GLASS ENGINEERING / THERMAL
WINDOW THERMAL
PERFORMANCE
Uw is a property of the complete window, not only the frame or IGU. Glass and frame areas, the glass edge, element geometry, threshold and — separately — the installation thermal bridge all affect performance.
PRINCIPLE
Ug + Uf + ψg → Uw.
Ug describes the glazing, Uf the frame and ψg the linear thermal bridge at the glass edge. Uw is calculated for the geometry of a specific element. This is why the same profile system can deliver different Uw values in a small window and a large glazed unit.
Glazing thermal transmittance
Frame thermal transmittance
Linear glass-edge thermal bridge
Whole-window thermal transmittance
The formulas and examples on this page show where Uw comes from. A project calculation needs the actual frame and glass areas together with the chosen Uf, Ug and spacer-edge value.
WHAT WE CHECK
Specification starts with the problem.
Thermal performance should not be reduced to one catalogue number. A small opening window, a large fixed pane and a door with a low threshold have very different frame-to-glass and edge proportions.
Glass-to-frame ratio changes Uw
As the glazing share increases, Ug has more influence on the result. In highly divided windows, Uf and edge zones become more important.
Warm-edge spacers improve the glass perimeter
Reducing ψg improves edge temperature and whole-window Uw, especially in elements with a large glazing perimeter.
Thresholds need their own thermal detail
Low thresholds, support profiles and bearing points can become the weakest thermal zone even in a high-performance system.
A good Uw does not remove the installation thermal bridge
Declared window Uw does not automatically describe the frame-to-wall junction. Frame position and installation-zone materials must be analysed separately.
PRACTICAL EXAMPLE
The basic Uw formula
Uw = (Ag×Ug + Af×Uf + lg×ψg) / (Ag + Af). It shows why a marketed profile Uf value cannot be presented as the performance of the complete window. The installation junction adds a separate thermal-bridge problem.
TECHNICAL SOURCES
START A PROJECT
We calculate the assembly, not a single attractive number.
Send element sizes, system, glazing and the wall junction. We can identify where the real thermal result is formed.
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