Do not assume that laminate over a wood subfloor needs an impermeable plastic sheet. First identify what is below the wood, whether the wood is dry and sound, and what the exact laminate and underlayment installation instructions require. An impermeable layer in the wrong place can trap moisture in wood instead of protecting it.
Separate a wood subfloor from a concrete slab
Plywood or OSB over joists is a different assembly from a wood panel placed over concrete. A floating laminate floor may need a compatible underlayment for support and acoustics, but that does not automatically mean a separate polyethylene vapor barrier belongs directly on the wood. A slab may need a defined moisture-control layer beneath the floor system; the slab, wood layer and finish must be assessed together.
The North American Laminate Flooring Association (NALFA) describes stable, flat hardwood, plywood and OSB as potentially suitable bases. Its public guidance states a vapor barrier requirement for concrete separately. That concrete instruction should not be transferred to every wood subfloor. Product-specific installation directions and local building requirements still govern the actual assembly.
NALFA: laminate subfloor and underlayment guidance, with separate concrete and wood examples.
| Assembly found on site | Check before selecting underlayment | Default decision boundary |
|---|---|---|
| Wood panel over a ventilated or conditioned space | Wood condition, flatness, measured moisture and the space below | Use the laminate system specified for wood; do not add an impermeable sheet by habit |
| Wood floor over a crawl space | Ground water, ground vapor, air leakage, wood condition and crawl-space design | Address the source below the wood and verify the floor assembly before adding a top layer |
| Wood panel or sleeper over concrete | Slab moisture, existing below-slab controls, wood moisture and the designed floor layers | Design the slab-side control and drying path for the complete assembly |
| Concrete directly below laminate | Manufacturer-approved slab test and underlayment/barrier specification | Follow the concrete installation instructions; this is not a wood-subfloor rule |
Inspect the wood and find the moisture source
Check for a current or previous leak, staining, swelling, soft panels, odor or visible mold. Inspect plumbing, exterior drainage and the space below the floor. Measure wood moisture with a meter suitable for the material, at multiple representative points, and record locations and conditions. A single room RH reading does not establish that the subfloor is ready for covering.
Compare the findings with the exact flooring and underlayment instructions. Allow the wood and new materials to acclimate as directed, and correct flatness or structural problems before installation. If the wood is wet, damaged or mold-affected, stop and fix the source and material condition first. Placing a low-permeance sheet over wet wood is not a drying method.
For a floor above a crawl space, first determine whether ground vapor or bulk water is reaching the floor system. Ground-cover and crawl-space air-control decisions belong to that assembly, not to a blanket polyethylene-over-wood recipe. The related crawl-space vapor-barrier guide addresses ground-side control; use it only after identifying the actual source and applicable design.
Related guide: diagnose and place a crawl-space ground vapor barrier.
If a wood subfloor sits over a slab, investigate the slab as well as the wood. A controlled slab/insulation/subfloor arrangement may place a moisture layer at the slab side, but the details depend on the existing slab, climate, material layers and intended drying direction. The US Department of Energy illustrates one existing-slab arrangement; it is a design example, not a universal laminate installation detail.
US DOE Building Science Education: one existing-slab insulation and moisture-control assembly.
After a known leak, do not cover either wood or concrete until the source is fixed and appropriate material or floor-system criteria are met. The separate concrete-floor drying guide covers cleanup and verification after a leak; its testing steps are not a substitute for the laminate manufacturer’s wood-subfloor limits.
Related guide: dry and verify a concrete floor after a leak.
Choose underlayment and any vapor control as one system
Read the instructions for the exact laminate product and its attached pad, if present. Record allowed subfloor types, required underlayment, moisture-test method and limit, permitted vapor-control material, overlap or tape details if specified, and warranty conditions. A combined pad may already contain a vapor-control layer; adding another one without approval can change the intended assembly.
For wood, do not prescribe a universal 6-mil, 12-mil or 20-mil sheet, seam overlap or liquid membrane. If the product documentation explicitly requires an impermeable layer over a particular wood assembly, follow that documented detail and confirm that the underlying moisture source and drying path have been assessed. Otherwise select the approved underlayment without inventing a vapor-barrier requirement.
For concrete, use the slab-moisture assessment and barrier or underlayment specified for that system. NALFA’s concrete guidance includes a 6-mil example, but the value is not a rule for wood and does not prove a wet slab is suitable. A plastic-sheet observation on wood is not a substitute for wood moisture measurements; concrete tests must likewise follow the product specification.
Install only after the assembly passes its checks
- Document the exact laminate, underlayment and subfloor instructions that apply.
- Repair water entry and damaged wood; verify the assembly is dry and structurally suitable.
- Measure and record subfloor condition using the specified method, locations and limits.
- Prepare the surface for the required flatness, cleanliness and movement clearances.
- Install the approved underlayment and, only where specified, vapor-control layer and seam details.
- Keep the product instructions, measurements and installation record for future diagnosis.
The US Environmental Protection Agency warns that a vapor barrier placed or installed incorrectly can hold moisture in a wall or floor long enough to cause mold problems. No barrier thickness or taped seam can compensate for an unresolved leak or for wood that was covered before it dried.
US EPA: building design, misplaced vapor barriers and drying of wet materials.
Sources
North American Laminate Flooring Association, Laminate Flooring FAQs.
US Environmental Protection Agency, Mold Course Chapter 2.
US Department of Energy Building Science Education, Insulating Existing Slabs.
FAQ
Frequently Asked Questions
Do I always need a plastic vapor barrier over a wood subfloor?
No. Check the exact laminate and underlayment instructions and the floor assembly. An impermeable sheet directly on wood can trap moisture, so do not add it as a universal step.
Is the 6-mil concrete rule also a wood-subfloor rule?
No. Concrete and wood are different assemblies. NALFA gives a concrete-specific barrier example; wood needs its own condition check and product-specific underlayment decision.
What if the wood floor is above a crawl space?
Inspect ground and bulk-water sources, crawl-space design and wood condition first. Address the moisture source below the floor; a sheet over the wood should not be used to hide a wet crawl space.
What if plywood sits over a concrete slab?
Assess slab moisture, wood moisture and the complete layer sequence. Follow an assembly designed for the slab and the chosen flooring, rather than assuming a top-side sheet on plywood is sufficient.
Can I cover a wood subfloor after a leak if it feels dry?
Do not rely on touch alone. Repair the source, inspect for damage, measure representative wood locations with an appropriate meter and compare with the exact flooring instructions before covering.
Does an attached-pad laminate need another underlayment?
Only if the product instructions call for it. An attached pad may already provide part of the specified system; an extra layer can alter support or moisture behavior.
Does a taped barrier guarantee mold prevention or the warranty?
No. Moisture-source control, a suitable and dry subfloor, correct material placement and the exact product instructions all matter. Keep the installation record, but do not treat a barrier as a guarantee.
