Uw, Uf and Ug: what the numbers on a window mean

Q: Why do two quotations state a different Uw for the same system?

Because Uw depends on the typology and the dimension, not on the series alone. It is declared per typology, and in the Declaration of Performance it refers to a reference size — for windows to EN 14351-1 usually 1.23 × 1.48 m.

Uf is the thermal transmittance of the frame, Ug that of the glazing unit and Uw that of the complete window. Uw depends on the specific dimensions, on the ratio of frame to glass, and on the thermal bridge around the perimeter of the glazing unit.

That is why we do not compare two windows on the Uf of the series alone, or on the Ug of the glass alone.

U_f concerns the frame alone, U_g the glazing alone and U_w the whole window — and only U_w compares two quotations. It is not an average of the other two: to EN ISO 10077-1 it follows from the areas and the perimeter of the glazing, so the same series gives a different U_w at a different size. The smaller the window, the greater the proportion taken up by the frame and the more the U_f weighs. That is why U_w is declared per typology and dimension — in the Declaration of Performance at a reference size, for windows to EN 14351-1 usually 1.23 × 1.48 m.

How Uw is arrived at

Uw is not an average of Uf and Ug. To EN ISO 10077-1 it is calculated from the areas and the perimeter of the glazing:

U_w = (A_g·U_g + A_f·U_f + l_g·Ψ_g) / (A_g + A_f)

A_g the glazed area, A_f the frame area,
l_g the perimeter of the glazing,
Ψ_g the linear transmittance at the glazing edge — which is exactly where a warm edge spacer acts.

Why the same system gives a different Uw

The smaller the unit, the larger the share taken by the frame, and the more the Uf weighs. A small opening window has a worse Uw than a large fixed light of the same series with the same glass. That is why Uw is declared per type and size, not per series.

In a Declaration of Performance, Uw refers to a defined reference size — for windows to EN 14351-1 usually 1.23 × 1.48 m. Your own opening will rarely be that size.

Uw says nothing about air permeability, watertightness or wind resistance. Those are separate classes to EN 12207, EN 12208 and EN 12210, declared separately.

Uw, Uf and Ug side by side

Quantity	What it covers	When we use it
Uf	The frame only	Characterises the profile series, not your window
Ug	The glazing unit only	Characterises the build-up of the glass
Uw	The complete window	The only quantity by which two quotations can be compared

Uw depends on the specific dimensions, on the ratio of frame to glass and on the thermal bridge around the perimeter of the glazing unit.

Frequently asked questions

Why do two quotations state a different Uw for the same system?

Because U_w depends on the typology and the dimension, not on the series alone. It is declared per typology, and in the Declaration of Performance it refers to a reference size — for windows to EN 14351-1 usually 1.23 × 1.48 m.

Does Uw say anything about air tightness?

No. Air permeability, watertightness and resistance to wind load are separate classes to EN 12207, EN 12208 and EN 12210, and are declared separately.