U-value is the single number that tells you how much heat a window or door loses. Lower is better. But the way it is quoted can mislead you, so it is worth understanding before you compare products.
Ug, Uf and Uw
Ug is the glass alone. Uf is the frame. Uw is the whole window — the figure that actually matters, because it combines glass, frame and spacer. A shiny Ug on the glass means little if the Uw is quoted separately or not at all. Always compare Uw to Uw.
- Ug
- glass centre-pane only
- Uf
- frame only
- Uw
- whole window — use this
- Lower Uw
- less heat loss
- Triple glazing
- lower Uw, more weight
Frame depth and warm edges
Deeper thermally broken frames — such as the higher AWS depths or ASE 80 doors — leave more room for insulation and post better numbers. Warm-edge spacer bars around the glass unit shave off further loss and reduce condensation at the edges.
Glazing does the heavy lifting
For most homes the glass specification changes the Uw more than the frame. Low-emissivity coatings, argon fill and the right spacer make a bigger difference than an extra few millimetres of profile. Solar gain is a related question — see solar control.
Reading a quote honestly
Ask every supplier for the whole-door Uw on the exact configuration you are buying. If one quote only gives a glass figure, it is not comparable. A supplier used to specifying to a target — like premium aluminium systems — should give you the Uw without hesitating.