In harsh climates or for Passivhaus architecture, double glazing is no longer sufficient. Inserting a third central pane revolutionises the thermodynamics of the window, creating a transparent element that insulates almost as well as a brick wall.
In a double-glazed unit, the optimal space for Argon gas is 16 mm; beyond this measure, the gas triggers vertical convective motion that worsens insulation. The third pane divides a very thick cavity into two separate, sealed chambers, cancelling out convection. Above all, it provides an extra surface to host the magnetron coatings: using two Low-E coatings (Faces 3 and 5), a double trap for thermal infrared is obtained, halving transmittance compared to the standard.
Three layers of normal glass would create a dense visual filter with a greenish light. The central pane in extra-clear (low-iron) glass, free of iron oxides, is therefore mandatory. Structurally, each pane adds 2.5 kg/m2 per millimetre: a large French door can weigh over 100 kg, requiring heavy-duty hardware.
Standards
European and international references applicable.
Physical properties
Usage environment
Cold climates, Passivhaus and nZEB standards. Caution: the central third pane can overheat in hot climates. Use toughened glass (ESG) for the central pane on south-facing facades.