Windows · Thermal

Window U-value calculator

A window is always worse than its glass, and the reason is the edge. U_w is the area-weighted average of the glazing and the frame, plus a penalty for the perimeter where the spacer bridges the two panes: U_w = (A_gU_g + A_fU_f + l_gΨ_g) / A_w. Split the same window into more lights and it gets worse, because you have added frame and edge without adding area.

The window

Overall width (mm)

Overall height (mm)

Frame face width (mm)

The visible width of the frame all round, seen from the front.

Lights across

Lights up

Glass

U_g of the glazing — W/(m²·K)

Centre-pane value. Work it out in the EN 673 calculator and bring it here.

Frame and spacer

Frame

Typical ranges to orient you. Use the manufacturer's declared U_f when you have it — profiles vary a lot within a family.

Spacer — Ψ_g W/(m·K)

Default tabulated values from EN ISO 10077-1 Annex E. A tested Ψ from the spacer manufacturer is always better.

—

Whole window U_w

— W/(m²·K)

Penalty over the glass

— W/m²K

Glazing ratio

— %

Edge length l_g

— m

Need the U_g first? The insulating glass calculator works it out to EN 673 from the panes, the cavity and the coating.

How EN ISO 10077-1 does it

U_w = (A_g·U_g + A_f·U_f + l_g·Ψ_g) / A_w // A_w = A_g + A_f A_g glazing area, the visible glass A_f frame area, everything else in the opening l_g total perimeter of the visible glazing edge Ψ_g linear transmittance of the glazing edge — the spacer's fault

The third term is the one people forget, and it is the one that separates a good window from a bad one built out of identical parts. It is a length multiplied by a coefficient, so it does not scale with area: the smaller the window, the more it hurts. A small window with a metal spacer can lose a couple of tenths of a W/m²K to the edge alone.

Why splitting a window makes it worse

Every mullion or transom you add does two things at once: it converts glass into frame, and it adds another glazed perimeter. Since the frame is nearly always worse than the glass, and the perimeter always carries a penalty, a window divided into four lights is meaningfully worse than the same opening as one. Change "lights across" above and watch it happen.

Typical values, to orient you

Frame	U_f W/(m²·K)
Aluminium, no thermal break	5.5 – 7.0
Aluminium with thermal break	2.0 – 3.5
PVC, 3 chambers	1.6 – 2.0
PVC, 5–6 chambers	1.0 – 1.4
Timber, softwood 70 mm	1.4 – 1.8

Worked example you can check by hand

1230 × 1480 mm, one light, 70 mm frame face, U_g = 1.10, U_f = 1.30, Ψ_g = 0.06 — the reference window size of EN 14351-1.

A_w = 1.230 · 1.480 = 1.8204 m² glass light = (1230−140)·(1480−140) = 1090 × 1340 A_g = 1.090 · 1.340 = 1.4606 m² A_f = 1.8204 − 1.4606 = 0.3598 m² l_g = 2 · (1.090 + 1.340) = 4.860 m U_w = (1.4606·1.10 + 0.3598·1.30 + 4.860·0.06) / 1.8204 = (1.6067 + 0.4677 + 0.2916) / 1.8204 = 1.30 W/(m²·K)

So a window with 1.10 glass in it performs at 1.30. The edge alone accounts for 0.16 of that — more than a tenth of the total, from a strip of metal a few millimetres wide.

What this page does not do

Simplified method. EN ISO 10077-1 is the hand calculation. The detailed route, EN ISO 10077-2, models the frame section by finite elements and is what gives a manufacturer their declared U_f.
No installation. The gap between the frame and the wall has its own linear transmittance, and a badly fitted good window beats nothing. That is Ψ_install, and it is not here.
No solar gain, no air leakage. U-value is heat loss by conduction only. A window's real energy balance also depends on the g-value and the airtightness class.
Rectangular, regular grids only. Arched heads, trapezoids and mixed light sizes need to be done light by light.
Typical U_f and Ψ_g are typical. They are here to let you sketch. For anything you declare or sell, use the tested values.

Read this. A pre-dimensioning and comparison aid, not a declaration of performance. A U_w that goes on a label or into a building regulations submission comes from tested component values and, usually, from the manufacturer's own certified calculation.