Single, double, and triple pane heat loss rates
A single-pane window allows 5.7 BTU per hour per square foot per degree Fahrenheit temperature difference, meaning heat floods out rapidly in winter. Double-pane with air gap cuts that nearly in half to 2.8. Triple-pane with two air gaps (or two gaps plus an inert gas fill) compresses it further to 0.8, a seven-fold improvement. The gains come from stacking insulating layers and using low-conductivity gases (argon or krypton) instead of air. Low-emissivity coatings further suppress radiative heat transfer.
Diminishing returns beyond double pane
The jump from single to double pane yields dramatic savings at modest cost, explaining why double-pane became code minimum decades ago. Moving from double to triple adds 30-50% upfront cost for only a 70% reduction in U-value. In moderate climates, the math favors high-performance double pane (low-E coatings, inert gas fill) over triple, balancing comfort, solar gain in winter, and cost. Far north and alpine climates lean triple for winter survival and occupant comfort during extended cold.