Concrete never really stops drying. A slab poured in 1978 is still moving water vapour upward today, pulled from the soil it sits on, and it will keep doing that for as long as the house stands. Put a floor on top without accounting for it and the vapour does not go away — it collects at the one interface where it does the most damage.
This is the question behind most cupped basement floors, most failed adhesive, and a good share of denied warranty claims in Ontario. It is also entirely solvable, and cheaply, as long as it is decided before the floor is ordered and not after.
Table of Contents
- The Short Answer
- What a Vapour Barrier Actually Does
- When You Definitely Need One
- When You May Not
- Vapour Barrier Options Compared
- How to Test a Slab Before You Buy Anything
- Where Vapour Barriers Go Wrong
- Getting It Right
- Related Guides
- Talk to a Consultant Before You Install
- Frequently Asked Questions
The Short Answer
If the flooring is going over concrete, assume you need vapour management until a test says otherwise. Below grade, slab-on-grade, and concrete condo subfloors all qualify. The only common exception is a plywood subfloor above a conditioned space.
For most Ontario basements, a dimpled membrane with a subfloor panel over it is the right specification — it handles vapour, adds a thermal break, and gives you a flat surface to work from. Where ceiling height rules that out, 6-mil polyethylene taped continuously at the seams is the minimum that most floating-floor manufacturers will accept.
Whichever you use, the number that decides it comes from a test, not from how the slab feels underfoot.
What a Vapour Barrier Actually Does
A vapour barrier is a thin layer — polyethylene sheet, a foil-faced underlayment, or a peel-and-stick membrane — that stops water in its gaseous form diffusing up into the flooring assembly. It is not waterproofing. Waterproofing resists liquid water under pressure. A vapour barrier deals with something quieter and more continuous.
Ontario slabs commonly emit somewhere between 3 and 8 pounds of moisture vapour per 1,000 square feet over 24 hours on a calcium chloride test, decades after curing. Engineered hardwood, laminate and several vinyl products set limits below the top of that range. The gap between what the slab does and what the product tolerates is the whole reason this layer exists.
Vapour Barrier, Moisture Barrier, Waterproofing: The Terms Are Not Interchangeable
The trade uses these loosely and it causes real confusion at the point of sale. A moisture barrier usually means liquid water — a shower pan membrane, a moisture-barrier carpet pad. A vapour barrier means water as gas. Waterproofing is a system built to hold back hydrostatic pressure, which is foundation work, not flooring work. A basement floor typically needs the second, sometimes the first, and cannot be rescued by either if it actually needs the third.
Does the Ontario Building Code Require One Under a Floor?
The Code deals with vapour barriers in walls and ceilings far more directly than in floor assemblies. In practice the binding document is the manufacturer’s installation instruction, because installing against it is what voids the warranty. For engineered hardwood, laminate or vinyl plank over a slab, that instruction almost always specifies vapour management, and it will specify the maximum slab reading it is written for.
When You Definitely Need One
- Any below-grade slab — basements, walk-out lower levels, anything in contact with earth. Worst in spring, when the surrounding soil is saturated and the slab is at its most active.
- Slab-on-grade construction — common in newer bungalows and additions. A dry-feeling slab routinely tests above the threshold for floating floors.
- Concrete condo subfloors — including on upper storeys, where people assume height means dryness. Newer buildings can still be releasing construction moisture years in.
- Engineered hardwood or laminate over concrete, without exception. Both are moisture-sensitive from below, and skipping the barrier voids the warranty in nearly every case.
When You May Not
- Plywood subfloor above a conditioned space — second storeys, main floors over a heated basement. The assembly below is dry and the substrate manages moisture differently.
- Certain rigid-core vinyl products with integrated IXPE or built-in moisture protection, rated by the manufacturer for direct installation on concrete. Product-specific, and worth confirming in writing before ordering.
- Tile. A thinset and grout system manages moisture differently and does not want a sheet barrier under it. Crack-isolation and decoupling membranes are often recommended for tile over a slab, but for movement, not for vapour.
On that second item we would still put something under it in a basement. The products are rated as claimed, but the rating assumes a slab within a stated limit, and most people never test to find out whether theirs is.
Vapour Barrier Options Compared
| Option | Added height | Also insulates | Handles incidental water | Best for |
|---|---|---|---|---|
| 6-mil polyethylene sheet | None | No | No | Tight ceiling height, dry slab |
| Combination underlayment | 2–3 mm | Slightly | No | Laminate and click engineered |
| Dimpled membrane | About 1/4 inch | Slightly | Yes, laterally | Damp Ontario basements |
| Dimpled membrane plus panel | About 1 inch | Yes | Yes | Cold and damp basements |
| Liquid-applied or peel-and-stick | Negligible | No | No | Slabs testing above 8 lbs |
The default for a straightforward job is 6-mil poly: cheap, available anywhere, and specified by name in most floating-floor installation guides. Lap the seams six to eight inches, tape them with vapour-rated tape, and run it a couple of inches up the wall before the baseboard goes on.
