Underlay and vapour barrier – what goes under the floor?

A floating floor, meaning laminate or click parquet, always needs an underlay. On concrete it also needs a vapour barrier (a damp-proof membrane) to stop moisture coming up from below. On a dry timber floor between two heated storeys, the underlay is usually enough. Here is what applies to your subfloor, which underlay to choose and when you can lay straight over the old floor.

You are laying onVapour barrier?Underlay
Concrete or screed, even old, and basement floorsYesFoam or acoustic underlay on top of the membrane
Aerated concreteYesAs above
Timber floor over a crawl space, boiler room or utility roomYesAs above
Underfloor heatingYesThin underlay approved for underfloor heating
Floorboards or chipboard between two heated storeysUsually notFoam or acoustic underlay
Vinyl, lino or tilesSame as for the floor underneathFoam or acoustic underlay
CarpetTake the carpet up first

Always follow the instructions for your floor if they say something different. Click vinyl often has its own rules, so read the pack.

Do I need a vapour barrier?

Concrete gives off moisture for years, even when it feels dry. That is why Kährs requires a vapour barrier on a ground-bearing concrete slab, aerated concrete, floors over a ventilated crawl space, floors over warm or damp rooms such as a boiler room or utility room, and on underfloor heating, however old the subfloor is. Pergo requires a moisture barrier on every mineral subfloor and always on underfloor heating. Tarkett says the same: a vapour barrier on concrete and wherever you suspect damp.

On a timber floor between two heated storeys you usually don’t need one. Kährs writes that moisture protection is normally not needed on floors with a relative humidity below 60%.

New concrete or screed always gets a vapour barrier and must be properly dry first. Above 90% relative humidity a plastic sheet is not enough, and the damp has to be dealt with before you lay the floor. If in doubt, have the moisture measured.

How to lay the vapour barrier

If you choose an underlay with a built-in vapour barrier (often sold as “with DPM”), you don’t need a separate sheet. Lay it edge to edge and tape the seams, or as the manufacturer says.

Which underlay should I choose?

The underlay evens out small bumps and reduces noise. Two kinds of sound are worth telling apart: impact sound is what the neighbours below or next door hear, and walking sound is the clatter you hear yourself in the room. Underlays often list both.

Three rules that apply to almost every floor:

  1. Never two layers of underlay. The floor gets too soft and the click joints can break.
  2. If the planks have an underlay attached, don’t add another, but you still need the vapour barrier on concrete.
  3. Thicker is not better. A soft underlay makes the floor bounce. Underlay doesn’t fix big dips; levelling compound or sanding does.

Underfloor heating: choose a thin underlay approved for it. Haro says the floor and underlay together should ideally not insulate more than about 0.15 m²K/W, or the heat won’t get through.

Can I lay over the old floor?

How flat must the subfloor be?

Lay a straight batten or straightedge on the floor and measure the gap underneath. Kährs accepts at most ±3 mm under 2 m, ±2 mm under 1 m and ±1.2 mm under 25 cm. Pergo allows at most 2 mm under 1 m. Sand down high spots and fill or level dips before you lay the membrane and underlay. An uneven subfloor is the most common reason a floor creaks, opens up at the joints or breaks its click locks.

Draw your room and Parquet Planner works out how many square metres of underlay and vapour barrier you need, along with planks, packs and skirting.

Plan my floor

Frequently asked questions

Do you need a vapour barrier under laminate flooring?

Yes on concrete and screed, however old, on aerated concrete, on floors over a crawl space or over a boiler or utility room, and always on underfloor heating. On a dry timber floor between two heated storeys you usually don’t. Use 0.2 mm polythene with at least 20 cm overlap, or an underlay with a built-in vapour barrier.

Which underlay should I use under laminate?

In a house, 2–3 mm foam is usually enough. In a flat, choose an acoustic underlay, because it reduces the noise reaching the neighbours below. If the planks already have an underlay attached, don’t add another. Never lay two layers of underlay.

Can you lay laminate over vinyl?

Yes, if the vinyl is stuck down, undamaged and flat. If it sits on concrete, you still need a vapour barrier. Carpet and other soft coverings must always come up.

How flat does the subfloor need to be?

Kährs specifies at most ±2 mm under a 1 m straightedge and ±3 mm under a 2 m straightedge. Pergo specifies at most 2 mm over 1 m. Sand down high spots and fill dips before you lay the underlay.

Does the vapour barrier go above or below the underlay?

Below. The polythene goes straight onto the concrete with at least 20 cm overlap and taped seams, and the underlay is rolled out on top.

More guides

Sources

Updated 8 October 2026.