Question: We are designing a facility in the Southwest where the water table is hundreds of feet below grade. Do we really need a vapor retarder beneath the slab?
Answer: Yes. Regardless of where a project is being constructed, effective below-slab moisture protection is an absolute necessity for any slab that is to receive a moisture-sensitive floor covering or coating. It does not matter if the water table is 10 feet or 510 feet down. Once the building is constructed, the structure itself will inhibit evaporation of moisture from the ground.
Regardless of the depth of the water table, once the slab is placed and covered with a low-permeance flooring material or coating, moisture beneath the slab will increase over time and most often reach close to 100% relative humidity.
In addition, to comply with published flooring industry guidelines, using a vapor retarder beneath floor coverings is mandatory. To omit or remove a vapor retarder from the design or construction process places those responsible for such action at risk of being cited for non-compliance with industry standards, should a moisture-related flooring problem develop.
Question: How can we determine if it is safe to proceed with our flooring installation over a concrete slab?
Answer: Several field tests will be required to establish if the moisture level within the concrete is low enough for the flooring installation to proceed safely. Historically, the calcium chloride, moisture vapor emission rate (MVER) test (ASTM F 1869) has been the most commonly used and referenced method of testing the slab moisture level.
However in recent years, much has been learned about what the MVER test does and does not measure. The calcium chloride MVER test has been shown to measure moisture present in the near surface region or top ¾ inches of the slab only. The test does not tell one anything about how much moisture is present deeper within the concrete.
While it is helpful to know the level of moisture near the slab surface, it is as, if not more important to know how much moisture is present deeper within the concrete that will rise and increase the moisture level at the slab surface once the flooring is installed.
To measure moisture deeper within the concrete, relative humidity sleeves are installed into holes drilled into the hardened slab. Measurements are then taken with a sensor placed into the sleeve after a minimum exposure period of 72 hours. Testing the internal relative humidity within a concrete slab has been done in Europe for many years and is done now in this country and covered by ASTM F 2170.
Using these two tests together can give those responsible for making the decision about moisture-related suitability of the slab a much better picture than one would have with MVER test results alone.
