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Cover image for Quick Tip: Dew Point vs Relative Humidity for Condensation Risk
Evgenii Konkin
Evgenii Konkin

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Quick Tip: Dew Point vs Relative Humidity for Condensation Risk

#hvac #engineering #quicktip #psychrometrics

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
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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
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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
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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)
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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
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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.

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