Insulated Glass Unit Calculator

This calculator models an insulated glass unit's (IGU) thermal performance using a simplified center-of-glass heat-transfer model in the spirit of ISO 10292/EN 673. Total resistance sums fixed interior and exterior surface resistances (0.13 and 0.04 m²K/W), each pane's glass resistance (thickness divided by soda-lime's conductivity of about 1.0 W/mK), and one or two gas-filled gap resistances — a third pane and second gap only apply when you specify a nonzero third pane thickness, switching the unit from double to triple glazing. Each gap's resistance combines two heat paths: conduction/convection through the fill gas (its conductivity depends on which of air, argon, krypton, or xenon you choose — argon and the noble gases conduct less than air) and radiative transfer between the glass surfaces, which is where low-E coatings matter: dropping surface emissivity from about 0.84 (bare glass) to roughly 0.1 sharply cuts radiative heat flow across the gap. The reciprocal of total resistance gives the U-value; multiplying resistance by 5.678 converts it to imperial R-value units.

SHGC is a rough estimate — each clear pane transmits about 86% of incident solar energy and each low-E coating cuts that by roughly 15% — not a spectrally-resolved calculation. Condensation resistance is a simple heuristic ((1 − U/8) × 100) meant to rank relative performance, not to substitute for an AAMA-rated CRF. Treat all outputs as center-of-glass estimates: they exclude the spacer bar and edge-of-glass effects, which typically perform worse than center-of-glass numbers.