Insulating Glass U-Factor and Interior Condensation Screen

Everything in a glazing unit is in SERIES, so the resistances add and the U-factor is the reciprocal. What is striking is how little the glass contributes: a light of glass is about R-0.03, essentially nothing, and the entire performance of an insulating unit lives in the gap and in the air films -- which is exactly why a low-emissivity coating and an argon fill work, because they attack the gap, the only meaningful resistance in the stack-up. The surface temperature line is the useful one. Interior glass runs colder than the room by a fraction of the indoor-outdoor difference, and that fraction is U times the indoor film resistance. A poor unit puts the glass far below room temperature and it condenses; a good one keeps it up. At 70 F inside and 0 F outside, a clear double-glazed unit with a half-inch air gap comes to R-1.93, U-0.518, and 45.3 F at the interior surface, while a low-e argon unit comes to R-3.04, U-0.329, and 54.3 F -- nine degrees of interior surface temperature from one coating and one gas fill. At 40 percent indoor relative humidity the dew point is about 44.6 F, so the clear unit is within a degree of running with water on it and the low-e unit has nine degrees of margin. Raise the indoor humidity to 55 percent, an ordinary winter kitchen, and the dew point climbs to about 53 F: the clear unit condenses heavily and the low-e unit is now the one within a degree of trouble. This is the honest answer to a complaint that new windows are sweating, which is usually not a window defect at all but an indoor humidity the old, leaky windows were quietly removing. CENTER-OF-GLASS PERFORMANCE ONLY, which is the best part of a window. The whole-window U-factor is worse, because the edge of the glass, the spacer, and the frame all conduct more than the center, and an NFRC-rated whole-window U-factor is the number that belongs on a code compliance form. Gap resistances depend on gap width, fill gas, coating emissivity and its surface position, and the temperature difference itself, so published values are condition-specific. Solar heat gain, air leakage, condensation resistance ratings, and the frame and sill condensation that is usually worse than the glass are all separate. NFRC ratings, the manufacturer's data, and the adopted energy code govern.

R_total = R_indoor film + lites x R_lite + (lites - 1) x R_gap + R_outdoor film; U = 1 / R_total; T_surface = T_in - U x R_indoor film x (T_in - T_out); dew point by the Magnus relation; the critical relative humidity is the RH whose dew point equals T_surface.

Series-resistance summation with the standard ASHRAE indoor and outdoor air film resistances, U = 1 / R_total, and the interior-surface temperature relation T_surface = T_in - U x R_indoor film x (T_in - T_out), by name; dew point by the Magnus relation. Center-of-glass only: the whole-window U-factor including the edge, spacer, and frame is worse, and an NFRC rating is what a code compliance form wants. NFRC ratings, the manufacturer's data, and the adopted energy code govern.

The series summation, the surface-temperature relation, and the Magnus dew point are all public; the gap and film resistances are user-entered values from published manufacturer and ASHRAE data.

Estimate. AHJ and licensed professional govern.

Field names used by the API: lites, r_per_lite, r_gap, r_indoor_film, r_outdoor_film, indoor_temp_f, outdoor_temp_f, indoor_rh_pct, r_total, u_factor, surface_temp_f, dew_point_f, critical_rh_pct

• The gap is the whole story glass is about R-0.03 per light; the gap and the air films carry the performanceASHRAE Fundamentals
• Center of glass only the NFRC whole-window U-factor including edge, spacer, and frame is worse and is what a code form wantsNFRC
• Gap R is condition-specific it depends on gap width, fill gas, coating emissivity and position, and the temperature difference itselfmanufacturer data