A.Energy and sustainable design consultant Andy Shapiro responds: The dewpoint is not a location; it is the temperature at which water will condense out of the air. Since the dewpoint changes with the amount of humidity in the air, as well as the air temperature, the dewpoint for a particular temperature and relative humidity is best looked up in a table or a psychrometric chart (see below).

Water from the air will condense on building components when they are below the dewpoint of the air that’s in contact with them. Cold water pipes in hot, humid summers condense water and drip. Uninsulated basement floors in hot, humid summers are often below the dewpoint of the hot, moist outside air, so water condenses on them if the space is...

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