Aluminium system catalogues and quotations are full of Uf, Ug, Uw and Ψ. They are often confused, or incomparable values are compared — Uf of one system against Uw of another. Here is what each coefficient means.
What U means
Thermal transmittance U shows how much heat (in watts) passes through 1 m² of a construction for a 1 K temperature difference between inside and outside air. The unit is W/(m²·K). The lower the value, the better the unit keeps heat in.
Ukrainian building codes use the inverse — thermal resistance R in m²·K/W. For a quick estimate, R ≈ 1/U.
Ug — glass
Ug describes the centre of the glazing unit, without edge effects. It is calculated to EN 673. For orientation:
single glass — about 5.7 W/(m²·K);
ordinary double glazing without coatings — 1.8–3;
triple glazing — 0.5 to 1 depending on coatings and gas fill.
The biggest improvements come from low-emissivity coatings and inert gas fills (argon or krypton).
Uf — frame
Uf is the thermal transmittance of the frame and sash profile, calculated to EN ISO 10077-2 by numerical modelling of the section. For aluminium, Uf is governed mainly by the thermal break — the polyamide insert between the outer and inner profile halves. For comparison, an aluminium frame without a thermal break has a Uf of around 6.8, and with a 42 mm insulating profile around 1.3 W/(m²·K).
Ψ — glass edge
Ψg (psi) is the linear thermal transmittance where the glazing unit sits in the frame. Here heat bypasses the insulation through the spacer and glass edge. Ψ is measured in W/(m·K) and multiplied by the glass perimeter. An aluminium spacer gives a noticeably higher Ψ than a warm-edge spacer made of polymer composites or stainless steel.
Uw — the whole window
Uw is the resulting coefficient for the whole window. Per EN ISO 10077-1:
Uw = (Ag·Ug + Af·Uf + lg·Ψg) / (Ag + Af)
where Ag and Af are the glass and frame areas and lg is the visible glass perimeter.
An important consequence: Uw depends on window size and division. A narrow window with several sashes has a larger frame share and a longer glass perimeter, so its Uw is worse than that of a large fixed window with the same profile and glass. That is why EN 14351-1 uses a reference window size of 1.23 × 1.48 m for declared values.
Worked example
Take a 1.23 × 1.48 m (1.82 m²) single-sash window. Illustrative inputs:
glass area Ag = 1.32 m², Ug = 0.6;
frame area Af = 0.50 m², Uf = 1.6;
glass perimeter lg = 4.62 m.
With a warm-edge spacer (Ψ = 0.04): Uw = (0.79 + 0.80 + 0.18) / 1.82 ≈ 0.98 W/(m²·K), so R ≈ 1.02 m²·K/W — the zone I requirement (0.90) is met.
With an aluminium spacer (Ψ = 0.11): Uw ≈ (0.79 + 0.80 + 0.51) / 1.82 ≈ 1.15 W/(m²·K), R ≈ 0.87 — the zone I requirement is not met.
Same profile, same glass — and the outcome depends on a spacer costing a few percent of the window.
The values in the example are illustrative and only show the method. For a real project use data from the system and glazing manufacturers.
How to read catalogues
Compare like with like: Uf with Uf, Uw with Uw.
Check which size and configuration a Uw value refers to.
Check whether Ψ is included and which spacer was assumed.
Facades use the equivalent Ucw, calculated to EN ISO 12631.
Common myths
“More chambers in the profile means warmer.” For aluminium, the width and design of the thermal break matter, not the chamber count as such.
“A four-pane unit is always better than triple glazing.” Coatings, gas and cavity width matter as much as the number of panes; each extra pane also adds weight.
“The catalogue Uw is my window’s Uw.” The catalogue value refers to the reference size; a narrow window with mullions will perform worse.
“The spacer is a detail.” As the example above shows, it can decide whether the window meets the code.
“Low U protects against heat.” U describes heat loss; protection from overheating comes from solar-control coatings, which are described by other values.
Summary
Uf describes the profile, Ug the glass, Ψ their junction, and Uw the result that is checked against the code. Good selection balances all three, rather than chasing a record value for one of them.