QUESTIONWhat is the Maximum Acceptable Vapor Transmission for Porcelain Tile with an Epoxy Grout - We're installing porcelain tile with epoxy grout on concrete substrate. What is the maximum acceptable vapor emission rate?
ANSWERANSWER - The allowable vapor emission rate depends on what installation and tile products you are using. Generally speaking the standard for the floor covering industry and what most installation product manufacturers publish is 3 lbs per 1000 square feet per 24 hours according to the ASTM F-1869 Calcium Chloride Vapor Transmission test. This is really considered very low and unrealistic for ceramic tile installations, although it is a requirement for carpet, vinyl, and hardwood products.
Most manufacturers of installation products for thin-set mortars will say anywhere from 10 to 15 lbs per 1000 square feet per 24 hours is acceptable. If you are installing over a waterproof membrane or a crack isolation membrane they might say no higher than 8 to 10lbs. You have to go to the manufacturer and ask them what their limitations are for the products you are using.
Although porcelain tile is much denser (less absorbing) than some tiles, most clay tiles are at least vitreous and moisture doesn't readily pass through them either. So whether you are using an impervious porcelain tile or a vitreous clay body tile it doesn't affect what the acceptable vapor transmission is for the concrete to which it is attached. Some stones are moisture sensitive so it is always recommended to install a waterproof/crack isolation membrane over concrete slabs on-grade prior to installing the stone.
Having the epoxy grout that is impervious with the impervious porcelain tile should not be an issue as long as you meet the vapor transmission requirements for the setting materials. Obviously the tile and epoxy grout will trap moisture in and the thin-set mortar won't cure as readily. Vapor pressure does not build up pressure, as is often mistakenly thought, whereas hydrostatic pressure from a water source above the level of the tile can build up pressure. Vapor will not pass unless there is a path, and it will take the path of least resistance just as electricity does. If there is no path then it becomes static.
The ASTM F-1869 Calcium Chloride Vapor Transmission test is the most common test referenced for vapor transmission, although it can be unreliable because there are other factors that can affect vapor transmission (e.g. air circulation, temperature) that might not be indicative if the moisture source is temporary from curing or if it is a perpetual source of moisture indicating there is a problem. The test that is becoming more valid and acceptable is the ASTM F-2170 Relative Humidity (RH) test method which the normal acceptable values are between 75% RH and 85% RH.