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Vapor Barriers Under Floating Subfloors
Lay a sheet of plastic on a concrete slab, tape it down, and come back two days later. If the underside is beaded with water, the slab is breathing moisture upward. That simple test explains why a vapor barrier sits beneath nearly every raised or floating floor built over concrete, and why leaving it out is one of the more expensive shortcuts in a renovation.
What moisture does inside a slab
Concrete is porous. Water that is in the soil beneath it, along with the water left over from the mix, migrates upward as vapor through tiny capillaries and escapes at the surface. In an open garage that vapor disperses into the air. Cover the slab with an airtight or nearly airtight floor assembly, and the vapor has nowhere to go. It condenses on the first cool surface it meets, which is typically the underside of a wooden subfloor or the back of a foam panel.
A barrier is a layer that slows that movement. It does not stop water in liquid form, and it is not a cure for a slab with active leaks. Its job is to control the slow, invisible vapor drive that is present on most ground contact slabs.
What goes wrong without one
Wood absorbs vapor and swells. A plywood panel resting on damp concrete picks up moisture from below and begins to cup, soften and eventually rot. Fasteners rust. Mold finds a ready home in a dark, damp, enclosed cavity. Odors rise through gaps at the walls. Because the damage happens under the finished floor, the first signs are often a musty smell or boards that creak and feel springy underfoot. By then a good portion of the assembly may need removal.
Even a floor built entirely from plastic or foam can suffer. Vapor condenses on the cold surface under the panels, pools in the seams and becomes a reservoir of standing moisture that never dries.
Types of barrier material
- Polyethylene sheeting. The standard choice. Thicker grades resist punctures better. It is laid across the slab with overlapping seams that are taped, and it is carried slightly up the wall edges.
- Foil faced or reflective membranes. Combine a vapor block with some thermal reflection, but are more expensive and need careful seam taping.
- Dimpled drainage membranes. Plastic sheets with raised bumps that create an air channel between the slab and the floor. They allow vapor to drift to the edges and be vented rather than trapped.
- Liquid applied sealers and primers. Rolled directly onto the concrete to reduce vapor emission. These suit situations where height is very limited.
Each has a place, and the choice depends on the measured moisture level, the available height and the type of floor going on top.
Measure first, then choose
A barrier is a response to a measurement. The plastic sheet test is quick, though rough. More reliable readings come from calcium chloride kits left on the slab for a set period, or relative humidity probes set into drilled holes. The results tell whether the slab is dry enough for a plain barrier or whether it needs a heavier mitigation system. Manufacturers of floor products publish limits, and staying under them is what protects a warranty. A slab that tests very wet might need time to dry, drainage improvements outside or even a repair to a leaking pipe before any floor goes on.
Laying the barrier correctly
Poor installation defeats a good material. Sweep and vacuum the slab so that nothing sharp pokes the sheet. Roll out the plastic with overlaps of six to twelve inches and seal each seam with a tape made for the purpose. Extend the sheet up the walls a few inches, where it will later be hidden behind baseboards. Seal around any pipes or posts that pass through. Avoid walking on it with dirty shoes or dragging materials across it, because small punctures reduce its effectiveness. Do not leave it exposed in direct sunlight for long, as it degrades.
The air gap question
Many raised floors benefit from a small air space under the structure. A dimpled membrane or sleeper system creates one. The gap lets any vapor that does get through diffuse sideways rather than concentrate under the panels. It also allows an easy path for a leak to run to the edge, where it can be noticed. In rooms with serious moisture issues, a mix of barrier and air gap has a better track record than either alone.
Insulation and the dew point
Where there is insulation between the slab and the floor, the location of the barrier matters. Warm moist indoor air can pass down through the floor, reach the cold surface near the slab and condense. Placing the barrier on the cold side, directly over the concrete, keeps ground vapor from entering the insulation. Placing an additional layer on the warm side could trap moisture between two barriers, so builders usually use one carefully placed layer instead of two. Local building rules and the product manufacturer's instructions give the specific recommendation for each assembly.
Edges and perimeter
The perimeter is where problems begin. Walls built over the slab, plates fastened to concrete and the joint between old and new construction all offer paths for vapor and air. Use treated lumber or a sill gasket under any wood in contact with concrete. Seal the base of the wall to the barrier. Leave a small expansion gap at the edge of floating floors so they can move, and cover it with trim rather than caulk that restricts movement.
Checking the work
After installation and before covering everything permanently, inspect. Look for tears, uncovered seams and places where the sheet pulled away from the wall. Once the floor is in use, stay alert to signs of trouble: a musty smell, floor panels that feel damp or an unusual rise in indoor humidity. A small hygrometer in the room gives a steady reading, and numbers consistently above comfortable levels suggest that something below needs attention.
The short version
A vapor barrier under a floating subfloor is cheap insurance relative to the cost of the floor above it. Test the slab, pick a barrier that matches the result, install it with taped seams and a clean surface, and give the assembly a way to dry. Done right, nobody ever thinks about it again, which is exactly the point.