Dimpled membranes are the step up, and below grade they are what we recommend. The moulded profile holds an air gap against the slab so vapour can move sideways and equalise, and stops pressing against the underside of the floor. Put a panel on top and you have solved vapour, warmth and flatness in one assembly — the subfloor options guide compares those systems directly.
Liquid-applied and peel-and-stick retarders are the answer for a slab testing above about 8 lbs per 1,000 sq ft per 24 hours. They cost considerably more and they are the right call when the number says so.
Bring your slab reading and the flooring you are considering to an appointment — matching the two is a five-minute conversation that prevents the most expensive mistake in a basement build.
How to Test a Slab Before You Buy Anything
Two methods, both giving a number you can hold against a spec sheet:
- Calcium chloride test — a sealed dish of desiccant left on the bare slab for 60 to 72 hours, then weighed. Reports in pounds per 1,000 square feet per 24 hours. Inexpensive, widely used, and measures the surface, not the depth.
- In-slab relative humidity probe — drilled into the slab to measure conditions inside it. More reliable, particularly on a thick slab or one that has been sealed, and increasingly what manufacturers specify.
Two caveats on both. The slab has to be bare and at service temperature, so testing through an old floor or in an unheated basement in January tells you very little. And a single reading is a snapshot — the same slab will read differently in March than in September, so a borderline result taken in the driest month deserves a second look before you commit to a moisture-sensitive product.
Where Vapour Barriers Go Wrong
- Discontinuity. Gaps, unlapped seams, a sheet stopping short of the wall. Vapour finds the weakest path, so a barrier that is 95 percent complete performs close to a barrier that is not there in the five percent that matters.
- Two impermeable layers with a gap between them. Poly over concrete, then a glue-down with its own impermeable backing, traps vapour at the interface. Adhesive failure and mould both start here. Match the barrier to the flooring; do not add one on principle.
- No test. Looking at a slab tells you nothing about its moisture state. This is the shortcut that turns into a replacement.
- Puncturing it during the rest of the build. A barrier laid early and then walked on by five trades is not the barrier that was specified.
The second one is the one people find counter-intuitive, and it is worth restating plainly: more barrier is not always better. Why basement flooring fails covers what each of these looks like when it eventually surfaces.
Getting It Right
- Test the bare slab, at service temperature, ideally not in the driest month.
- Get the installation instruction for the specific product you want and find its moisture limit.
- Choose the barrier to the gap between those two numbers, and to the ceiling height you can spare.
- Install it continuously, lapped, taped and turned up at the walls.
- Confirm you are not sandwiching vapour between two impermeable layers.
- Protect it until the floor goes down.
Everything that goes over the top of this is in Best Flooring for Basements in Ontario. Outside the basement, what you can install over concrete asks the same question of condos and slab-on-grade.
Related Guides
A few other guides connected to this topic:
- Best Flooring for Basements in Ontario: What Actually Works (and What Doesn’t) — The complete basement flooring decision pillar.
- Best Flooring for Damp Basements: Moisture Solutions That Work — When the slab test comes back high.
- Subfloor Options for Basements: DRIcore vs Plywood vs Foam-Backed Panels — How subfloor systems and vapour management combine.
- What Flooring Can You Install Over Concrete? (Basements, Condos & Slabs) — The companion guide for concrete anywhere in the house.
- Why Basement Flooring Fails (Moisture, Subfloors & Installation Mistakes) — What each failure looks like and how long it takes.
- Best Underlayment for Basement Flooring (Moisture & Concrete Guide) — The layer that sits above the barrier.
- Flooring Warranties Explained — Why the installation instruction is the binding document.
- Flooring Installation Services — What our installation work covers.
Talk to a Consultant Before You Install
The best way to choose what’s right for your home is to see the options side by side. Book an in-store appointment at one of our 17 Ontario showrooms and one of our consultants will walk you through what fits your space, your needs, and your budget — or request an in-home visit if that’s easier.
Frequently Asked Questions
Do I need a vapour barrier in a finished, heated basement?
Yes. Finishing and heating the room changes the air, not the slab. The concrete keeps releasing vapour year-round regardless of how dry the space feels, and the barrier is protecting the flooring assembly, not the room.
Can I add a vapour barrier under an existing floor?
Not without lifting it. The barrier has to sit between the slab and the floor. If the existing floor is already showing moisture damage, lifting it is the right answer anyway — and it is the moment to test the slab properly.
Can I just use plastic sheeting from the hardware store?
Yes. Six-mil polyethylene from any home centre is the same product the installation instructions name. What matters is that it is continuous, lapped, taped with vapour-rated tape, and turned up at the walls. Failures come from the execution, not the material.
Does a dimpled subfloor system replace a separate vapour barrier?
Usually. Dimpled panel systems include their own vapour retarder and add an air gap that handles incidental moisture as well. Check it against the flooring manufacturer’s requirement, and do not assume, but for a typical Ontario basement one assembly does both jobs.
Will a vapour barrier stop mould in my basement?
It closes one path — moisture moving from the slab into the flooring. It does nothing about bulk water entry, condensation on cold walls, or humid air. Those need grading, drainage and dehumidification, and a barrier installed over an unsolved water problem simply moves where the damage appears.
