Choose the Moisture-Control Layer That Fits the Whole Floor Assembly

Choose the moisture-control layer that fits the whole floor assembly.

The short answer: identify the assembly before adding a barrier

A vapor barrier for a subfloor is not an automatic layer. Vapor protection is commonly considered above existing concrete and over exposed crawl-space soil, but there is no universal rule requiring polyethylene directly above or below every plywood or OSB subfloor.

The correct decision depends on five variables:

1. Substrate type: existing concrete, new slab-on-ground, plywood, OSB, or suspended concrete.
2. Moisture source: liquid water, ground moisture, slab moisture, humid-air leakage, or vapor diffusion.
3. Finish-floor permeability: whether the finished floor permits or restricts drying.
4. Climate and conditioning: outdoor humidity, heating and cooling patterns, indoor setpoints, and crawl-space design.
5. Drying path: whether the completed assembly can dry upward, downward, or in either direction.

Before selecting a product, map the floor from top to bottom:

• Finish flooring
• Flooring adhesive or mortar, if applicable
• Underlayment or membrane
• Plywood, OSB, or concrete subfloor
• Insulation, rigid foam, or insulation facing
• Floor framing
• Crawl space, slab base, or exterior air
• Ground or soil

This inventory matters because “vapor barrier” can refer to several products that are not interchangeable. It may mean a flooring underlayment directly below a finish floor, a membrane beneath a new concrete slab, a liner covering crawl-space soil, or a vapor-control layer within a wood-framed floor.

Use this quick routing guide:

• Existing concrete slab: Identify the flooring system, perform the required slab-moisture testing, and use a membrane, coating, or underlayment approved for that system.
• New slab-on-ground: Follow the plans and project specifications for the below-slab vapor retarder.
• Exposed crawl-space soil: Evaluate a ground liner as a way to reduce soil-vapor entry.
• Plywood or OSB: Review the complete assembly and its intended drying direction before adding polyethylene.
• Conditioned crawl space: Treat the crawl space as part of the enclosure rather than merely as an empty area below the floor.

The central principle is to treat the finish floor, subfloor, insulation, framing, and crawl space as one moisture-management system. Reviewing those components separately can create a “moisture sandwich,” especially when vapor-resistant flooring is combined with another low-permeance layer below the wood, according to Huber Engineered Woods’ floor-assembly guidance.

Terms also require care. “Vapor barrier,” “vapor retarder,” “moisture barrier,” and “low-permeance layer” are not reliable substitutes for product data. Check the documented permeance, intended installation location, compatible substrates, seam details, and approved flooring assembly instead of choosing by package wording alone.

Find the moisture source before choosing a membrane

A wet subfloor does not prove that vapor diffusion is the cause.

Bulk liquid water includes roof or plumbing leaks, runoff, flooding, groundwater entry, and standing water. A sheet intended to slow vapor does not repair these defects.

Capillary movement occurs when porous materials such as concrete draw liquid moisture from adjacent damp materials or soil. This is a particular concern for slabs in contact with the ground.

Air transport occurs when humid air leaks through gaps and carries moisture into a cooler location. Joints, rim areas, service openings, mechanical penetrations, and poorly detailed access doors can all become air paths.

Vapor diffusion is water-vapor movement through materials. A low-permeance layer is intended principally to limit this pathway.

Deal with bulk water first. Check roof and plumbing leaks, site grading, gutters and downspouts, foundation drainage, stains on walls, low points in the crawl space, and evidence of previous standing water. Correct runoff and drainage defects before covering the ground or floor.

Humid-air movement deserves separate attention. In warm, humid weather, outside air entering a cooler crawl space near an air-conditioned floor can condense on the underside of the subfloor. This mechanism is specifically identified in floor-assembly guidance addressing humid crawl-space air.

Air sealing and vapor control perform different jobs. Conversely, an air-control layer can reduce moisture carried by air even if it is not a polyethylene-style vapor barrier.

A crawl-space ground liner also has a limited but valuable purpose: reducing vapor migration from exposed soil. It does not stop plumbing leaks, runoff, groundwater collecting above the liner, or water entering through foundation walls.

