To install a vapor barrier on a subfloor (like plywood or concrete), clean the surface thoroughly and roll out a continuous sheet of 6-mil polyethylene plastic. Overlap seams by at least 6 inches and seal them completely with waterproof vapor barrier tape, then run the edges a few inches up the baseboard.
Laying the Vapor Barrier
Roll out the vapor barrier, ensuring it covers the entire concrete floor. Overlap seams by at least six inches to create a continuous moisture barrier. Secure the seams using seam tape, pressing firmly to eliminate air pockets and gaps.
Don't Skip Sealing Seams, Edges, and Penetrations
A vapor barrier with unsealed seams fails at every gap. Moisture and soil gases enter through even small openings. Overlap panels by at least 12 inches and tape every seam.
No, you generally do not need a vapor barrier over a plywood subfloor. In fact, adding a plastic or impermeable vapor barrier directly over wood can trap moisture, preventing the subfloor from breathing and leading to mold, mildew, and wood rot.
Yes, mold can absolutely grow under a vapor barrier. While vapor barriers are designed to block moisture, any moisture that gets trapped beneath the barrier—due to a plumbing leak, groundwater seepage, or poor installation—will create a prime environment for mold to grow, out of sight.
You should avoid using a traditional, impermeable vapor barrier (like polyethylene plastic sheet) when it traps moisture instead of letting the wall system dry. If moisture gets into a wall, the barrier prevents it from escaping, leading to mold, mildew, and wood rot.
Options include:
No, standard Tyvek cannot be used as a traditional vapor barrier. Tyvek is a housewrap designed as an air and moisture barrier. It keeps bulk water out but is highly vapor-permeable, allowing moisture to escape the wall to prevent rot and mold.
Protecting a subfloor from moisture requires stopping water from both below and above. First, ensure adequate ventilation in crawlspaces, apply a high-quality penetrating sealer or vapor barrier directly over the subfloor, and install a waterproof underlayment before laying your finished floor.
Pressure treated lumber is infused with chemical preservatives to resist rot and insects, making it great for decks, fences, and landscaping. But those same chemicals can off-gas or leach into the air and dust of your home, which isn't safe for framing, shelving, or furniture.
There are a wide number of materials available to create effective vapor barriers, including:
Mold is generally at its worst from early spring through early fall (specifically July through October), peaking during the warmest and most humid months. Mold thrives in warm temperatures (60∘F to 80∘F) combined with relative humidity levels above 60%.
For most crawl space applications, a 10-mil to 20-mil thick, reinforced poly vapor barrier is recommended for durability, while 6-mil is the typical minimum code requirement. For enhanced protection, energy efficiency, and high-traffic areas, 16-mil or 20-mil is preferred.
Crafted with a moisture-resistant PE film backing and a synthetic rubber-based pressure-sensitive adhesive coating, this tape ensures a strong and lasting seal for plastic poly liners, vapor barriers, crawl space wall liners, and more.
Yes, but with significant caveats. While 4 mil plastic is acceptable for temporary use or light-duty general projects, it tears very easily. Professionals and building codes generally prefer or require 6 mil plastic for durability and long-term performance.
Proper vapor barrier installation requires placing the material on the warm side of the wall (facing inward in cold climates, outward in hot climates). Use a minimum 6-mil polyethylene sheet (or higher for durability), overlap all seams by at least 6 inches, and completely seal the edges, cuts, and overlaps using acoustical sealant or vapor barrier tape.
Whether you need a moisture barrier under your subfloor depends entirely on what your subfloor is sitting on, and what type of finished flooring you are installing.
Yes, you can put Kilz on a plywood subfloor. It is highly recommended for sealing in stubborn odors (like pet urine or smoke) and blocking water stains before installing new carpet, laminate, or hardwood.
Latex-based or polyurethane-based paint is one of the most accessible forms of waterproofing plywood. There are plenty of colours to choose from, which can be applied using a spray gun or painted with a roller. You must apply two to three coats of paint for maximum effectiveness.
Yes, mold can absolutely grow under a vapor barrier. While vapor barriers are designed to block moisture, any moisture that gets trapped beneath the barrier—due to a plumbing leak, groundwater seepage, or poor installation—will create a prime environment for mold to grow, out of sight.
The ZIP System and Tyvek both act as weather-resistive barriers (WRBs), but they work very differently. ZIP replaces standard sheathing and housewrap with an all-in-one coated board and taped seams. Tyvek is a breathable housewrap layered over standard oriented strand board (OSB) or plywood.
Because spray foam can trap moisture, hide the condition of the roof, make it difficult for surveyors to inspect, which means risks like timber decay, dry rot, and not forgetting increased fire risk. If you've got it or you're buying a property that does, this is something you need to know upfront.
DryBarrier® subfloor system is mold and mildew resistant and provides a moisture barrier to protect and add warmth to finished basement floors on concrete slabs. Convenient and simple to install, DryBarrier® makes it easy to transform a cool, damp area into a warm, dry living space for your family.
Symptoms from black mold exposure can appear anywhere from a few hours to several weeks. The exact timeline depends heavily on your immune system, any pre-existing allergies, and the concentration of airborne spores.
Vinegar is the better choice for killing mold. While bleach removes surface stains and kills mold on hard, non-porous surfaces, it fails to penetrate porous materials like wood or drywall, allowing the mold roots to survive. Vinegar penetrates these materials, killing the roots and preventing the mold from easily returning.