IndoorComfortKit What the window estimate actually means
The model balances heat flow through a simplified, uniform window. It uses Ts = Tin − U × 0.13 × (Tin − Tout), with temperatures in °C and U in W/(m²·K). The indoor surface resistance of 0.13 m²·K/W is a fixed, site-defined modeling assumption here; it is not a measured air film in your room.
The condensation threshold is 100 × es(Ts) / es(Tin), capped at 100% RH. Saturation pressure is es(T) = 6.112 × exp(17.62T / (243.12 + T)) hPa. This is the same Magnus-form family as our dew point tool. Below freezing it describes liquid-water saturation, not frost formation.
A whole-window U-factor averages the glazing, frame and other components. Applying that number here gives an equivalent uniform-surface scenario. It does not calculate the center-of-glass temperature, the coldest edge or the NFRC Condensation Resistance rating. The error is not guaranteed to be conservative. US U-factor values are multiplied by 5.678 to convert to SI units.
A worked cold-night example
With 21 °C indoors, 0 °C outdoors and U = 2.8, the model surface is about 13.4 °C and the liquid-water saturation threshold is about 61.7% RH. This is calculated, not a recommended room setting. A colder edge could become wet earlier. Changing U to 1.8 raises the modeled surface to about 16.1 °C and its threshold to about 73.5%.
Use a measured surface when you can
Measure where water actually appears, and record the indoor reading at the same time. A surface reading replaces the estimate for the current result. It does not establish how that surface will change on another night, so the outdoor-temperature table is omitted in measured mode.
Reflective glass can mislead infrared instruments. Follow your instrument's surface and emissivity instructions; do not treat an unverified infrared reading as the true glass temperature. The room-and-window dew point guide explains this measurement distinction.
Edges, curtains and changing weather
Frame bridges, glazing spacers and sheltered air behind curtains may make a small region colder than this scenario. Wind, sunlight, radiative cooling, indoor airflow and heat stored in the frame are not modeled. No fixed safety margin is supplied. Visible wetness takes priority over a reassuring calculated number.
Wipe accumulated water and investigate repeated dampness. If you were adding moisture, pause and reassess before raising the setting. Fog between sealed panes and water from a leak require a different check from room-side condensation.
References for these winter assumptions
- NFRC: windows and condensation
Supports: Whole-window U-factor and the distinction between glass, edges and frame.
Does not support: Our surface-temperature estimate, a safe RH, or a condensation guarantee.
Source checked:
- Pilkington: indicative whole-window U values (historical table)
Supports: The three specified wood/PVC-frame examples: 4.8, 2.8 and 1.8 W/m²·K.
Does not support: Current regulations, a generic value for every glazing type, or measured glass temperature.
Source checked:
The examples are educational planning scenarios, not building diagnosis, medical advice or whole-house HVAC design. See fixed arithmetic test cases and the source registry.