According to ASHRAE standard 62.1, what relative humidity should the occupied space be limited to by design?

• A. 65%
• B. 60%
• C. 70%
• D. 75%

Answer: (A)

The thing being tested is the practical upper limit for humidity in spaces that are in use, to keep IAQ acceptable and avoid moisture problems. ASHRAE 62.1 specifies an upper bound on relative humidity for occupied spaces so that surface condensation, mold growth, and comfort issues don’t arise while still allowing feasible ventilation and energy use.

The best answer is 65% because the standard sets 65% as the design ceiling for indoor RH in occupied spaces. Keeping RH at or below this level helps prevent condensation on cooler surfaces and reduces the risk of mold and other moisture-related IAQ problems, while not being so restrictive as to make control systems unnecessarily burdensome. Higher values like 70% or 75% would raise the risk of condensation and microbial growth, and 60% would be more conservative than the standard’s stated design limit.