Dew point depends only on how much moisture the air holds, so it is a direct measure of absolute humidity. Any surface colder than the air's dew point gets wet: windows, cold water pipes, ducts in a humid attic, and a cooling coil. That last case is the point: a coil must run below the entering dew point to dehumidify. Dew points above about 60 degrees F feel muggy indoors.
Attic dew point was 72 and those supply ducts run at 58, of course they're sweating through the ceiling.
Duct insulation in a humid attic must have an intact vapor jacket; one torn seam and the duct sweats.
Also heard as
- dewpoint
- DP
- dew point temperature
Related terms
Relative humidity
The moisture in air as a percentage of what would saturate it at the same temperature, the number a humidistat shows.
Humidity ratio
The weight of water vapor per weight of dry air, usually given in grains per pound.
Latent heat
Heat tied up in moisture, added or removed when water evaporates or condenses without changing air temperature.
Infiltration
Outdoor air that sneaks into a building through gaps in its shell, pushed by wind, stack effect and fans rather than by design.