Uw is the heat-loss rate of the whole window — frame and glass together. The lower the number, the less energy escapes. For passive house and net-zero work, it’s the number that decides whether the envelope closes.
Two values matter. Ug is the U-value of the glass alone; Uw is the U-value of the whole window, and it varies with unit size because the frame-to-glass ratio changes. A specification that quotes only Ug is telling you the best case, not the installed one.
What the numbers mean
Our triple-glazed, argon-filled glass reaches a Ug as low as 0.09, with whole-window Uw values installed in the 0.12–0.16 range — an R-8 to R-10 equivalent. A ‘warm edge’ spacer at the glass perimeter stops the cold bridge that otherwise drives condensation and heat loss at the edge of the unit.
Why it adds up to comfort
Low Uw isn’t only about the energy bill. A warm interior glass surface means no cold draft falling off the window, no condensation line in winter, and a room that’s usable right up to the glass. Pair that with the air-tightness of a tilt-turn seal and the comfort gain is larger than the raw numbers suggest.
A long head start
We brought our first Passive House–approved window to market nearly two decades ago, and have been refining the systems since. For projects chasing net-zero or certification, we’ll model the glazing to your target rather than quoting a catalog number.