Quick answer: Usually not directly on a wood subfloor. Plywood and OSB need to release moisture, and a sheet of plastic on top can trap it against the wood. Use the underlayment the laminate manufacturer specifies, and only add a vapor barrier if their instructions require it for your situation (for example over a crawl space or an unheated basement). The place to stop ground moisture is usually below the floor: a vapor retarder on the crawl space ground, plus ventilation or dehumidification. Concrete is different: laminate over concrete almost always needs a vapor barrier.
Quick answer
- Wood subfloor over a heated space: specified underlayment only, no plastic sheet.
- Wood subfloor over a crawl space: fix moisture in the crawl space; follow the laminate maker's instructions.
- Concrete slab: vapor barrier required (typically 6-mil polyethylene or an underlayment with a built-in barrier).
Wood subfloor vs concrete: why the answer differs
| Subfloor | Moisture source | Barrier on top? |
|---|---|---|
| Plywood/OSB over a heated room | Little; room air | Usually no; underlayment only |
| Plywood/OSB over a crawl space | Ground moisture and humid crawl space air from below | Depends on manufacturer; fix the crawl space first |
| Plywood/OSB over an unheated basement or garage | Humid or cold air from below | Check manufacturer instructions |
| Concrete slab | Moisture vapor rising through the slab | Yes, almost always |
The North American Laminate Flooring Association treats wood subfloors and concrete separately: flat, stable plywood, OSB and hardwood can be suitable bases, and the vapor barrier requirement applies to concrete (NALFA FAQs). The laminate manufacturer's installation instructions are the deciding document.
Moisture barrier, vapor barrier, underlayment: the terms
| Term | Meaning |
|---|---|
| Underlayment | The pad under laminate: cushions, levels small bumps, reduces noise |
| Vapor barrier / vapor retarder | A layer that slows water vapour, rated in perms. Building codes class vapour retarders as Class I (≤ 0.1 perm, e.g. polyethylene), Class II (0.1–1 perm) and Class III (1–10 perm) |
| Moisture barrier | Usually used loosely for the same thing; sometimes means a waterproof layer against liquid water |
| Combined underlayment | Pad with an attached film; one product does both jobs |
Polyethylene sheet (6-mil) is a Class I retarder: it blocks almost all vapour, which is why it suits concrete and can trap moisture on wood.
Check the wood subfloor first
Before laying laminate over wood:
- Measure moisture content with a pin or pinless wood moisture meter at several points. Many flooring manufacturers set a maximum of around 12 % for wood subfloors; use your manufacturer's figure.
- Look for signs of moisture: dark stains, cupped or swollen panels, soft spots, musty smell, rusty fasteners.
- Check underneath if there is a crawl space or basement: wet ground, condensation on ducts or pipes, high humidity.
- Check flatness and firmness; fix squeaks and loose panels.
If the subfloor reads high, find the cause and dry it before installing. Covering a wet subfloor leads to cupping, swelling at joints and mold.
Choosing the underlayment
| Underlayment type | Use over wood subfloor? |
|---|---|
| Foam or felt underlayment without a barrier | Yes, the usual choice over wood |
| Underlayment with an attached vapor barrier film | Only if the manufacturer calls for it, for example over a crawl space |
| Separate 6-mil polyethylene sheet | Generally for concrete, not on top of wood |
| Laminate with pre-attached pad | Usually no extra underlayment; check instructions |
Laminate over concrete: installing the vapor barrier
- Test the slab for moisture with the method and limit in the laminate instructions (often in-situ RH probes, ASTM F2170); see drying a concrete floor.
- Lay 6-mil polyethylene (or the specified film) over the whole slab, running a few centimetres up the walls.
- Overlap seams by the amount the instructions state (often about 200 mm) and tape them.
- Lay the underlayment on top (unless it has the film built in, in which case overlap and tape its lip).
- Leave the expansion gap at the walls; trim the film after the skirting is on.
Hardwood and other wood floors over a wood subfloor
Solid and engineered hardwood nailed to a wood subfloor is usually installed over a vapour-permeable layer such as asphalt felt or rosin paper rather than plastic, so the subfloor can still release moisture. Over crawl spaces, the moisture control goes below: a ground vapour retarder and dry crawl space air. Follow the flooring maker's and local code requirements.
How to protect a subfloor from moisture
- Keep the space below dry: ground vapour retarder, drainage, ventilation or dehumidification.
- Fix plumbing leaks quickly; dry spills before they soak in.
- Keep indoor humidity steady (about 35–55 % RH suits most wood floors).
- In bathrooms and kitchens, use water-resistant products and seal edges where the maker allows.
- Insulate cold floors over unheated spaces to avoid condensation on the underside.
Fix moisture from below
Most moisture problems in wood-framed floors start in the crawl space or basement:
- Ground vapor retarder: cover the crawl space soil with polyethylene sheeting, overlapped and sealed; see how to install a crawl space vapor barrier.
- Drainage: keep gutters, downspouts and grading directing water away from the foundation.
- Humidity control: in sealed crawl spaces, a dehumidifier holds humidity below about 60 %; see crawl space humidity control.
The US EPA notes that moisture control is the key to mold control in buildings (US EPA, Mold course chapter 2).
Commercial installations
On fit-outs and renovations, flooring contractors often need to dry subfloors and the building quickly before installation. A dehumidifier in the space keeps the wood and the air at the conditions the flooring will see in service, and helps subfloors and adhesive-bonded layers reach the manufacturer's moisture limit. Our commercial portable dehumidifiers are used for this kind of temporary drying.
FAQ
Frequently asked questions
Do I need a vapor barrier under laminate on plywood?
Usually not over a heated space. Use the underlayment the laminate maker specifies; add a barrier only when their instructions require it.
Can a moisture barrier on a wood subfloor cause problems?
Yes. A plastic sheet can trap moisture coming from below against the wood, causing swelling and mold.
What moisture content should a wood subfloor have before laminate?
Many manufacturers specify a maximum of around 12 %. Measure at several points and follow your manufacturer's limit.
Is a vapor barrier necessary for laminate flooring?
Over concrete, yes, almost always. Over a dry wood subfloor, usually not, unless the laminate maker's instructions call for it (for example above a crawl space or unheated basement).
What is the best vapor barrier for laminate flooring?
Over concrete, a 6-mil polyethylene sheet or a combined underlayment with an integrated vapor barrier, as the laminate maker specifies. Over wood, the maker's recommended underlayment.
How do I fix subfloor moisture before installing a floor (moisture mitigation)?
Find and fix the source first (leaks, crawl space humidity, ground moisture), dry the subfloor to the moisture content the flooring maker specifies, and only then install; see crawl space humidity control.
Should I put a vapor barrier under the subfloor?
Under a wood floor over a crawl space, the vapour barrier goes on the crawl space ground, not stapled under the subfloor where it can trap moisture.
Do I need a vapor barrier for laminate over concrete?
Yes, almost always: 6-mil polyethylene or an underlayment with a built-in film, seams overlapped and taped.
Do I need a moisture barrier for laminate over a crawl space?
Fix the crawl space first: ground vapor barrier, drainage and humidity control. Then follow the laminate maker's instructions for underlayment.
Sources
- North American Laminate Flooring Association, Laminate flooring FAQs.
- US EPA, Mold course chapter 2.
- US DOE Building Energy Codes resources, Insulating existing slabs.
