Acoustic underlay for tiles: sound, height and wet areas

A tiled floor can make an apartment feel composed and continuous. Beneath that finish, the acoustic mat, adhesive and substrate need to work as one specified system.

The detail becomes particularly important at bathrooms, thresholds and doors. Acoustic performance cannot be considered separately from waterproofing, support or finished height.

Why is tile the loudest floor on a concrete slab?

A tile bonded to a slab adds a hard layer and no cushion. When a heel or a dropped pan hits it, the energy goes straight into the concrete and comes out as sound in the apartment below.

The on-site numbers show how large the gap is. In 18 Kingsford Street [2016] QBCCMCmr 78, an owner replaced carpet with porcelain tiles laid without acoustic underlay. An acoustic engineer measured LnT,w 75 to 80 in the unit below. The adjudicator ordered the owner to fix it with carpet and underlay, or with “new tiles laid over an appropriate acoustic membrane”.

In Body Corporate for Admiralty Towers II v Johnson [2017] QCATA 29, the tribunal recorded carpeted areas in an apartment tower at about LnT,w 28 and tiled kitchens at about 61 to 65. It also noted that a 10 dB difference is typically heard as halving or doubling the sound. Lower LnT,w is quieter, and these are field results, measured in real buildings.

How does an acoustic mat under tiles work?

The mat is a resilient layer that separates the tile, or a screed, from the slab. It absorbs part of each impact before it reaches the concrete. It only works if nothing hard bridges around it, so makers ask for gaps at the walls. ARDEX TB147 asks for a gap of at least 6 mm at walls and protrusions “to allow total isolation of the tiles from the adjacent building structure”, and the gap is filled with silicone.

There are two ways to build it:

1. Mat under the tile adhesive (direct stick). The mat is glued to the slab and the tile is adhered to the mat. This keeps the floor low.
2. Mat under a screed. The mat goes on the slab, a sand and cement screed goes on the mat, and the tile goes on the screed. This is higher and heavier, and it tested better in the reports below.

What acoustic underlay is used under tiles in Australia?

These are the products we found with an Australian data sheet that shows a tile test. ΔLw is the laboratory improvement over the bare slab, so higher is better. Ln,w is the laboratory result for the whole floor, so lower is better.

Sources: Damtec Standard tiling brochure, Damtec under-screed brochure, REGUPOL sonus multi 4.5, sonus core 5 and sonus core 10-S technical data, the ARDEX DS 40 datasheet and Embelton’s ImpactaMat tile brochure. REGUPOL also lists sonus curve 6, a 6 mm profiled mat for screeds, at ΔLw 24 dB under tile and screed.

Read the table with care. Damtec tested on a 140 mm slab and REGUPOL on a 150 mm slab, in different laboratories. A 1 dB gap between two reports does not make one product better. ARDEX also quotes a “field result 18 dB” with its X 56 adhesive, but the datasheet does not say which building it came from. If you want to compare the two biggest names directly, see Regupol vs Damtec.

The National Construction Code (NCC 2025, F7D5) sets a laboratory Ln,w of 62 or lower for a floor between apartments in a new Class 2 or 3 building. Your by-law may ask for a field LnT,w number instead, which includes your ceiling and your building. Our guide to acoustic ratings explained shows how the two differ.

Does a thicker mat make tiles quieter?

Not under bonded tile, going by the published tests. In Damtec’s series under 9 mm ceramic, 2 mm gave ΔLw 18 dB and 3 mm and 4 mm gave 17 dB. Under a screed, thickness helped in REGUPOL’s series: 21 dB for the 5 mm mat and 23 dB for the 10 mm mat.

Thicker mats can also move more under a tile. MAPEI’s work method statement C06 recommends a screed over the mat “especially for acoustic matting thicknesses above 4.5 mm which are usually more compressible”. Damtec asks for a trial installation before its Multi mat at 4 mm or more goes under direct-stick tile (Damtec Multi tiling brochure).

Can you use acoustic underlay under tiles in a bathroom?

Often, but the waterproofing has to be designed with the mat, not around it. MAPEI’s C06 statement cites AS 3740-2021, the standard for waterproofing domestic wet areas, and asks for a screed with falls to it. Each maker then shows where the membrane goes:

• ARDEX DS 40. TB147 gives two options. In one, the screed to falls goes down first, the mat on top, then the membrane over the mat. In the other, the mat goes on the slab, the screed over it, then the membrane on the screed. The datasheet says it suits domestic bathrooms, but not permanently wet areas or public bathrooms and toilets.
• Vibramat. The Acoustic Supplies wet area detail puts the membrane on the screed, above the mat, to stop efflorescence from the sand and cement. It asks for a bond breaker at the floor to wall junction and notes that acoustic engineers normally require a 10 mm gap between screed and wall.
• DAMTEC standard. KRAIBURG’s data sheet says it suits tiles in wet areas “provided an appropriate waterproofing system is used”.
• ImpactaMat. Embelton says the mat is porous and recommends a moisture barrier in wet areas.

Crack isolation and tile limits

Some mats also act as a crack isolation layer. The ARDEX DS 40 datasheet says it bridges cracks up to 1 mm and carries loads up to 2 kN/m². It also sets tile limits: at least 15 x 15 cm, stoneware at least 8 mm thick and natural stone at least 15 mm thick, with movement joints carried up through the tiles.

Damtec’s tiling page says its Multi mat, at about 970 kg/m³, gives a more rigid base that reduces the risk of grout cracking. That is the maker’s claim, not a test result.

How much height does acoustic underlay add under tiles?

With a direct-stick mat, you add the mat and its adhesive. ARDEX DS 40 is 4.5 mm, and TB147 asks for a tile adhesive bed of at least 3 mm over it. Damtec Standard is 2 or 3 mm.

A screed system adds much more. REGUPOL’s tests used a 30 mm screed. Damtec’s under-screed system is for screeds “greater than 35mm thick at waste” and was tested with 60 mm. Check door clearances, thresholds and your shower falls before you choose.

What to check on the data sheet

• A tile test, not only a timber or vinyl test, with the tile thickness stated
• Slab thickness and whether a ceiling was present
• Lab (ΔLw, Ln,w) or field (LnT,w) result, and the report number
• The approved tile adhesive and trowel notch
• The maker's wet area detail, if the room has a floor waste
• Perimeter isolation: the gap size and the sealant
• Minimum tile size and thickness

The AAAC’s explanatory note warns that no underlay on its own can be guaranteed to reach a rating in your building. If the by-law number is tight, ask an acoustic consultant about a sample test on site. Our strata flooring rules guide explains the approval steps.

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