The Thermal Break Window
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Problem
Windows are the great contradiction of northern building: we need them for light, for connection to the world, for sanity through the long dark — but every window is a hole in the envelope, a place where heat pours out and cold pours in. When the interior surface of a window drops below the dew point, condensation forms. When it drops below freezing, ice grows on the glass. The sill rots. The wall around it grows mold. People push furniture away from windows, hang curtains to block the cold draft, and the window becomes a problem to manage rather than a source of light and life.
Evidence and Discussion
The physics are unforgiving. A standard double-glazed window with an aluminum frame has a U-value around 2.5 W/m²K — roughly ten times worse than a well-insulated wall. But the frame matters as much as the glass. Aluminum conducts heat 1,000 times faster than wood; an aluminum frame without a thermal break becomes a direct thermal bridge from inside to outside. At -30°C, the interior surface of an unbroken aluminum frame can drop to -5°C or colder. Condensation is inevitable. Ice is likely.
The thermal break — a strip of low-conductivity material (usually polyamide) that separates the interior and exterior aluminum sections — was developed in Europe in the 1970s and has become standard in Scandinavian and German construction. Combined with triple glazing and insulated spacers between the panes, modern high-performance windows achieve U-values of 0.8 W/m²K or better. The Passive House Institute certifies windows below 0.85 W/m²K for cold climates. At this level, the interior glass surface stays warm enough — above 15°C even at -30°C outside — that condensation rarely forms and occupants can sit within arm's reach of the window without feeling a cold draft.
Edmonton's 5,200 heating degree days demand this level of performance. The city's building code has steadily tightened window requirements, but code-minimum windows still condense at extreme temperatures. The difference is felt most acutely in the shoulder seasons — October and March — when interior humidity rises and outdoor temperatures hover near zero. A window that passes code may still weep for weeks. The old rule holds: the coldest surface in the room determines where moisture collects. Make the window warm, and the problem moves elsewhere — ideally, out the ventilation system.
Installation matters as much as specification. A high-performance window poorly installed — with gaps in the air barrier, thermal bridges through the mounting screws, or insufficient insulation in the rough opening — loses much of its advantage. The window must be set in the insulation plane of the wall, not at the exterior sheathing. The Passive House approach seats the window in the middle third of the wall depth, surrounded by insulation on all sides except the interior face. This creates the deep reveal that Light on Two Sides (159) and Deep Walls (46) call for, and eliminates the thermal bridge at the frame's edge.
Therefore
In cold climates, specify windows with a whole-unit U-value of 0.85 W/m²K or better — triple-glazed, with insulated spacers and thermally broken frames. Set the window in the insulation plane of the wall, not at the exterior sheathing. Insulate the rough opening continuously. Test by measuring the interior glass surface temperature during the coldest week of winter: it should stay above 15°C when the room is heated to 20°C. If ice forms, or if condensation persists for more than an hour after cooking or bathing, the window or its installation has failed.