How to Lay a Vapor Barrier
A vapor barrier is the thin polyethylene 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 vapor 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 vapor barrier in 5 steps
Decide whether you need a barrier at all
A vapor 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 wood floor on an upper story 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 calcium carbide moisture test and wait until it is below the limit for your floor type. Only cover it once the reading is dry enough.
Lay the polyethylene film across the floor
Roll the film out over the whole floor and turn it up a couple of inches 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 baseboard will sit.
Overlap the sheets and tape them airtight
Lay each new length so it overlaps the last by about 8 in, then seal the seam along its full length with the tape made for it. A vapor barrier only works as one closed skin – keep the film free of punctures and tears, and patch any nick before you cover it.
Install the underlayment and floor, then trim the film
Put the impact-sound underlayment and the floating floor down on top of the sealed film. Leave the turned-up edges standing until the very end; once the baseboards 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 feet 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 vapor 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 wood 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 8 in and tape the seam along its full length, so the barrier forms one airtight skin. Turn the film up a couple of inches at the walls as well. Remember the overlaps when you buy – they add a little to the bare floor area from the planner.
- My underlayment already has a barrier built in – do I still need a separate film?
- Some underlayments come with an integrated vapor 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 polyethylene film underneath is the safer route.