Underlay is the part of a flooring job nobody gets excited about, and the part that quietly decides how the floor sounds, feels and lasts. There is no single best underlay. It depends on the floor you are laying, the subfloor under it, and what you want: quieter rooms, softer footfall, protection from moisture, or a floor that stays flat and tight for twenty years.

The short version

  • Check the subfloor for moisture first. Concrete slabs must be sealed, every time, for every floor type.
  • Boards with attached underlay need no other underlay. Just a builder's plastic moisture barrier on concrete.
  • Laminate and floating engineered timber need a separate underlay, ideally a 2mm foam with a foil barrier built in.
  • Glue down floors get no underlay at all. The moisture protection is a roll on sealer under the adhesive.
150μmMinimum polyethylene barrier ATFA recommends under hybrid on concrete
200μmThickness of our Moisture Barrier Film, tougher to puncture on site
2%Maximum moisture meter reading on a slab before any floor goes down
1Underlay layer, ever. Two underlays under a click floor voids the warranty

The quick answer, by floor type

Floor Underlay Moisture barrier
Hybrid (SPC) with attached underlay None. It is already on the board. Yes on concrete: 200μm film over the whole slab.
Laminate 2mm foam, ideally foil backed. Built into the foil underlay, or film under plain foam.
Engineered timber, floating 2mm foam or acoustic underlay. Film under the underlay, or a 2 in 1 foil underlay.
Engineered timber, glued down None. Roll on barrier or primer sealed over the slab.
Vinyl plank, glued down None. Roll on sealer before the adhesive.

What underlay actually does

Four jobs. Different underlays prioritise different ones, which is why there are so many on the shelf.

  • Sound. Softens footsteps travelling to the room below and kills the hollow click inside the room. In apartments this is the whole reason strata rules specify a rating. Our 5 star acoustics range exists for exactly that.
  • Comfort. A couple of millimetres of foam takes the hardness and the cold out of a slab. You feel it in bare feet and in your knees at the kitchen bench.
  • Moisture protection. Slabs release vapour for years. A film or foil layer stops it reaching the boards. Without it laminate swells, timber cups, and hybrid weakens over time.
  • Longevity. Even support under the click joints means less flex, fewer creaks and less wear on the locking system. Underlay is part of caring for the floor, laid once, before the first board.

Rule one: check the subfloor for moisture

Before anything else

A slab that feels dry can still push enough vapour to swell a laminate board or cup a timber one. You cannot tell by looking. Hire a moisture meter for the day and seal every concrete slab. A thicker barrier is not a fix for a wet slab: find the water first.

  • Concrete: 2% or under on the meter (3.5% if previously covered), relative humidity under 75%, and new slabs cured for at least 60 days.
  • Timber, ply and particleboard: 8% or under (10% if previously covered), underfloor space ventilated, anything loose screwed down.
  • Existing tiles: fine under a floating floor if none are loose. Fill grout lines deeper than 3mm.

Flatness matters just as much as moisture. Our subfloor preparation guide covers levelling.

Boards with attached underlay

Every board in our hybrid range has a built in IXPE underlay, and it is spreading to premium laminate too. It is the better system: the underlay is matched to the board, it cannot bunch or shift, and there is one less thing to lay. It changes the shopping list in two ways.

  1. No other underlay. Foam under a board that already has underlay is the most common mistake we see. Two soft layers let the joints flex, the click system works loose, and the warranty is void.
  2. Still a moisture barrier on concrete. Attached underlay handles sound and comfort but is not a vapour seal. Lay 200μm builder's plastic over the slab, tape the joins, then lay the boards straight on top.

What ATFA found when it tested hybrid boards

The Australasian Timber Flooring Association ran independent testing on 22 hybrid products. It matters because hybrid is often sold as if it needs nothing underneath it.

Although hybrid products generally have low moisture absorption, moisture may still penetrate the boards and reach the stone fillers. In high alkalinity environments, such as new concrete slabs, prolonged moisture exposure may weaken the internal structure of the boards and potentially lead to issues such as warping or deformation. For this reason, the industry currently recommends installing a minimum 150μm polyethylene moisture barrier during installation.

In plain terms: hybrid is water resistant, not immune. Spills on top are fine. Years of vapour from underneath are not, and a new slab is the highest risk because it is damp and alkaline at the same time. Cure time plus a barrier is the answer.

The two products that cover every floating floor

Moisture Barrier Film 200 micron roll
Builder's plasticMoisture Barrier Film 200μmPolyethylene film for every concrete slab. Exceeds the 150μm ATFA minimum. 100m² roll, enough for a typical three bedroom home. Use under hybrid on its own, or under any foam underlay.View product
Silver Foil Foam Underlay 2mm roll
2 in 1 underlaySilver Foil Foam Underlay 2mmDense 2mm foam with a reflective foil moisture barrier. For laminate and floating engineered timber: cushion, sound and vapour protection in one layer. 20m² roll.View product

Underlay for hybrid flooring

The simplest case. Dry slab, moisture barrier film, boards. On a timber subfloor lay the boards directly, as long as it is dry, flat to 3mm over a metre and screwed down. Over tiles the attached underlay is enough on its own. Laying upstairs or in an apartment with an acoustic requirement? Ask us for the acoustic report on the range you like. The hybrid installation guide shows where the barrier goes.

Underlay for laminate flooring

Most laminate has nothing attached, so it needs a 2mm foam underlay with foil: cushion and echo control from the foam, vapour protection from the foil, one product. Foil side down, edges butted (never overlapped), joins taped, sheets running the same way as the boards. Never use carpet underlay: it is far too thick and soft and the joints fail. More in the laminate underlay guide and the laminate installation walkthrough.

