What is Permeability?
Permeability & Moisture Transmission:
All building materials have a degree of permeability and where a building envelope is involved, it is the passage of moisture that is a primary concern. Some moisture is acceptable, but this varies with each material and engineered building system.
The most common method for evaluating the moisture vapor diffusion rate of a building material is ASTM E-96 (Standard Test Method for Water Vapor Transmission of Materials). E-96 determines the "water vapor permeance" for a given material at a given thickness. The permeance is often referred to as the "Perm rating"; the higher the Perm rating, the faster the diffusion rate of water vapor through the material.
Moisture Vapour Transmission (MVT)
Excessive concrete moisture vapor emission is known to cause distress or failure of floor covering and coating systems. Most manufacturers of floor covering materials and adhesive systems state limits of tolerable concrete moisture content or vapor emission volume (2-4 lbs / 1,000 sq ft / 24 hrs) and concrete surface pH levels. While pH levels are primarily controlled by factors within the concrete itself, moisture vapor emission is not.
The first potential moisture source is the concrete itself. Construction schedules rarely contain sufficient time to facilitate natural drying of concrete. An old "rule-of-thumb" exists that suggests allowing one month of drying per inch of slab thickness, under ideal conditions. The ideal ambient conditions are a minimum temperature of 70° F, maximum 30% relative humidity and constant air movement at 15 m.p.h. Neither "rule" accounts for mix design water/cement (W/C) ratio that will dramatically impact the required dry time. In his 1965 study entitled "Moisture Migration – Concrete Slab-On-Ground Construction" H.W. Brewer tracked moisture outflow of concrete as it dried. His study shows that a high W/C concrete takes longer to achieve low level outflow than drier mix designs.
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