In high-performance buildings, we have to manage energy and moisture with equal intensity. The psychrometric chart helps us understand how they are linked and gives us key data points for managing moisture. Using just the vertical temperature lines, the horizontal humidity lines, and the curved relative humidity lines, we can do two important things.
Determine the dew point. Every point on the psychrometric chart has coordinates that give us the dry-bulb temperature and the relative humidity (RH). By moving leftward in a straight line from any coordinate, we can travel all the way to the saturation curve and from there straight down to determine the dew point of that air. At 68°F and 60% RH, the dew point is around 53°F; at 70% RH, the dewpoint is 58°F—a temperature that surfaces like windows or HVAC ducts could easily reach, resulting in condensation. The relationship between temperature and relative humidity is important for design of both HVAC systems and the building envelope.
Determine the mass of water in any volume of air. By moving rightward in a straight line from any point on the chart, we can determine the weight of the water in a given volume of air. This helps determine how well a building space can accommodate expected increases in the water content of the air, which affects HVAC system design and specifications.
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