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Vinyl Flooring Over Concrete: Moisture Testing, the 75% RH and 3 lb Numbers, Vapour Retarders, and What Grows Under a “Waterproof” Plank on a Damp Slab
Flooring & Tile
Vinyl Over Concrete: The Plank Is Waterproof, the Slab Is Not, and the Space Between Them Is Where the Trouble Lives
“Waterproof” on a box of vinyl plank means the plank will not swell or rot if it gets wet. It does not mean the floor is a vapour barrier for your basement, and it certainly does not mean the slab under it is dry. Lay a waterproof plank over a slab that is passing water vapour and you have made a lid. The vapour comes up, hits the plank, condenses, and sits there in the dark with the dust and the paper from the box. Three years later someone lifts a plank and finds black fuzz. The plank is fine. The floor is a write-off. Here is how to test the slab, what to put between, and what it costs.
How a slab gets wet, and stays wet
Concrete is a sponge with the texture of a rock. When it is poured it is full of water, and it takes months to give that up; the old rule of thumb is a month per inch of thickness in good drying conditions, and a basement in October is not good drying conditions. A 4 in. slab poured in September is still giving off water the following spring. After that it settles down to whatever the ground under it is doing. With 6-mil poly under the slab, which code has required for new houses for years and which I put under every slab I pour, the ground stays out and the slab dries to the house. With gravel and no poly, which is most houses built before the 1990s, the slab is a wick to the water table for the life of the house.
You cannot see any of this. A slab that passes a lot of vapour looks like a slab that passes none. The only way to know is to test it, and the test is cheap next to the floor.
The two tests and the two numbers
| Test | How it works | Pass for most vinyl plank | Notes |
|---|---|---|---|
| In-situ relative humidity probe (ASTM F2170) | A hole drilled to 40% of slab depth, a sleeve, a sensor left 24 hours, read in % RH | Under 75% RH | Measures the inside of the slab, not the surface. The one the manufacturers trust. Three probes for the first 1,000 sq ft, one more per 1,000 after. |
| Calcium chloride (ASTM F1869) | A dish of salt under a sealed dome for 60 to 72 hours, weighed before and after | Under 3 lb per 1,000 sq ft per 24 hours | Measures what the top inch is giving off right now. Older method, still on many sheets, fooled by a slab that was just mopped. |
| Plastic sheet test | A 2 ft square of poly taped down for 48 hours | Dry and no colour change under it | Free. Not a test anyone will warranty against. If it fails, do not bother with the others; if it passes, do one of them anyway. |
The room has to be at living conditions, 18 to 29 C and 35 to 55% RH, for 48 hours before and during the test, or the numbers are wrong. Glue-down products often have tighter limits than floating ones; read the sheet for the plank you bought, not the one on the shelf beside it.
What goes between, and what each costs
Taped 6-mil poly, $0.35 a square foot. Under a floating floor on a slab that passes the test, this is the standard: polyethylene across the whole floor, seams overlapped and taped, run up the wall an inch behind the baseboard. It stops the small amount of vapour a good slab passes from reaching the plank, so there is no condensation plane under the floor. Some planks come with a pad that claims to do this. A pad with a vapour film on it is better than nothing; it is not a taped sheet, and the manufacturer’s sheet will tell you whether they accept it alone.
Epoxy moisture mitigation, $4.00 a square foot. For a slab that fails the test and you still want a glued or floating floor on it. The slab is ground, a two-part epoxy is rolled on, and sometimes a skim of levelling compound goes over that. It works and it is warrantied. It also costs two thirds of the floor itself, so on a 700 sq ft basement it is $2,800 before the $4,200 of mid-grade vinyl. Before writing that cheque, find out whether the slab is damp from vapour, which the epoxy fixes, or from water, which nothing on top of the slab fixes. Why basements leak starts that conversation.
A raised panel subfloor, $3.50 a square foot. Lifts the floor off the slab on a plastic base with air channels, so the vapour has somewhere to go instead of into the plank. A good answer for a borderline slab that you do not want to coat, and a good answer for cold. The basement flooring page compares them.
