EN ISO 10077-1 explained: the U-value of a window (Uw)

What the calculator works out with it

• The whole-window U_w, in W/(m²·K).
• How much worse the window is than its glass, and what share of the loss goes through the glass, the frame and the edge.
• The glazing ratio and the length of the glazing edge.

What it asks for

• The overall width and height, one frame face width all round, the lights across and up and the mullion or transom width.
• The U_g of the glazing, from the EN 673 calculator.
• The frame's U_f: your manufacturer's figure, or one of the typical values the calculator offers to orient you: Aluminium, no thermal break — Uf ≈ 7.0; Aluminium, thermal break — Uf ≈ 3.2; Aluminium, high-performance break — Uf ≈ 2.2; PVC, 3 chambers — Uf ≈ 1.8; PVC, 5–6 chambers — Uf ≈ 1.3; PVC, reinforced high performance — Uf ≈ 1.0; Timber, softwood 70 mm — Uf ≈ 1.6; Timber, 90 mm or laminated — Uf ≈ 1.2.
• The spacer's Ψ_g: a tested figure, or the default values the calculator takes from annex E of the standard: Aluminium spacer, metal frame with break — 0.08; Aluminium spacer, PVC or timber frame — 0.06; Warm edge, metal frame with break — 0.06; Warm edge, PVC or timber frame — 0.05; Aluminium spacer, metal frame with no break — 0.02.

How it is calculated, and what the site assumes

The simplified method of EN ISO 10077-1: U_w = (A_g·U_g + A_f·U_f + l_g·Ψ_g) / (A_g + A_f), with the areas and the length of the glazing edge worked out from the overall size, one frame face width all round and a regular grid of lights separated by mullions or transoms.

An example worked by the calculator

The reference window size of EN 14351-1, 1230 × 1480 mm, one light, a 70 mm frame face, U_g = 1.10, U_f = 1.30 and Ψ_g = 0.06:

• window area A_w = 1.8204 m², glazing A_g = 1.4606 m², glazing edge l_g = 4.86 m;
• U_w = 1.30 W/(m²·K), 0.20 worse than the glass;
• of the heat lost, 68 % goes through the glass, 20 % through the frame and 12 % through the glazing edge.

Open this example in the calculator. The calculator page works the same window out by hand.

What it leaves out

• One frame face width all round: a wider bottom rail or a different mullion cannot be entered as such.
• Rectangular, regular grids only.
• No installation thermal bridge (Ψ_install), no solar gain, no air leakage.
• The typical U_f and Ψ_g are typical: a declared value comes from tested components or from EN ISO 10077-2.

The calculator is run against 1 published reference case, side by side with the published figure and the tolerance we accept, on how we check the calculations.

A pre-dimensioning and comparison aid, not a declaration of performance: a U_w for a label comes from tested components and the manufacturer's certified calculation.

Sources

• NSAI Standards Store, I.S. EN ISO 10077-1:2017 (the Irish adoption; a corrected version followed in 2020)

Questions people ask

What does EN ISO 10077-1 calculate?

The U-value of a whole window or door, Uw, from the U-value of the glazing, the U-value of the frame and the linear thermal transmittance of the glazing edge, weighted by their areas and the edge length.

What Uw does a 1230 × 1480 mm window with Ug 1.1 glass get?

1.30 W/(m²·K) by the site's calculator, with a 70 mm frame face, a frame of Uf 1.3 and a spacer of Ψ 0.06: 12 % of the loss goes through the glazing edge.

Does EN ISO 10077-1 include the fitting of the window?

Not in the calculator: the thermal bridge at the joint between frame and wall, solar gain and air leakage are left out, as the calculator page and how we check the calculations say.