🪟 Ug (Glass Only)
Refers solely to the glass surface. An old single pane has Ug ≈ 5.8, a double with Argon drops to 1.0-1.1, while a Passivhaus triple reaches 0.5.
Windows & Energy Glass: The Ultimate Guide to U-Value, G-Value and Thermal Breaks
Walls may be thick and insulated, but windows are literally "holes" covered with a thin layer of glass. If you don't choose the right frames, your insulation and your heat pump will "fly away" through the pane.
Let's decode the manufacturers' jargon: U-Value, g-value, Low-E, Argon - so you know exactly what you're buying.
1. The U-Value (Thermal Transmittance)
The U-Value measures how much heat "escapes" through the window. Unit: W/(m²K). The rule: the lower, the better the insulation. In spec sheets you'll see three indicators: Ug (glass), Uf (frame), Uw (whole window). The one that matters is always Uw.
🔲 Uf (Frame Only)
Refers to the frame (aluminium, PVC or timber). An old aluminium without thermal break: Uf ≈ 7.0. A modern PVC or thermally-broken aluminium: Uf ≈ 1.2-1.8.
📐 Uw (Whole Window)
The truly important indicator. An old single- glazed aluminium window: Uw ≈ 5.0. Modern energy-rated: Uw ≈ 1.5. Passivhaus: Uw below 0.8!
📏 Order of magnitude
Replacing old single-glazed windows (Uw=5.0) with modern energy-rated ones (Uw=1.5) reduces heat losses through windows by 70%. This directly translates to a smaller heating system and a lower bill.
2. The Frame: Thermally-Broken Aluminium vs PVC
Old aluminium was an excellent heat conductor: the frame froze, "sweated" and transferred cold inside. The modern solution: the profile is "cut" down the middle with polyamide (thermal break) - the outer aluminium freezes, the inner stays warm.
🔩 Thermally-broken aluminium
Between the outer and inner sections, a rigid plastic strip (polyamide) "cuts" the thermal bridge. Uf drops to 1.5-2.0 W/(m²K) - a huge improvement.
🏗️ PVC (Synthetic)
The plastic itself is a poor heat conductor - therefore an excellent insulator. Uf ≈ 1.0-1.4. Usually more affordable, but with limitations on very large openings due to structural strength.
🪵 Timber frames
Excellent insulation (Uf ≈ 1.2-1.6), aesthetically beautiful, but require regular maintenance (painting, varnishing). Ideal for listed buildings or Scandinavian-style homes.
💡 What to ask for
Always ask the manufacturer: "What is the frame Uf?" If they don't know or won't answer, keep your distance. A reputable factory always provides certified technical data sheets.
3. Glass Panes: Double/Triple, Argon and Low-E
The glass covers 80% of the window surface. Insulation isn't provided by the glass itself, but by the cavity between the layers - filled with Argon (or Krypton) gas for even higher performance.
🔬 Argon gas
Argon is denser than air and hinders heat transfer through the cavity. It reduces Ug by 15-20% compared to ordinary air. In premium applications, Krypton is used (costlier, even more effective).
🪞 Low-E (Low Emissivity)
An invisible, microscopic silver coating on the inner surface of the glass. In winter, heat escaping towards the glass is reflected back into the room like a mirror. Reduces radiative losses by 70%.
📐 Double vs Triple
Double glazing with Argon and Low-E (4/16/4) is the baseline of every modern construction. Triple glazing (4/14/4/14/4 Argon) offers even lower Ug ≈ 0.5 - but is heavier and more expensive.
🔧 Spacer Bar
The spacer between the panes makes an enormous difference. A cold aluminium spacer creates a thermal bridge at the edge of the glass. Ask for a Warm Edge Spacer to reduce edge condensation by 65%.
4. The Solar G-Value: Watch Out for the Greek Summer!
The g-value (expressed as a fraction, e.g. 0.40 or 40%) indicates how much solar radiation passes through the glass. It's the biggest trap for homes in Greece - if you don't factor in orientation.
☀️ High g-value (0.60)
Lets the sun in for free. Perfect for Northern Europe and for south-facing windows in winter. But in Greek July, the south side becomes a greenhouse - the air conditioner never stops.
🛡️ Low g-value (0.35)
Blocks the sun. "Four-Season Energy Glass" with g ≈ 0.30-0.40 is the ideal choice for the Mediterranean climate - lower cooling loads, smaller A/C unit, lower bill.
🧭 Strategy per façade
South/West: low g-value + external shutters or awnings. North/East: medium g-value, as it receives little intense sun. The engineer should design different glass per façade.
🏠 The Greek dilemma
In Greece, summer cooling loads often exceed winter heating loads. The right g-value choice (combined with shading) reduces A/C kW by 30-50%.
🪟 Choosing windows is about balance. Always check Uw, insist on thermal breaks, demand Argon, and discuss g-value with your engineer - so you don't lose the sun in winter, but don't "bake" in summer.
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Insulation & Heat Pump: The Biggest Renovation Mistake External vs Internal Insulation: Thermal Mass The Envelope & Passive Buildings (Passivhaus) Building Airtightness & Blower Door Test
Energy Design, Studies & Legislation
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