Hardwood Floors April/May 2017

You should know, however, that common construction grade poly film tears and punctures easily, is easily perforated, and may deteriorate or dissolve due to bacteria in moist soil which eats it. These issues can result in ineffective moisture control. From my perspective, a better choice would be a more durable 6-mil or thicker 100 percent virgin resin plastic. A membrane with high tensile, tear, puncture, and bacteria resistance that meets ASTM E1745 for plastic vapor retarders in contact with the soil. It only costs a few cents more per square foot and may eliminate future moisture issues. It’s important to note that beneath old concrete there may never have been a barrier or it may have deteriorated. Even if you know a moisture barrier is beneath the slab, test the concrete for moisture. A vapor retarder is always required when the moisture tests are >3 lb. by the Calcium Chloride test (ASTM 1869) method or 75 percent by the relative humidity (ASTM 2170) method. Simply using the testing method you are most familiar with or is most convenient will not necessarily coincide with the products you are using and may get you in trouble if the floor fails. Even when readings fall within the suggested parameters, it is always suggested to use a moisture control system to minimize the risk of future moisture infiltration. The NWFA Guidelines define an acceptable vapor retarder over concrete as an “impermeable” vapor retarder. A Class I membrane according to building codes. A few options for an adequate vapor retarder over concrete include: 1. Aminimum 6-mil poly, or better, more durable E1745 membrane over the slab or screeds. Usually used with a nail down floated plywood or screed system. It offers an economical, effective, Class I membrane allowing immediate installation of floors. 2. Poly film/mastic method— 6-mil polyethylene film in 48-inch wide rolls or wider— applied over a “skim” coat of asphalt mastic. The mastic should be troweled with a straight-edge trowel to skim coat the slab, with coverage of about 80-100-square-feet per gallon. After about 30 minutes,

ASPHALT/MASTIC Asphalt roofing felt, or asphalt saturated paper in asphalt mastic applied with a notched trowel at the rate of 50-square-feet per gallon.

the magazine of the national wood flooring association

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