Condensation Physics: Dew Points, Relative Humidity, and Psychrometric Charts

## The Psychrometric Science of Condensation Condensation is governed by the laws of psychrometrics—the thermodynamic study of moist air. Air at any given temperature has a maximum capacity to hold water vapor. When warm, humid indoor air is cooled by contact with a cold window pane, its relative humidity reaches 100% (saturation), and water vapor condenses into liquid droplets. ### The Dew Point Formula & Indoor Boundaries: The dew point $T_d$ can be approximated using the Magnus-Tetens formula: $$T_d = \frac{b \cdot \alpha(T, RH)}{a - \alpha(T, RH)}$$ where $a = 17.27$, $b = 237.7^\circ\text{C}$, and $\alpha(T, RH) = \frac{a T}{b + T} + \ln(RH/100)$. ### Why Single-Pane Windows Condense Constantly: - **Outdoor Temperature:** 20°F | **Indoor Temperature:** 70°F (40% RH, Dew Point = 45°F). - **Single-Pane Interior Glass Temp:** ~ 32°F (Well below 45°F dew point $\rightarrow$ heavy condensation & frost). - **Energy Star Double-Pane Interior Glass Temp:** ~ 58°F (Well above 45°F dew point $\rightarrow$ zero condensation). Contact ServiceFind Windows Pro to upgrade your thermal comfort with our upfront **Fair Quote**.