Short answer: most modern click-lock vinyl already has underlay pre-attached to the back of every plank, so adding a separate underlay underneath is usually wrong — and on some products it voids the warranty. You only need a separate underlay if your vinyl is bare-backed (no pre-attached pad), or if you need a moisture barrier over a concrete slab, or if you’re chasing extra acoustic performance in an apartment. Glue-down and loose-lay sheet vinyl never use underlay.
The first question: which type of vinyl are you laying?
The underlay answer changes completely depending on the install method. Three categories:
- Click-lock SPC hybrid or LVP — floats over the subfloor, planks click together at the edges. Almost always has IXPE or EVA foam underlay pre-attached. Do not add a second layer.
- Glue-down LVP — bonded directly to the subfloor with adhesive. Never uses underlay. The adhesive needs to stick to a hard, flat substrate.
- Loose-lay sheet vinyl — rolled out across the room, held by perimeter adhesive or its own weight. No underlay, but the subfloor needs to be very flat because every imperfection telegraphs through.
If you’re not sure which type you’ve bought, look at the back of a sample plank. A grey or black foam layer bonded to the underside is pre-attached underlay. A bare PVC back means it’s either glue-down or you’ll need a separate underlay.
Why doubling up on underlay is a mistake
Click-lock vinyl is engineered to sit on a precise amount of give. Add a second layer of foam or rubber and the planks flex too much at the joints. Three things go wrong:
- The click joints work loose under foot traffic, leaving visible gaps.
- Heavy furniture sinks unevenly, creating dips around chair and table legs.
- Most manufacturers list “installed over additional underlay” as a warranty exclusion, especially for SPC hybrid.
If your vinyl spec sheet calls for a maximum underlay thickness (commonly 1.5 mm IXPE), that’s the cap — not a starting point you add to. The pre-attached pad usually counts toward that limit on its own.
When you do need a separate underlay
Bare-backed click-lock vinyl
Some entry-level click-lock LVP doesn’t come with underlay attached. In that case, a 1-1.5 mm closed-cell foam or IXPE underlay rated for vinyl is the right pick. Avoid carpet underlay — it’s too thick and too soft, and it’s the single most common reason DIY vinyl installs fail.
Concrete slab subfloors
Concrete slabs release residual moisture for years after they’re poured, and even older slabs can wick moisture from the ground. If you’re laying click-lock vinyl over concrete, run a separate 200-micron polyethylene moisture barrier underneath, even if your vinyl already has pad attached. The moisture barrier is plastic film, not foam, so it doesn’t add cushioning. We’ve covered the full slab-prep process in our guide on how to lay vinyl on a concrete floor.
Apartments and acoustic ratings
Body corporates and strata bylaws often require a minimum impact-noise rating (commonly Ln,w 55 dB or lower) on hard floors above another dwelling. The pre-attached IXPE on most click-lock vinyl is around 1 mm and won’t hit that number on its own. A 5 mm rubber acoustic underlay — Regupol, 6 Star, or similar — is the usual fix, but you have to check that your vinyl manufacturer permits it. Some allow rubber underlay only with specific products; others void the warranty entirely. Get this in writing before you order.
Subfloor flatness still matters more than underlay
The most common reason vinyl floors fail isn’t the underlay choice — it’s the subfloor underneath. Click-lock vinyl needs a subfloor flat to within roughly 3 mm over a 2 metre span. Underlay won’t fix a wavy slab or a dipping particleboard sheet; foam compresses unevenly under load, and the joints fail at the low spots first. If your subfloor is out, level it with a self-levelling compound (concrete) or sheet patching (timber) before you start. Also leave the correct expansion gap at the perimeter — the expansion gap rules for loose-lay vinyl apply to most click-lock products too.
Quick decision guide
- Click-lock SPC or LVP with pre-attached pad, over timber subfloor: no separate underlay.
- Click-lock SPC or LVP with pre-attached pad, over concrete: 200-micron poly moisture barrier only, no extra foam.
- Bare-backed click-lock vinyl: 1-1.5 mm IXPE underlay rated for vinyl, plus moisture barrier on concrete.
- Apartment install needing acoustic compliance: rubber acoustic underlay, but only if the vinyl manufacturer approves it in writing.
- Glue-down LVP or loose-lay sheet vinyl: never use underlay. Focus on subfloor preparation instead.
For more on which vinyl product fits which room, our waterproof flooring guide covers wet-area performance, and the SPC vs RCB flooring breakdown explains the core differences that affect underlay choice. If you’re still not sure what’s right for your subfloor, bring a photo into one of our Sydney or Brisbane showrooms — we’d rather spend ten minutes checking before you order than rip the floor up afterwards.