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ISO 10077-1Whole-window UwFree

Window U-Value
Calculator

Combine the centre-of-glass, frame, and edge contributions into a whole-window U-value (Uw) — the area-weighted figure used for energy compliance.

Who it's for: architects and energy consultants combining certified glass, frame, and spacer data into a whole-window Uw at construction documents for compliance submissions.

Uw = (Ag·Ug + Af·Uf + Lg·Ψg) / Aw
Ag = glass area, Ug = centre-of-glass U; Af = frame area, Uf = frame U; Lg = glass perimeter, Ψg = edge-of-glass linear transmittance (≈ 0.04–0.08 W/m·K for warm-edge spacers). Aw = Ag + Af.
Window
Uw

How to use this calculator

1
Enter the glass area Ag
The sight-glass area — the visible glazing excluding the frame. Find it on the product datasheet or measure from your drawing.
2
Enter the glass U-value Ug
The certified centre-of-glass thermal transmittance from the glazing manufacturer or NFRC data; triple-low-e glass typically runs 0.6–1.1 W/m²K.
3
Enter the frame area Af
The total opaque frame area: total window area (width × height) minus the glass area Ag. A typical frame adds 15–25% of the overall window area.
4
Enter the frame U-value Uf
From the frame manufacturer's test certificate or simulation; timber and uPVC frames commonly range 1.0–2.0 W/m²K, aluminium frames higher.
5
Enter the glass perimeter Lg and edge factor Ψg
Lg is the perimeter of the glass pane (m); Ψg is the spacer bar linear transmittance — 0.04–0.06 for warm-edge types, 0.08–0.12 for aluminium.
6
Read the whole-window Uw
The area-weighted result is the single U-value required for energy compliance declarations and building-regulation submissions.

The formula

The whole-window U-value is an area-weighted combination of the three heat-loss paths through a window: glass, frame, and edge-of-glass strip.

Uw = (Ag · Ug + Af · Uf + Lg · Ψg) / Aw

Uw — whole-window thermal transmittance (W/m²K)
Ag — glass (sight) area (m²)
Ug — centre-of-glass U-value (W/m²K)
Af — frame area (m²); Aw = Ag + Af
Uf — frame U-value (W/m²K)
Lg — glass perimeter (m) — the linear length of the edge-of-glass zone
Ψg — linear thermal transmittance of the spacer bar (W/m·K)

Worked example

Example
A window with glass area Ag = 1.40 m², Ug = 1.10 W/m²K, frame area Af = 0.40 m², Uf = 1.40 W/m²K, perimeter Lg = 4.8 m, and warm-edge spacer Ψg = 0.06 W/m·K gives: Aw = 1.80 m²; heat flows = 1.54 + 0.56 + 0.29 = 2.39 W/K; Uw = 2.39 / 1.80 = 1.327 W/m²K (frame fraction 22%).

When this estimate will be off

  • Uses certified product values — substituting catalogue approximations for Ug or Uf gives a rough estimate, not a compliance declaration.
  • Does not include installation thermal bridges at the rough opening (reveal Ψ), which can raise the installed window U-value by 0.1–0.3 W/m²K.
  • Assumes a single homogeneous glazing type; windows with mixed pane sizes or different glass specifications need a separate Uw per zone.
  • Frame and spacer U-values vary significantly by product and fixing method; a value calculated from catalogue data is not a certified result for building permits.

Frequently asked questions

A common mistake: Uses certified product values — substituting catalogue approximations for Ug or Uf gives a rough estimate, not a compliance declaration.

Ug is the centre-of-glass U-value — the thermal performance of the glazing unit measured at its centre, away from edge effects. Uw is the whole-window U-value, which adds frame and edge-of-glass losses weighted by their areas. Uw is always higher (worse) than Ug and is the figure required for energy compliance and building regulations.

Warm-edge spacers (foam, silicone-foam, or thin stainless-steel types) achieve Ψg 0.04–0.06 W/m·K. Standard aluminium spacers run 0.08–0.12 W/m·K. Use the certified value from the spacer manufacturer. ISO 10077-1 Annex E provides default values when test data is unavailable.

Af is the total window opening area minus the visible glass area Ag. For a window without a datasheet, a typical frame width is 80–120 mm. Multiply the frame perimeter length by the frame width to estimate Af. The result can also be read off many certified window test reports.

No. The ISO 10077-1 Uw covers the window product only — glass, frame, and edge spacer. Heat loss through the joint between frame and rough opening (the reveal thermal bridge) is assessed separately using a linear transmittance value from ISO 10211 testing or a national annex default.

Sources

  • ISO 10077-1:2017 — Thermal performance of windows, doors and shutters — Calculation of thermal transmittance — Part 1: General (area-weighted Uw method)

Sources and review record

Sources and method basis

Limitations

  • Requires certified user inputs; excludes installation/reveal bridges, mixed zones, geometric verification, local targets, and certification declarations.

This free Window U-Value Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.