Pause the flooring or membrane installation if you find:

• Standing water or evidence of recurring flooding
• Repeated condensation on framing or ducts
• Mold or suspected microbial growth
• Rot, softness, swelling, or delamination
• Wet or fallen insulation
• Damaged subfloor panels
• Elevated moisture readings without an explained source
• No identifiable direction in which the assembly can dry

Manufacturer guidance for crawl-space liners likewise calls for addressing standing water, leaks, wet insulation, debris, and mold before installation begins (crawl-space preparation guidance). Covering the evidence may conceal the symptom without correcting the moisture mechanism.

Do not confuse underlayment, ground liners, and under-slab barriers

Product labels are used inconsistently. Selection should be based on installation location, documented performance, and approval for the complete assembly—not the largest wording on the package.

A flooring underlayment with vapor-control properties is installed directly beneath an approved finish floor. Some products combine cushioning, acoustic performance, and vapor resistance. Attached-barrier underlayments are available for certain floating-floor systems, but availability does not establish compatibility with every substrate, adhesive, fastener, or finish.

A crawl-space ground liner covers exposed soil. In an encapsulation system, it may also connect to foundation walls, columns, piers, access openings, and penetrations. Its immediate target is vapor coming from the ground, not moisture already trapped in plywood.

An under-slab vapor retarder is installed beneath a new slab-on-ground. ASTM E1745 applies to vapor retarders in contact with soil or granular fill beneath concrete, while ASTM E1643 addresses installation practices for under-slab vapor retarders. Those scopes do not make either standard a universal specification for membranes installed against plywood or OSB (trade coverage of the ASTM standards’ under-slab application).

A vapor-control layer within a wood floor is part of a climate- and assembly-specific design. It cannot be selected correctly without considering insulation, air sealing, indoor conditioning, crawl-space conditions, existing resistant layers, and the finish floor.

Layer or product category	Installation location	Moisture source addressed	Typical application	What it does not solve
Flooring underlayment with vapor control	Directly below an approved finish floor, often above concrete	Vapor moving toward moisture-sensitive flooring	Approved floating laminate or engineered-wood assembly	Active leaks, standing water, incompatible adhesives, or a wet wood assembly
Above-slab sheet or membrane	On an existing concrete slab	Vapor moving through the slab	Retrofit flooring installation over concrete	Water entering over the slab or bypassing untreated edges and penetrations
Under-slab vapor retarder	Beneath a new slab-on-ground	Soil vapor and capillary moisture moving toward the slab	New concrete construction	Later leaks from above or defective site drainage
Crawl-space ground liner	Over exposed soil; sometimes connected to walls and columns	Vapor from the ground	Crawl-space moisture control or encapsulation	Runoff, plumbing leaks, standing water, or humid-air leakage
Wood-assembly vapor-control layer	At a designed location within the framed floor	Assembly-specific vapor diffusion	Climate-specific framed-floor design	Air leakage, bulk water, or moisture trapped by another resistant layer
Housewrap	Normally within a wall or enclosure water- and air-control strategy	Product- and detail-dependent	Exterior enclosure assemblies	It should not be assumed to replace a specified polyethylene or vapor-retarder layer

Also distinguish water vapor from liquid water. Some manufacturers use “moisture barrier” as a synonym for “vapor barrier,” while others use it for a material that resists liquid water but may permit vapor transmission. The product data sheet and approved assembly should settle the question.

Do not assume housewrap performs like polyethylene. A community answer discussing Tyvek beneath a raised wood floor characterizes it as vapor-transmitting rather than a vapor barrier. That answer is not a code, professional assessment, or manufacturer specification, so the final decision should rely on the exact product’s documented permeance and intended use (community discussion of Tyvek in a raised-floor assembly).

Decision matrix by subfloor and foundation type

The likely control location changes with the foundation and subfloor. The matrix below is a starting point, not a substitute for plans, current product instructions, flooring-manufacturer requirements, or local rules.

