Relative humidity tells you a ratio. Dew point tells you where condensation will form.
RH = how full the air is relative to its capacity at that temperature
Dew point = the temperature at which moisture starts condensing
The critical difference: RH changes when you heat or cool the air. Dew point does not (unless you add or remove moisture).
Same room, different RH, different condensation risk
Scenario A:
Room: 75°F, 50% RH
Dew point: 55°F
Cold water pipe surface: 50°F
50°F < 55°F → condensation forms
Scenario B:
Room: 75°F, 30% RH
Dew point: 41°F
Cold water pipe surface: 50°F
50°F > 41°F → pipe stays dry
The rule: condensation forms when a surface temperature drops below the dew point. Not below some RH percentage.
Dew point depression as a quick diagnostic
Depression = Dry-bulb temp − Dew point temp
< 5°F → near saturation, high condensation risk
15–35°F → typical comfortable range
> 55°F → very dry air
Comfort boundary
ASHRAE 55 upper comfort limit: ~62°F dew point
Typical comfortable range: 50–60°F dew point
(~40–55% RH at 75°F)
Above 62°F dew point, most people feel the air is too humid regardless of the thermostat setting.
Why RH alone fails
Heat the air from 75°F to 95°F without adding moisture:
Before: 75°F, 50% RH, dew point 55°F
After: 95°F, 23% RH, dew point 55°F
RH dropped from 50% to 23%. Feels drier. But the dew point is still 55°F. The same cold pipe will still sweat.
For quick dew point checks from dry-bulb and RH or wet-bulb readings, there is a dew point calculator on CalcEngineer.
