Short answer: yes, in almost every Australian install you’ll want an underlay beneath an engineered timber floor. The exception is engineered planks that ship with a pre-attached acoustic backing, and even then the subfloor type often dictates an extra moisture barrier. This guide covers when underlay is essential, when it’s optional, and what spec to ask for based on your subfloor and install method.
What underlay actually does
Underlay is the thin layer that sits between your subfloor and the engineered planks. It does three jobs at once:
- Moisture barrier. Stops vapour rising from a concrete slab into the timber, which is the number-one cause of cupping and edge swell in engineered floors laid on slab.
- Acoustic dampening. Cuts impact noise — footfall, dropped cutlery, dog claws — both within the room and to the room below in apartments.
- Subfloor smoothing. Bridges minor undulation up to about 3 mm over a 2 m span, so small imperfections don’t telegraph through the click joints.
Underlay does not fix a bad subfloor. Anything outside that 3 mm tolerance needs self-levelling compound before you start laying.
Subfloor decides the spec
Concrete slab
Most ground-floor Australian homes are on slab. Concrete carries moisture for years after pour, and even old slabs cycle moisture with the seasons. Use an underlay with a built-in moisture barrier — typically a foam or rubber underlay laminated to a 200-micron polyethylene film, taped at the seams. Skip this layer and the manufacturer’s warranty against cupping and delamination is voided. We’ve covered the broader picture of laying flooring on a concrete subfloor if you want the full story.
Plywood or particleboard
Timber subfloors don’t carry the same vapour load as concrete, so the moisture barrier is less critical. A standard 2-3 mm acoustic foam underlay is usually enough. If the subfloor is over an unventilated crawlspace or the timber feels damp to the touch, add a moisture barrier anyway — the cost is trivial compared to lifting a finished floor.
Existing tile or vinyl
If the existing floor is sound, level and well-bonded, you can float over it. Use a thinner underlay (around 2 mm) so you don’t lift door clearances and skirting reveals more than necessary. Check the height of dishwashers and fridges in cabinetry before committing — adding a floating floor on top of an existing one can raise the finished height by 12-15 mm.
Install method matters too
Floating install (click-lock)
This is the most common method for engineered timber in Australia and the one where underlay is non-negotiable. The planks click together using a 5G or 2G system (here’s our explainer on 5G and 2G click systems) and float as one sheet over the subfloor. Without underlay you get hollow-sounding footfall, joint failure over minor undulation, and no moisture protection.
Glue-down install
Engineered planks bonded directly to the subfloor with a flexible polyurethane adhesive don’t use a separate underlay — the adhesive itself acts as the moisture barrier and acoustic layer. This is the better-performing install method on slab where acoustic transfer matters (apartments) or where wide planks need maximum dimensional stability. It costs more in labour but eliminates the hollow feel of a floating floor.
Secret-nailed install
Used over plywood or batten subfloors. A red rosin or building paper layer goes between the subfloor and the planks to reduce squeaks and provide a minor moisture buffer. Conventional foam underlay isn’t compatible with nailed installs — the nails won’t bite reliably through soft underlay.
Pre-attached underlay: convenient, but check the spec
Some engineered ranges come with an IXPE or cork backing already bonded to each plank. That covers the acoustic layer, but it’s usually only 1-1.5 mm and doesn’t include a moisture barrier. On slab you’ll still need to roll out a polyethylene moisture film underneath. On timber subfloors the pre-attached backing is generally enough on its own.
What we recommend, in plain terms
Floating engineered floor on slab: foam-plus-moisture-barrier underlay, around 2 mm thick. Floating engineered floor on timber: standard 2-3 mm acoustic foam. Apartment with a body-corporate acoustic spec: a rated acoustic underlay (often 5 mm rubber) or a glue-down install. Glue-down or nail-down: no separate underlay, but use the adhesive or rosin paper the manufacturer specifies. If you want a sense of where underlay sits in the overall budget, our engineered timber flooring prices guide has the m² costs broken out, and our underlay for vinyl flooring piece walks through how the same decision plays out for hybrid and SPC floors.
If you’re not sure which underlay matches your subfloor or your specific engineered range, bring the product spec sheet into our Sydney or Brisbane showrooms and we’ll match it to the right underlay on the spot.