Subfloor or foundation condition	Likely control location	Required checks	Major moisture-trap risk
Existing concrete slab	Approved membrane, coating, or underlayment above the slab and within the flooring system	Flooring manufacturer’s moisture tests and limits; slab condition; installation method; membrane compatibility	Covering an inadequately assessed slab or allowing vapor to bypass seams, edges, and penetrations
New slab-on-ground	Continuous, durable vapor retarder beneath the slab as detailed in project documents	Intended finish and future use; project specification; designer’s placement detail; puncture and tensile resistance; penetrations	Damaged membrane, unsealed penetrations, or treating sheet thickness as proof of complete performance
Plywood or OSB over vented or open crawl space	No default location; evaluate ground control, air sealing, insulation, and drying direction	Climate; outdoor humidity; air conditioning; insulation; finish-floor permeability; exposed soil; ventilation strategy	Wood trapped between a low-permeance finish and another resistant layer below
Plywood or OSB over conditioned crawl space	Usually at the crawl-space enclosure or another designed location—not automatically against the subfloor	Enclosure boundary; ground liner; wall and rim details; humidity control; access door; mechanical strategy	Combining incomplete vented- and conditioned-crawl approaches or leaving humid-air bypasses
Crawl space with exposed soil	Sealed ground liner over soil; wall connection depends on design and local requirements	Standing water; drainage; leaks; debris; walls; columns; pipes; inspection access	Mistaking soil-vapor control for a cure for liquid water or subfloor condensation
Suspended concrete topping over plywood and rigid foam	Assembly-specific; existing foam may already contribute vapor resistance	Foam type, thickness, permeance, seam treatment; topping design; crawl-space condition; penetrations	Adding polyethylene without knowing whether it creates redundant resistant layers

Existing concrete slab

Start with the finish-floor manufacturer. Determine the approved installation method and required slab-moisture tests. The solution may be a sheet underlayment, liquid-applied mitigation system, adhesive system, or another proprietary assembly; a generic polyethylene roll is not automatically acceptable.

Record the flooring product, substrate requirements, testing method, acceptable results, approved moisture-control system, seam details, and warranty conditions before materials are ordered.

New slab-on-ground

When a new slab will support moisture-sensitive flooring, coatings, stored materials, or humidity-controlled space, commercial technical guidance recommends a durable below-slab vapor retarder. Placement and detailing must still follow the project designer and specifications because concrete placement, curing, penetrations, and membrane protection are project-specific (below-slab application and placement considerations).

Do not transfer new-construction under-slab requirements to an existing slab or wood floor. Those are different locations with different exposures and installation demands.

Plywood or OSB over a vented or open crawl space

Do not default to polyethylene stapled against the wood. First determine:

• Is the finish floor vapor resistant?
• Is the home heavily air-conditioned during humid weather?
• Does outdoor air enter the crawl space?
• Is the floor insulated?
• Do the insulation or foam products have resistant facings?
• Is soil exposed?
• Can the subfloor dry upward or downward?
• Is condensation seasonal, continuous, or associated with a leak?

The response may involve drainage, ground-vapor control, air sealing, revised insulation, or a different crawl-space strategy rather than plastic attached to the subfloor.

Plywood or OSB over a conditioned crawl space

Treat the crawl space as part of the building enclosure. Review the ground liner, wall insulation, rim and penetration air sealing, access door, drainage, humidity control, and mechanical plan together.

Do not assume that calling a crawl space “conditioned” proves that the enclosure is complete. Verify where the air, thermal, water, and vapor-control layers are intended to be and whether outdoor air has uncontrolled paths into the space.

Crawl space with exposed soil

A sealed ground liner addresses vapor at its source. That is a different intervention from placing plastic beneath the joists.

The liner does not replace drainage, leak repair, air sealing, or humidity management.

Suspended concrete topping over plywood and rigid foam

An archived Green Building Advisor case involved a suspended concrete topping over plywood and rigid foam above a conditioned crawl space. For that particular assembly, the editor concluded that additional polyethylene between the plywood and foam was unnecessary because the foam already functioned as a vapor retarder. Taping the foam seams was recommended to reduce concrete bleed-water leakage (assembly-specific suspended-floor discussion).

That is an example, not a general rule. The foam type, thickness, documented permeance, facing, seam treatment, fastening penetrations, and complete floor configuration must be verified before applying the same conclusion elsewhere.

Protect wood subfloors without creating a moisture sandwich

When wood is placed between two low-permeance layers, its ability to dry can be sharply reduced. If construction moisture, a leak, condensation, or humid air introduces water, the assembly may remain damp longer because both drying directions are restricted.

The finish floor is therefore part of the vapor-control design. It is not merely decoration selected after the subfloor details are complete.

The supplied assembly guidance broadly characterizes vinyl plank and vinyl tile as highly vapor resistant and warns that these finishes can limit upward drying. That does not justify assigning every vinyl product the same permeance. Use product-specific data, but include vinyl and other resistant finishes in the moisture-sandwich review.

