You've just received three different quotes for replacing your old windows. Every manufacturer promises "top energy performance" and "perfect insulation". But when you read the fine print, you spot strange letters and numbers: Uw = 1.4, Uf = 1.8, Ug = 1.0.
What are all these codes? Is 1.4 good, or is 1.8 better? For the average homeowner, this jargon feels like another language. For engineers, however, these values are a window's ultimate "identity card". Let us decode these terms in plain language so you can compare quotes like a professional.
The letter U (short for U-value) represents the Thermal Transmittance Coefficient. In simple terms, the U-value measures how much heat manages to escape through a material (for example, through your window) when it is cold outside and the heating is on inside. It is expressed in Watts per square metre per degree Kelvin (W/m²K).
The absolute, unbreakable rule you must remember is this: the LOWER the U number, the BETTER the insulation! A material with U = 3.0 "leaks" a lot of cold air in. A material with U = 1.0 keeps the house warm like a flask. This rule always applies, regardless of the material or manufacturer.
The U-value is the only objective metric that lets any homeowner compare two windows "apples to apples", regardless of brand names, colours, or marketing slogans. If one manufacturer quotes U = 1.2 and another quotes U = 2.4, you instantly know the first offers twice the insulation.
A window is not made from a single material. It consists of the glass and the frame (the aluminium or PVC profile). Each of these parts has its own insulating ability. That is why the letter U always comes with a small lower-case subscript that tells us which part it refers to.
The letter "g" comes from the word Glass. This index tells us how well the glass pane alone insulates, without accounting for the frame. Since the glass typically occupies 70% to 80% of a window's surface, Ug is extremely important. A simple single pane has a Ug around 5.8, while a modern energy double pane with Argon gas drops to an impressive 1.0. The difference is enormous and directly reflects on your heating bill.
The letter "f" comes from Frame. This index measures the insulation provided by the aluminium or PVC profile alone. If the aluminium is old (without a thermal break), the Uf is enormous (e.g. 7.0), meaning it is an energy "hole". If it is a modern thermally broken profile with large polyamide strips, the Uf drops to 1.5 or even lower, effectively closing this heat "bridge".
The letter "w" comes from Window. This is the KING of all indices! It is the weighted average of the glass insulation (Ug) and the frame insulation (Uf) combined. WARNING: When comparing quotes or applying for the "Exoikonomo" subsidy programme, the only number that truly matters is Uw. Many sellers, in order to impress you, may say "we have U = 1.0", slyly referring only to the glass (Ug), while the total window (Uw) may be far worse!
For more discerning buyers and engineers, the overall Uw is not a simple average. It is derived from a strict mathematical formula that accounts for the area of each material and the thermal bridges at the junctions.
The official calculation per the EN ISO 10077-1 standard is: Uw = (Ag · Ug + Af · Uf + lg · Ψg) / Aw. Where Ag and Af are the areas of the glass and the frame respectively, Aw is the total window area, lg is the glass perimeter, and Ψg (Psi) is the linear thermal bridge of the spacer bar (the strip between the panes). This formula proves that Uw depends not only on glass quality, but also on the window's geometry and small components (spacers).
One critical detail most people overlook: the exact same aluminium system with the exact same glass will produce a different Uw depending on the window size! A small window (e.g. 60×60 cm) has a larger frame-to-glass ratio, meaning the "poor" Uf value has a greater influence on the total. A large window (e.g. 150×180 cm) has more glass, so the "good" Ug value dominates, pulling the overall Uw down to a more favourable level. This is why reputable quotes always state the exact window size at which the Uw was measured.
To get a clear market picture and evaluate your quotes properly, here is where the overall Uw values stand across the Greek market today.
Old wooden windows or basic aluminium frames with single glazing. The thermal transmittance exceeds 4.5 units, meaning heat escapes almost unimpeded. These windows are no longer acceptable in any new build or major renovation under the current KENAK regulations.
Older aluminium frames without a thermal break, fitted with plain double glazing (no Low-E coating). Although slightly better than single panes, they remain energy-inefficient. In a cold winter, the inner side of the profile literally freezes, creating condensation and mould around the frame.
Good modern thermally broken aluminium frames with energy double glazing (Low-E + Argon gas). This is the "sweet spot" for most residential renovations in Greece. The combination of a thermally broken frame with energy glass delivers 20–30% savings on energy bills, meets KENAK requirements, and provides genuine thermal comfort next to the window.
Premium, heavy-duty thermally broken systems with triple energy glass panes. Ideal for northern Greece (Florina, Kastoria, Grevena), highland villages, or buildings targeting Passive House certification. These systems achieve values below 1.0 W/m²K, minimising heat loss to the absolute technological minimum.
Next time you look at a window replacement quote, ignore the impressive words and go straight to the numbers. Look for the Uw value. It is the only objective judge that tells you the truth about how much electricity or heating oil you will save every winter. Don't fall for marketing "tricks" where someone quotes only the glass Ug without declaring the overall Uw.
Before you sign, check the following: (1) Request the Uw coefficient in writing (not just Ug). (2) Ask at which window size it was measured. (3) Verify that Uw meets the KENAK limits for your climate zone. (4) If you plan to apply for the "Exoikonomo" subsidy, get confirmation that the stricter programme criteria are met, as they often require a lower Uw than what KENAK mandates.
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