Hardwood Floors April/May 2017

AT THE SITE

By Roger Barker

Subfloor Focus: Minimizing Moisture PART 2: USE OF VAPOR RETARDERS OVER CONCRETE SUBFLOORS

As an installer, it’s critical to understand the different types of moisture control membranes and the advantages of each. Part Two of this series will address installation methods utilizing the NWFA guidelines for moisture control membranes over concrete subfloors. Many flooring manufacturers recommend specific underlayment materials to be used below their flooring. Follow manufacturer recommendations whenever applicable. The installation of flooring over concrete is not without its intricacies. Concrete appears to the untrained eye to be a solid, impermeable foundation, yet it’s actually a source in itself for moisture. Water is a critical part of the concrete mixture. Even after it has been placed, concrete absorbs moisture in humid weather or by exposure to rain. Concrete does not release this moisture easily and may never dry out enough to accept flooring without a vapor barrier. A vapor barrier is a Class I membrane with a perm rating of <.10. All concrete surfaces regardless of age or grade level will emit or conduct some degree of moisture, usually in the form of vapor. This is a very natural and necessary function of healthy concrete — it’s like continual “breathing.” However, too much moisture emission without proper moisture control can result in flooring failures. Many times the blame for this is placed on a faulty product, improper specification, or faulty workmanship when the real reason lies with slab vapor emission conditions and inadequate moisture controls in place. Moisture conditions at the time of installation are the flooring contractor’s responsibility. When these conditions fall outside the necessary requirements, the contractor should take the initiative to determine the sources and advise the customer of adequate solutions and costs involved before the start of installation. Then let the client choose based on potential risks of each method you have presented and funding available to address the problems. Finally, have them agree to those choices in the contract. Because moisture can rise through concrete by capillary action, moisture-vapor barriers and moisture retarders need to be part of the installation process. Laid between the base of gravel or crushed stone and the slab, this barrier is usually in the form of heavy plastic, uninterrupted film. This film prevents the penetration of moisture through the slab to the interior surface, where it can ultimately damage hardwood floors. Moisture vapor also rises from the ground; a moisture barrier is required by IRC¹ and IBC² codes beneath new concrete slabs to prevent this. The IRC and IBC require a Class I vapor retarder³(aka barrier), minimum 6-mil poly under new concrete. Be sure to check local codes which may vary.

Liquid moisture control system.

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