If indoor conditions are 70°F (21°C) and 60% RH, at what temperature does condensation begin to occur on surfaces?

To understand at what temperature condensation begins to occur on surfaces, it is important to consider the concept of dew point. The dew point is the temperature at which air becomes saturated with moisture, meaning it can no longer hold all the water vapor present, leading to condensation.

In this question, the indoor conditions are 70°F (21°C) with a relative humidity of 60%. To determine the dew point, we can use a dew point calculator or refer to psychrometric charts. At 60% relative humidity and 70°F, the dew point is approximately 56°F (13°C). This indicates that any surface that cools to this temperature or lower will start to experience condensation since the air in contact with those surfaces cannot hold all of its moisture anymore.

Thus, the correct answer reflects that condensation begins when the temperature reaches this dew point of 56°F (13°C), making it the temperature at which moisture will start to visibly condense on surfaces. This highlights the critical relationship between temperature, humidity, and condensation in drying and moisture control processes.