Uw, Ug, window thermal transmittance: the right values for your climate zone

When a proper quote arrives, the first thing the customer sees is a strange code: Uw 1.1 W/m²K. It sounds arcane, but it’s the single most important figure in the whole document. It’s what determines whether the window you’re about to buy will actually make sense for your building and your climate zone. In this article I’ll try to explain it the way I do on a site visit: no university formulas, with practical references to Rome, Milan, Turin.

Three letters, three different things

The three codes you’ll find in quotes are:

• Uf (frame): thermal transmittance of the profile alone, without glazing. It’s a property of the material and the profile geometry. For a PVC Salamander 76 it’s around 1.1 W/m²K. For a modern thermal-break aluminium profile it’s around 1.4-2.0 W/m²K.
• Ug (glass): thermal transmittance of the glazing alone, without the frame. For double low-emissivity glazing with argon it’s 1.0-1.1 W/m²K. For triple low-emissivity glazing it’s 0.5-0.7 W/m²K.
• Uw (window): thermal transmittance of the whole assembly — frame + glazing + spacer — weighted by surface area. It’s the figure that matters for your bills and for tax incentives.

All three are measured in W/m²K (watts per square metre per kelvin of temperature difference). The lower the number, the less heat the window loses. A window with Uw 1.0 loses half as much heat as one with Uw 2.0.

What changes between double and triple glazing

The double-glazed unit is where most of the insulation happens. The typical build-ups you’ll see in quotes:

Standard double glazing:

• 4-16-4: two 4 mm panes with a 16 mm cavity
• Air-filled cavity: Ug ~2.7 W/m²K (old, poor)
• Argon-filled cavity with one low-emissivity pane: Ug 1.0-1.1 W/m²K

Triple glazing:

• 4-12-4-12-4 or 4-16-4-16-4: three panes with two cavities
• Argon plus two low-emissivity panes: Ug 0.6-0.7 W/m²K
• Krypton plus two low-emissivity panes plus warm edge: Ug 0.5 W/m²K

Practical differences between argon and krypton:

• Argon is the standard gas, low cost, with thermal conductivity 67% that of air. Used in 12-20 mm cavities.
• Krypton conducts even less (40% of air), allowing narrower cavities (6-10 mm) while maintaining high performance. It costs 3-4 times as much as argon and is only used in passive-house profiles (BlueEvolution 82 with Ug 0.5 glazing).
• Xenon exists but is prohibitively expensive; you’ll only find it in laboratory settings or in very specific certified phA+ windows.

Low-emissivity glass is the real game-changer. It’s an ordinary pane with a thin metallic coating (tin oxide, or silver deposited under vacuum) that reflects thermal infrared while letting visible light through. In winter it keeps heat inside; in summer it reflects solar heat outward. In quality units, two low-emissivity coatings are used, facing each other — one on the inner face of the outer cavity, one on the outer face of the inner cavity.

The statutory limit values in Italy

The Ministerial Decree (“DM”) of 26 June 2015, “Requisiti minimi” (“Minimum Requirements”) sets the maximum permitted Uw for windows in energy-efficiency renovation works, depending on the climate zone of the municipality. These are regulatory values, not commercial ones. For works aiming to claim tax deductions, the required Uw values are of the same order but can be slightly stricter — refer to ENEA for the updated table applicable to your project, and to the Italian Revenue Agency for the tax conditions.

The DM Requisiti Minimi values (maximum Uw) for a window-replacement project are:

Climate zone	Typical municipalities	Maximum Uw (DM Requisiti Minimi)
A-B	Lampedusa, Reggio Calabria, Palermo, southern coastal areas	≤ 3.00 W/m²K
C	Naples, Bari, Cagliari, south-central coast	≤ 2.20 W/m²K
D	Rome, Florence, Pescara, Ancona	≤ 1.80 W/m²K
E	Milan, Turin, Bologna, Padua, most of central-northern Italy	≤ 1.40 W/m²K
F	Belluno, Cuneo, Sondrio, Alpine municipalities	≤ 1.10 W/m²K

These are the statutory maximums: a window with a higher Uw cannot be installed in that zone under a subsidised scheme. They are minimum acceptability thresholds, not quality targets.

