywood?

Solid strip boards need a nailing base. In the ATFA’s apartment system, plywood is glued to the acoustic underlay and the boards are fixed to the plywood, so no nail reaches the slab. That matters: the ABCB handbook warns that a resilient rubber underlay must not be “bridged” by nails and screws running into the slab, because they “diminish its performance”.

If an installer proposes battens instead of a sheet, ask for a test report for that exact batten, pad and board system, and check how the battens are held down without fixings through the underlay.

Will the new floor be too high at doors and stairs?

It can be. The REGUPOL sonus curve 17 test build stacked 19 mm boards on 18 mm yellow tongue on a 17 mm rubber mat. That is about 54 mm before adhesive, enough to catch doors, clash with a tiled bathroom and make a step at every threshold. REGUPOL markets its thinner 6 mm sonus curve 6 as “suitable for constructions with limited floor heights”, and publishes ΔLw 23 dB for it under T&G timber on yellow tongue (product page).

The ABCB handbook asks designers to keep “sufficient ceiling height” and to avoid “an excessively high step” between carpet and hard floors. Before you choose an underlay, measure the height available at each door, the kitchen and bathroom thresholds, and the top and bottom of the stairs. Stairs are a separate detail; ask the timber maker how treads and nosings are fixed, and whether the underlay continues onto them.

Moisture: what does timber need on a slab?

Timber takes up moisture. The ATFA explains that if moisture vapour beneath a floor “is too high … then it will cause the flooring to absorb the moisture”, which can lead to cupping. It also warns that “Moisture vapour barriers are not ‘water proofing membranes’ and they will allow a small amount of moisture vapour transmission through them.”

So the slab has to be dry enough for the flooring system first. Ask the timber maker for its moisture limit and required barrier, and ask the underlay maker whether its product can sit on, or be glued over, that barrier.

Installation basics for timber on acoustic underlay

• Keep every edge off the walls. The ATFA explains that a floor hard up against walls lets noise travel through the walls into the next unit. For fixed floors it describes skirting set 2 to 5 mm above the floor with flexible sealant; for floating floors, “Do not use a sealant at skirtings” and keep a gap.
• Check flatness. The ATFA gives no more than 3 mm under a 1.5 m straight edge for adhesive and mechanically fixed floors, and 3 mm under a 1.0 m straight edge for floating floors other than laminate.
• Use the adhesives the underlay maker names. REGUPOL’s bonded test used its own one-part polyurethane adhesive.
• Get the whole system in writing. The ATFA says “both the flooring manufacturer and the acoustic underlay supplier need to be fully accepting of the installation method used”.

What to check on the data sheet

• Is the test result for timber, and for your fixing method: floating, glued, or on plywood?
• Laboratory ΔLw and Ln,w with the standard (ISO 10140-3 and ISO 717-2, or older ISO 140-8) and the lab named
• Full build-up: board type and thickness, any sheet layer, slab thickness, ceiling
• Total height of the finished floor
• Density or weight per square metre, and any compression and recovery figures
• Which adhesives and moisture barrier the maker approves
• Written acceptance of the system from the timber maker and the underlay supplier

For the wider picture across every floor type, read the acoustic underlay hub. For how timber compares with other floors in an apartment, see our guide to acoustic flooring.