Underlay for engineered timber

Real wood on top, so the most moisture sensitive of the floating floors. The underlay depends on how you fix it.

  • Floating: 2mm foam or acoustic underlay with a vapour barrier under it, or a 2 in 1 foil foam that does both. Overlap film joins 200mm, tape them, run the film 100mm up the wall. In apartments, strata usually demands an acoustic grade, so check the rating before you order.
  • Glued down: no underlay. The board is bedded in flexible timber adhesive; moisture protection is a roll on barrier sealed over the slab first. The only method over hydronic in slab heating.
  • Secret nailed over joists or ply: no underlay.

Every engineered timber product page has a downloadable installation guide for both methods, and the hybrid vs engineered timber vs hardwood comparison helps you pick.

Underlay for vinyl plank

Glue down vinyl goes straight onto the subfloor in adhesive, so no underlay. It needs a perfectly smooth, sealed surface because every grain of grit shows through a thin plank: roll on sealer on concrete, then a skim of levelling compound if the slab is rough. No floating joints and no underlay is also why glue down vinyl is the floor we recommend for bathrooms and wet areas.

Underlay by situation

  • Apartments and two storey homes. Get the body corporate's acoustic requirement in writing and match the underlay or the board to it. See best flooring for apartments and the apartment flooring collection.
  • Underfloor heating. Underlay insulates, which is the opposite of what a heated slab wants. Most floating floors are not warranted over it at all; the ones that are usually limit it to hydronic systems under a glued down floor. Check the product warranty and ask us.
  • Over tiles or old timber. A floating floor bridges both. Tiles over a slab still need the film, because tiles are not a vapour seal. Checklist in laying hybrid over existing floors.
  • Trades still coming through. A 2mm flooring protection layer over the finished floor stops paint, grit and dropped tools marking it. It comes up when the job is done.

Thickness, density and foil

Thicker is not better. Click floors are designed around a thin, dense underlay of 1.5 to 3mm. Go thicker or softer and the boards flex at every joint until the locking profile wears and opens. What you want:

  • Density: firm foam that barely compresses. If it feels like packaging foam, it is too soft.
  • Thickness: 2mm for laminate and floating timber. Attached IXPE on hybrid is 1 to 1.5mm and matched to the board.
  • Foil or film: anything over concrete needs a vapour barrier, built in or laid separately. Never foam alone.
  • Acoustic rating: if you need one, buy a tested rating and keep the report. "Sound reducing" with no number is not a rating.

How to lay a moisture barrier and underlay

  1. Vacuum twice. Grit under a barrier punctures it, and grit under an underlay telegraphs through thin boards.
  2. Roll out the film. Whole room, joins overlapped at least 200mm and taped their full length, and 100mm up every wall. Trim it flush after the skirting goes on.
  3. Tape around pipes and posts. The barrier only works if it is continuous.
  4. Roll out the underlay (laminate and floating timber only), same direction as the boards, edges butted with no overlap, seams taped.
  5. Lay only what you will cover that session so it is not walked on and torn while exposed.
  6. Lay the boards. Never fix the film or the underlay to the subfloor. The whole system floats.

Boards with attached underlay: stop after step 3 and start laying.

How much to order

Floor area of each room plus 10% for overlaps, cuts and the run up the walls. The Moisture Barrier Film is a 100m² roll and the Silver Foil Foam Underlay is 20m², so divide your total by 20 and round up. The room measuring guide has a worked example.

Do and do not

Do

  • Meter the slab, then seal it, every time.
  • Overlap film 200mm and tape every seam.
  • Run the film 100mm up the walls.
  • Butt foam underlay edges and tape them.
  • Match the acoustic rating to what strata asks for.

Do not

  • Lay a second underlay under a board that has one attached.
  • Trust "it felt dry". Feel is not a moisture reading.
  • Use carpet underlay under any hard floor.
  • Put underlay under a glue down floor.
  • Leave gaps around pipes and posts in the film.

Frequently asked questions

Do I need underlay under hybrid flooring?

Not a separate one. Our hybrid boards have IXPE underlay attached. On concrete you still need a polyethylene moisture barrier film underneath, which ATFA recommends at a minimum of 150μm.

Can I lay hybrid or laminate straight onto concrete?

Only with a moisture barrier between the slab and the floor. Concrete releases vapour for years, and new slabs are alkaline as well, which is what weakens boards over time.

Is a thicker underlay better for sound?

No. Density matters more than thickness. A dense 2mm underlay outperforms a soft 5mm one, and the thick one damages the click joints.

Does underlay make the floor warmer?

A little. Foam and foil slow the cold of a slab coming through the boards, which you notice in bare feet in winter. It is not a substitute for insulation.

Do I need underlay on a timber subfloor upstairs?

Laminate and floating engineered timber, yes, for sound and support. Hybrid with attached underlay, no. A moisture barrier is only needed if the underfloor space is damp or poorly ventilated.

What is the difference between builder's plastic and a moisture barrier film?

Builder's plastic is the generic name. What matters is the thickness (150μm minimum, ours is 200μm), that it is polyethylene, and that it is continuous and taped. Thin painter's drop sheets do not count.

Keep reading

Everything here is in stock in the underlay and moisture barrier range. Order free samples of the floor first and feel the attached underlay for yourself, or send us a photo and a moisture reading and we will tell you what your slab needs.