Q. We build the concrete floor for a building occupied by a tenant who requires tight control of temperature and relative humidity in the work area. The tenant must maintain a relative humidity of 30% to 50% at a temperature of 68° F. But even though HVAC equipment was installed, the relative humidity won't stay within the required range.
The bare concrete floor in the work area is nine months old. Using the calcium-chloride cup test, an engineering testing firm measured water-vapor emission rate through the floor and found it to be 6 pounds/square foot/24 hours. The firm also took samples of the subgrade and found that it wasn't excessively wet. However, I'm being blamed for the relative-humidity problem. Is a 6-pound water-vapor emission rate high enough to overwhelm the HVAC system?
A. We asked Howard Kanare, Construction Technology Laboratories, Skokie, Ill., about your problem because he has investigated many problems involving floor moisture emissions. Howard says he has seen significant increases in room relative humidity when the water-vapor emission rate is 12 pounds or more, but not when it's 6 pounds. He also notes that with the proper choice of HVAC equipment, room relative humidity usually can be easily held to within ±5% of a target value.
He advises that you hire an independent refrigeration engineer (not the one who installed the existing HVAC system) to evaluate the HVAC system and its operation.