Before adding polyethylene to a wood floor, inventory every potentially low-permeance layer:

• Vinyl sheet, tile, or plank flooring
• Rubber or membrane-backed flooring
• Waterproof flooring underlayments
• Self-adhered membranes
• Rigid foam
• Foil- or kraft-faced insulation
• Closed-cell spray foam
• Liquid coatings and sealers
• Roofing-type membranes
• Concrete toppings
• Existing polyethylene

Do not assume every item on that list is always a vapor barrier. Record the exact product, location, condition, seam treatment, and documented permeance when available.

Then identify the intended drying direction. Can the wood dry upward through the finish floor, downward toward the crawl space, or in both directions? If neither direction is available, do not add another resistant layer until the assembly has been reviewed.

The familiar instruction to put the vapor barrier on the “warm side” is too simple for many floors. Conditions can differ between a heating-dominated building and an air-conditioned building exposed to humid outdoor air. Crawl-space ventilation, enclosure design, insulation, and indoor temperature all affect the boundary conditions around the subfloor.

Air movement can also be the dominant problem. If humid outdoor air reaches a cool subfloor through vents or enclosure leaks, plastic stapled across the joists may leave bypasses and conceal condensation rather than stop the air source. The floor requires coordinated water, air, thermal, and vapor control.

Existing rigid foam deserves particular attention. Depending on its type, thickness, facing, and seam treatment, it may already contribute substantial vapor resistance. Do not use a generic foam thickness as the deciding rule; verify the product documentation and all transitions.

Pause and obtain qualified assessment when a wood assembly has recurring underside condensation, unexplained elevated moisture readings, panel swelling, edge ridging, delamination, mold, rot, wet insulation, corroded fasteners, or no reliable drying path. Do not treat a new membrane or finish floor as a repair for damaged structural material.

Concrete-subfloor workflow: test, prepare, cover, and inspect

For an existing concrete subfloor, begin with the finish-floor manufacturer’s current approved system and moisture limits. The flooring type and installation method govern whether the job needs a sheet membrane, attached-barrier underlayment, liquid treatment, compatible adhesive system, or another solution.

1. Confirm the flooring assembly

Record:

• Flooring product and manufacturer
• Floating, glue-down, mortar-set, nail-down, or other installation method
• Required underlayment, adhesive, or primer
• Approved vapor-control products
• Required concrete testing
• Maximum acceptable test results
• Seam, edge, and penetration details
• Substrate-preparation requirements
• Warranty conditions

Attached vapor-barrier underlayments are marketed for some floating laminate and engineered-hardwood floors. One manufacturer’s instructions call for preparing the substrate, placing the barrier side upward, taping seams, and then installing the flooring according to its manufacturer’s directions (floating-floor underlayment instructions).

That example does not establish compatibility with nail-down hardwood, glue-down flooring, tile, or mortar-set systems.

2. Test the slab as required

Calcium chloride testing and in-slab relative-humidity probes are two methods identified in commercial guidance for assessing concrete moisture (concrete moisture-assessment overview).

Do not select a test method, number of tests, conditioning procedure, or acceptance limit from a generic article. Follow the current requirements of the flooring manufacturer and any specified mitigation system.

If results exceed the approved limit, stop. Determine whether a specified mitigation system can address the condition or whether additional diagnosis is required.

3. Prepare the substrate

The slab should be:

• Clean and free of dust, debris, oil, and incompatible residues
• Dry to the applicable system specification
• Sound and sufficiently level for the flooring installation
• Free of projections likely to puncture a sheet
• Patched at cracks, spalls, or holes as directed
• Free of active liquid-water entry

“Dry” is not a visual judgment. A slab may look dry while still failing the flooring system’s moisture criteria.

4. Plan full coverage

Lay out sheets before cutting. The selected membrane should cover the required surface without gaps, with enough material for specified overlaps and transitions.

Plan around:

• Columns
• Doorways and thresholds
• Floor outlets
• Pipes and conduits
• Walls
• Irregular edges
• Changes in elevation

Minimize narrow patches and unnecessary seams. Use only the tape, sealant, primer, patch, or accessory approved for the selected membrane.

