Moisture barrier is also known as vapor barrier or vapor retarder. Moisture barrier is the preferred term when describing products installed between the subfloor–wood or concrete–and finish flooring. These barrier materials help prevent moisture penetration to the interior of the house–especially in cold and humid climates. Moisture damages framing members, and finishing products, and promotes mold and rot.
Types Of Moisture Barriers
Moisture barriers are divided into three classes based on permeability–how much moisture can penetrate the material. This unit of measurement is called perms. Lower numbers indicate better moisture protection.
Class I
Class I moisture barriers are rated at 0.1 perms or less–almost totally impermeable. They include materials such as heavy polyethylene–6 mils or heavier, rubber sheet, metal, and glass. These are the best options for cold and humid climates.
Class II
Class III moisture barriers include bitumen-coated kraft paper–the type used on paper-backed fiberglass insulation, 30-pound asphalt-coated paper–commonly used as roofing felt, unfaced expanded or extruded polystyrene rigid insulation, and some vapor retarding paint are semi-permeable. They have ratings of 0.1 – 1.0 perms and allow some moisture to penetrate. They can be used in a variety of applications in different climate conditions.
Class III
Class III moisture barriers are rated 1.0 – 10.0 perms–allowing moisture to pass through fairly easily. Examples include unfaced fiberglass batt insulation, drywall, concrete block, brick, 15 lb. tar paper, and house wrap. They are best used in milder dryer climates.
Benefits Of Using Moisture Barrier Under Flooring
There is no downside to using a moisture barrier under all flooring except the extra cost. The benefits are worth the cost and time.
- Mold Prevention. Mold grows anywhere there is moisture, warmth, and an organic food source–like wood.
- Rot Prevention. Wood and wood products rot when exposed to water. Mold accelerates the process. A well-installed moisture barrier eliminates the problem.
- Flooring Problems. Wood and wood products are hygroscopic. They absorb and release moisture. Too much moisture absorption causes cupping in hardwood floors and may delaminate other wood-based products.
- Noise Prevention. Moisture barrier is often incorporated into flooring underlayment. Underlayment–even 15 lb. tar paper–prevents wood-on-wood noise.
When You Need a Moisture Barrier
Whether you need a moisture barrier is determined by four factors.
- Climate. Moisture barrier installation is rarely used in dry climates and may not be required by local building codes. Installing moisture barriers in locations with cold snowy winters or lots of rain and humidity is usually necessary.
- Grade Level. Below-grade basements, crawl spaces, and even homes built on a pad are more susceptible to water infiltration and humidity buildup.
- Subfloor Material. Never install flooring over concrete without a moisture barrier. Concrete is porous and soil moisture wicks up from the ground and through the concrete into the flooring without a barrier. Hardwood flooring is very sensitive to moisture. Using a moisture barrier on a wood subfloor may not be necessary, but it can reduce potential problems.
- Building Codes. Virtually all construction is governed by local building codes. Make sure you check before installing new flooring. Non-compliance can result in an order to remove the floor.
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