Vapour barrier

How to Lay a Vapour Barrier

A vapour barrier is the thin PE film that keeps residual moisture in a screed or concrete slab from reaching a floating floor. This guide shows when you need one and how to lay it right – and how to work out how much film to buy for your room.

Work out your floor area first →

A vapour barrier is cheap, but a floor laid over trapped moisture is a write-off, so it pays to get this layer right. First find out whether your subfloor even needs one, then buy enough film to cover the room with generous overlaps. Draw your room in the free planner to get the exact floor area, add the overlap and follow the five steps below.

Laying a vapour barrier in 5 steps

Decide whether you need a barrier at all
A vapour barrier belongs on any mineral subfloor – screed or concrete – and on every ground floor or slab in contact with the ground, where moisture can rise from below. Over a dry timber floor on an upper storey you usually leave it out, so the construction can still breathe. When in doubt, add it.
Check the screed for moisture first
Never seal a subfloor that is still damp – the film would trap the moisture underneath and rot the floor from below. Have a fresh screed checked with a CM measurement and wait until it is below the limit for your floor type. Only cover it once the reading is dry enough.
Lay the PE film across the floor
Roll the film out over the whole floor and turn it up a few centimetres at every wall, so it forms a shallow tray that no moisture can creep around. Work it flat into the corners without stretching it tight, and let it run right up under where the skirting will sit.
Overlap the sheets and tape them airtight
Lay each new length so it overlaps the last by about 20 cm, then seal the seam along its full length with the tape made for it. A vapour barrier only works as one closed skin – keep the film free of punctures and tears, and patch any nick before you cover it.
Lay the underlay and floor, then trim the film
Put the impact-sound underlay and the floating floor down on top of the sealed film. Leave the turned-up edges standing until the very end; once the skirting boards are on, trim the film flush along the top so nothing shows.

Buy the right amount of film in one go

The free planner gives you the exact floor area of your room, so you know how many square metres of film to buy – just add a little for the overlaps and the turn-ups at the walls. No second trip to the shop.

Open the free planner →

Frequently asked questions

When do I need a vapour barrier?: You need one on any mineral subfloor – screed or concrete – and on ground floors or slabs sitting on the ground, where moisture can rise from below. On a dry timber floor upstairs you normally leave it out. If you can't be sure the subfloor is permanently dry, laying the barrier is the safe choice.
How much should the sheets overlap?: Overlap each length by about 20 cm and tape the seam along its full length, so the barrier forms one airtight skin. Turn the film up a few centimetres at the walls as well. Remember the overlaps when you buy – they add a little to the bare floor area from the planner.
My underlay already has a barrier built in – do I still need a separate film?: Some underlays come with an integrated vapour barrier, and on many subfloors that is enough on its own. But it only works if the overlaps are taped airtight, exactly like a separate film. On a damp-prone slab, or if you're unsure of the product, a dedicated PE film underneath is the safer route.

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Laying a vapour barrier in 5 steps

Decide whether you need a barrier at all

A vapour barrier belongs on any mineral subfloor – screed or concrete – and on every ground floor or slab in contact with the ground, where moisture can rise from below. Over a dry timber floor on an upper storey you usually leave it out, so the construction can still breathe. When in doubt, add it.

Check the screed for moisture first

Never seal a subfloor that is still damp – the film would trap the moisture underneath and rot the floor from below. Have a fresh screed checked with a CM measurement and wait until it is below the limit for your floor type. Only cover it once the reading is dry enough.

Lay the PE film across the floor

Roll the film out over the whole floor and turn it up a few centimetres at every wall, so it forms a shallow tray that no moisture can creep around. Work it flat into the corners without stretching it tight, and let it run right up under where the skirting will sit.

Overlap the sheets and tape them airtight

Lay each new length so it overlaps the last by about 20 cm, then seal the seam along its full length with the tape made for it. A vapour barrier only works as one closed skin – keep the film free of punctures and tears, and patch any nick before you cover it.

Lay the underlay and floor, then trim the film

Put the impact-sound underlay and the floating floor down on top of the sealed film. Leave the turned-up edges standing until the very end; once the skirting boards are on, trim the film flush along the top so nothing shows.

Frequently asked questions

When do I need a vapour barrier?

You need one on any mineral subfloor – screed or concrete – and on ground floors or slabs sitting on the ground, where moisture can rise from below. On a dry timber floor upstairs you normally leave it out. If you can't be sure the subfloor is permanently dry, laying the barrier is the safe choice.

How much should the sheets overlap?

Overlap each length by about 20 cm and tape the seam along its full length, so the barrier forms one airtight skin. Turn the film up a few centimetres at the walls as well. Remember the overlaps when you buy – they add a little to the bare floor area from the planner.

My underlay already has a barrier built in – do I still need a separate film?

Some underlays come with an integrated vapour barrier, and on many subfloors that is enough on its own. But it only works if the overlaps are taped airtight, exactly like a separate film. On a damp-prone slab, or if you're unsure of the product, a dedicated PE film underneath is the safer route.

How to Lay a Vapour Barrier

Laying a vapour barrier in 5 steps

Decide whether you need a barrier at all

Check the screed for moisture first

Lay the PE film across the floor

Overlap the sheets and tape them airtight

Lay the underlay and floor, then trim the film

Buy the right amount of film in one go

Frequently asked questions

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How to Lay a Vapour Barrier

Laying a vapour barrier in 5 steps

Decide whether you need a barrier at all

Check the screed for moisture first

Lay the PE film across the floor

Overlap the sheets and tape them airtight

Lay the underlay and floor, then trim the film

Buy the right amount of film in one go

Frequently asked questions