5. Follow the specified seam and edge details

Generic sources conflict. A brief demonstration uploaded in 2009 shows 6-mil polyethylene over concrete with approximately 8-inch overlaps and says the seams do not need to be taped (older concrete-floor demonstration). More detailed current commercial guidance calls for overlapped and taped seams, sealed edges and penetrations, and repairs before the floor is covered.

Do not average those instructions or choose whichever is easier. Follow the current directions and warranty requirements for the exact membrane, adhesive, underlayment, and flooring system. If a manufacturer requires taped seams or sealed transitions, an older untaped-overlap demonstration is not an acceptable substitute.

6. Select for performance, not thickness alone

One commercial source describes 6 mil as a minimum and says 10- to 20-mil sheets provide greater durability and puncture resistance. The same source cautions that selection also depends on permeability, application, mechanical exposure, flooring type, codes, and manufacturer instructions (concrete vapor-barrier selection guidance).

Those thickness descriptions are not universal specifications. Mil thickness alone does not establish permeance, tensile performance, puncture resistance, classification, chemical compatibility, seam durability, or approval for the intended flooring system.

Compare documented products approved for the actual location and installation method.

7. Inspect before flooring covers the work

Complete and document a final walk-through:

• No uncovered gaps
• Overlaps meet the selected system’s instructions
• Seams have not shifted
• Required tape or sealant is fully bonded
• Edges and transitions are detailed correctly
• Pipes and other penetrations are sealed
• Cuts and punctures are patched with compatible materials
• Foot traffic or material handling has not displaced the membrane
• No incompatible adhesive, tape, primer, or patching product was used
• The membrane orientation is correct
• Test records and installation photographs are retained when required

Crawl-space ground barriers: control soil vapor at the source

When a crawl space has exposed soil, the ground can add moisture to the space. A liner placed over the soil reduces that source. It does not perform the same job as polyethylene attached beneath a plywood or OSB subfloor.

Start with investigation and cleanup. Address standing water, plumbing leaks, foundation seepage, wet insulation, mold, and damaged wood. Remove construction debris and sharp objects that could puncture the liner. Where projections cannot be removed, use a protection layer compatible with the selected system.

The continuity objective is straightforward even though exact details vary:

1. Cover the exposed soil.
2. Arrange sheets to minimize seams and difficult patches.
3. Overlap and seal seams as specified.
4. Connect the floor liner to foundation walls when the selected design requires it.
5. Detail columns, piers, pipes, ducts, access openings, and other interruptions.
6. Repair tears and punctures with compatible materials.
7. Protect the completed liner from service traffic and stored items.

One manufacturer reports that local seam-overlap requirements may range from at least 6 inches to as much as 12 inches, which demonstrates why no single generic dimension should be applied to every crawl space (manufacturer crawl-space installation guidance).

A separate vendor guide recommends 10- to 12-inch floor overlaps for its example system. That dimension remains vendor guidance, not a universal requirement (commercial liner layout guide).

Wall attachment requires similar care. An encapsulation system may call for compatible wall tape, mechanical termination, sealant, or a combination. Local building and termite-inspection requirements may also affect wall coverage or require an exposed inspection area. Verify those details with the product instructions and applicable local officials rather than copying a generic clearance.

A ground liner is only one control layer. Depending on the site and crawl-space strategy, the project may still require:

• Exterior grading and drainage
• Foundation waterproofing or dampproofing
• A sump or other water-management system
• Air sealing
• Duct or plumbing repairs
• Dehumidification
• Designed ventilation
• Crawl-space conditioning
• Wall or rim insulation
• Condensate drainage

Climate, equipment location, combustion appliances, ducts, local requirements, inspection access, and the intended enclosure boundary all matter. Evaluate the complete space, not just the visible soil.

Product and preinstallation checklist

Before purchasing or installing a vapor barrier for a subfloor, write down the application and required performance. This prevents a common procurement error: buying a product labeled “moisture barrier” before deciding where it goes or what it must control.