Nothing. The commonest choice, because the plank is “waterproof”. On a new slab with poly under it that has had its drying time and passed a probe test, a glue-down floor with the right adhesive goes straight on and lives a long life. On anything else, nothing is the choice that produces the fuzz.
What the fuzz costs
A 700 sq ft basement, mid vinyl plank, floated on a damp slab with no poly to save $245. Three years in, the room smells and a corner plank comes up black underneath. Health Canada’s position on mould in a house is short: find the moisture, stop it, clean it up or take it out. So out it comes. Lifting a floating floor is $1.25 a square foot, $875; the planks are not reusable because the backs are contaminated. Treat and dry the slab, test it, poly at $0.35, $245. New planks, 770 sq ft at $3.25, $2,502.50. Labour again, 700 at $2.75, $1,925. Four transitions at $35, $140, and delivery $150. Total $5,837.50 before HST to fix a $245 omission, and that is before anyone has priced the drywall at the bottom of the walls.
The rules for a slab
- Test it. Probes, not opinions. Write the numbers down with the date and photograph the meter. This is the one piece of paper that settles a warranty claim later.
- Flatten it. 3/16 in. in 10 ft. Grinding is $2.00, self-leveller $3.00. Levelling costs.
- Clean it. No paint, no old adhesive, no curing compound, no dust. Glue does not stick to any of those and poly slides on them.
- Separate it or coat it. Taped poly under a floating floor on a passing slab; epoxy or a raised subfloor on a failing one; the manufacturer’s adhesive and nothing else under a glue-down.
- Leave the gap. 1/4 to 3/8 in. at every wall and post for a floating floor, with the poly behind the gap, not stopped short of it.
On the heated slabs in the houses I build the sequence is set at the start: poly under the slab, months of drying with the heat running, a probe test, then glue-down plank or tile straight on the concrete. The heated slab flooring page has the order of operations, and the temperature rule that goes with it is on the radiant compatibility article.
Questions people ask
Do I need a vapour barrier under vinyl plank on concrete?
Under a floating floor, yes: taped 6-mil poly at about $0.35 a square foot, unless the manufacturer accepts the attached pad alone in writing. Under a glue-down floor, no sheet, but the slab has to pass a moisture test for the adhesive.
How dry does a concrete slab need to be for vinyl flooring?
Under 75% relative humidity by the in-situ probe test, or under 3 lb per 1,000 sq ft per 24 hours by calcium chloride, for most products. Some glue-down planks and adhesives want 80% or 85%; a few want less. The sheet for your plank is the number.
Can mould grow under waterproof vinyl flooring?
Yes, and it is the commonest way a basement vinyl floor fails. The plank is unharmed; the dust, pad and slab surface under it stay damp and grow. The fix is removal, drying, a moisture test and a vapour retarder on the way back.
How long does a new slab take to dry before flooring?
Rule of thumb, a month per inch of thickness in decent conditions, so three to four months for a basement slab, longer in a cold, closed house. Test it rather than counting days; a slab with poly under it and heat in the house dries much faster than one without.
Should vinyl plank be glued or floated on concrete?
Floated with taped poly for a bedroom or rec room on a passing slab. Glued on a heated slab, in a long open room, or anywhere with rolling loads, with a slab that passes the adhesive maker’s moisture limit. What each costs.
Price the slab work with the floor
The flooring cost calculator carries poly, grinding, self-leveller, epoxy mitigation and the raised subfloor as line items so a basement number has the slab in it. If a quote you were given says “waterproof, no barrier needed”, the $99 review will say what I just said, in writing, with the page of the manufacturer’s sheet that agrees.
Sources and further reading
- Addressing moisture and mould in your home: Health Canada: what mould under a floor means, when it can be cleaned and when the floor comes out.
- Torlys: Canadian maker of engineered, vinyl and laminate floors; the installation and radiant-heat instructions are a good example of what the sheet in the box says.
- ICFPro.ca: the ICF foundations and slabs side of the business: what is under the floor.