What to actually choose by zone

Zone A-B (coastal Sicily, southern Calabria): The law states Uw ≤ 3.00. In practice no serious manufacturer makes PVC windows above 1.3-1.5 Uw any more, so any modern window passes. The problem in zone A-B isn’t the cold, it’s the summer heat. Here the solar factor g of the glazing (its ability to filter out solar heat) matters more than the Uw value. Selective glass with g 0.30-0.35 is chosen instead of pure low-emissivity glass with g 0.55-0.60.

Zone C (Naples, Bari, south-central): Uw ≤ 2.20 by law. A GreenEvolution 76 PVC window with 4-16Ar-4 low-emissivity double glazing reaches Uw 1.0-1.1, comfortably under the threshold. Triple glazing here only makes sense for very cold north-facing exposures or for those seeking acoustic comfort (for Uw purposes it’s already over-specified).

Zone D (Rome, Florence): Uw ≤ 1.80. In zone D, double low-emissivity argon glazing on 76 mm PVC is the economic sweet spot. Triple glazing starts to make sense if the house faces north or is heated all winter (not a second home). Real-world example: in Rome’s EUR district, replacing 6 single-glazed 1970s timber windows with GreenEvolution 76 double glazing 4-16Ar-4 took one of our customers from €1,450 to €850 a year in gas costs. Triple glazing would have saved a further €80-120/year — not worth the extra cost.

Zone E (Milan, Turin, Bologna, Padua): Uw ≤ 1.40. Here triple glazing starts to become rational. PVC BlueEvolution 72 with 4-12-4-12-4 argon triple glazing reaches Uw 0.85-0.90. Practical difference versus double glazing: €150-300/year saved on bills for a 90 m² flat. It pays back in 6-10 years against an average extra cost of €1,500-2,500 on the total works.

Zone F (mountain municipalities, Belluno, upper Cuneo): Uw ≤ 1.10. Triple glazing is mandatory on performance grounds, and in practice you move up to the BlueEvolution 82 with argon or krypton triple glazing to reach Uw 0.72-0.80. This is the bracket for mountain homes or the coldest northern municipalities: here the extra investment pays back in 4-7 years because heating costs so much.

Real-world examples

Case 1 — 90 m² flat in Milan (zone E): 6 windows plus 1 French door, old 1990s double glazing with an estimated Uw of 2.5. Replaced with BlueEvolution 72 triple glazing, Uw 0.85.

• Annual gas saving: ~€600
• Summer electricity saving (less solar gain with low-emissivity glass): ~€80
• Payback time net of tax deductions: 7-9 years

Case 2 — 180 m² detached house in Turin (zone E): 12 windows plus 3 French doors. From 1970s timber windows with single glazing to GreenEvolution 76 with triple glazing, Uw 0.80.

• Annual gas saving: ~€1,400
• Comfort: internal condensation disappeared, lower ambient humidity
• Payback: 5-7 years with incentives

Case 3 — 60 m² two-room flat in Rome (zone D): 3 windows. From single-glazed timber to 76 mm PVC with double glazing, Uw 1.05.

• Annual gas saving: ~€300
• Payback: 6-8 years

A note on the spacer bar (warm edge)

The panes are separated by spacer bars, the metal or plastic strips running around the perimeter of the double-glazed unit. Historically these were aluminium (highly conductive: a thermal bridge). For the last fifteen years, warm edge spacers have been used: stainless steel, fibre-reinforced plastic, or hybrid materials. On a Uw 1.1 window with a standard spacer, switching to warm edge gains 0.1-0.15 W/m²K — not negligible. Always check the quote specifies “warm edge spacer”, not “standard aluminium spacer”.

What to ask for in the quote

You should always see three things stated clearly in writing:

1. Declared Uw per window/French door (the value also depends on the dimensions: a large French door has a better Uw than a small window, because the glazing matters more than the frame)
2. CE marking from the manufacturer with declared values
3. Clear glazing composition: “4-16Ar-4 LE” or “4-12-4-12-4 LE + warm edge” or similar

If the quote just says “double glazing” without specifying gas fill, low-emissivity coating, or spacer type, it’s a quote that needs redoing.

For an overview of the wider decision, also read our complete guide to PVC windows and doors.

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Request a quote: we assess climate zone, exposure and energy class to recommend the right glazing configuration for you — without selling you unnecessary triple glazing where a well-made double glazing unit will do.