Product specification checklist

• Installation location: above an existing slab, beneath a new slab, directly below finish flooring, over crawl-space soil, or within a framed floor
• Documented permeance: tested value and test method relevant to the project specification
• Physical durability: puncture, tear, and tensile resistance where exposure warrants them
• Substrate compatibility: concrete, soil, plywood, OSB, foam, mortar, adhesive, or another material
• Finish-floor approval: exact flooring category and installation method
• System accessories: approved tape, sealant, primer, termination bar, boots, patches, or protection course
• Seam details: required overlap, cleaning, taping, rolling, and cure conditions
• Edge details: walls, thresholds, transitions, columns, and terminations
• Penetration details: pipes, conduits, drains, fasteners, and floor outlets
• Orientation: which face goes up or toward the moisture source
• Environmental limits: substrate temperature, ambient conditions, and storage requirements
• Warranty requirements: approved products, testing, records, and installer qualifications
• Project requirements: applicable building, energy, fire, termite, and design provisions

Mil thickness describes physical thickness. It does not, by itself, establish vapor permeance, puncture resistance, tensile strength, chemical compatibility, seam performance, or expected service durability.

For under-slab work, verify the classification and installation requirements stated in the project documents. ASTM E1745 addresses vapor retarders used beneath concrete in contact with soil or granular fill, while ASTM E1643 addresses under-slab installation practices (under-slab standards overview). Do not transfer those requirements automatically to membranes installed against plywood or OSB.

For flooring underlayment, confirm whether the finish floor is floating, nail-down, glue-down, or mortar-set. An attached barrier approved below a floating laminate floor may be inappropriate for another installation method.

Review every existing low-permeance layer before adding another. Include the finish floor, rigid foam, facings, coatings, membranes, and insulation—not just the material immediately beside the proposed barrier.

Jobsite readiness checklist

Before work begins, confirm:

• Required moisture testing is complete and documented.
• Test results meet the applicable flooring or mitigation-system criteria.
• Liquid-water sources and drainage defects have been corrected.
• The substrate is clean, sound, and prepared to specification.
• Damaged wood, wet insulation, mold, or rot has been assessed.
• Material quantities include laps, wall transitions, columns, penetrations, and repairs.
• Approved tape, sealant, primer, patches, and edge accessories are on site.
• Pipes, conduits, and irregular transitions have workable details.
• Installation will not leave wood between two unreviewed low-permeance layers.
• The completed assembly retains a defined drying direction.
• A final continuity inspection is assigned before the work is covered.

Consult the flooring manufacturer, membrane manufacturer, local inspector, project designer, or a qualified building professional when requirements conflict, moisture damage is present, or the drying path remains unclear.

Frequently asked questions

Do I need a vapor barrier over a concrete subfloor?

Often, but not automatically. Determine the finish-floor manufacturer’s moisture-testing requirements and approved installation system first. Concrete beneath moisture-sensitive flooring may need a compatible vapor-control approach, but that could be a sheet underlayment, attached barrier, liquid-applied system, adhesive system, or another specified product.

Test the slab as required and follow the current flooring and membrane instructions rather than assuming generic polyethylene will satisfy the installation or warranty.

Should polyethylene go above or below a plywood or OSB subfloor?

There is no universal placement. It depends on climate, heating and cooling, crawl-space configuration, insulation, air sealing, finish-floor permeability, and existing vapor-resistant layers.

Polyethylene above the wood may restrict upward drying; polyethylene below it may restrict downward drying. If both sides are already resistant, another sheet can create a moisture sandwich. Map the complete assembly before adding it.

Is attached vapor-barrier underlayment enough for laminate or engineered hardwood?

They are not automatically suitable for nail-down, glue-down, tile, or mortar-set installations.

Can Tyvek be used as a subfloor vapor barrier?

Do not assume so. Housewrap is generally selected for different enclosure functions and should not be treated as interchangeable with polyethylene.

Check the exact product’s documented permeance, approved uses, and the floor assembly’s requirements. Community guidance may offer a useful caution, but product documentation and the project design should govern the decision.

Do vapor-barrier seams need to be taped?

Follow the exact system instructions. A system intended to provide continuous vapor control commonly requires treatment of seams, edges, penetrations, and repairs, but details differ among products and applications.

Do not assume one overlap width or an untaped seam is acceptable across different membranes, substrates, flooring systems, warranties, and jurisdictions.

The final rule has three steps: identify the moisture source, map every layer and drying direction, and follow the approved assembly for the specific substrate and finish floor.

Over concrete, that usually means manufacturer-required testing and a compatible, continuous vapor-control system. In a crawl space, it may mean controlling soil vapor, liquid water, and humid-air entry rather than stapling plastic beneath the subfloor. With plywood or OSB, do not add another low-permeance layer until the complete assembly has a verified drying path.