Adverts promise "absolute insulation", "zero losses" and "the end of heating bills". Many homeowners invest thousands of euros in top energy windows and, when the first harsh winter arrives, are unpleasantly surprised: they still need to turn on the heating.
Let us speak with absolute honesty. Our job isn't to sell you illusions, but real solutions. Believing a window offers "100% insulation" is scientifically impossible. Here's why - and what you should realistically expect.
In a modern building with external wall insulation (ETICS), the brick-and-insulation wall achieves a U-value of 0.3–0.5 W/m²K - practically a "rock" that stops heat from escaping. In contrast, even the top-of-the-line heavy-duty aluminium window with triple glazing (Passivhaus grade) struggles to reach 0.8–1.0 W/m²K. Most "good" energy windows sit around 1.5 W/m²K.
Even the best window in the world loses 2–3 times more energy than an insulated wall. In engineering terms, a window will always remain an "energy hole" in the building envelope, simply because... it is transparent. Transparency means radiation, conduction and heat transfer through glass - laws of physics that cannot be overridden.
Don't expect "zero" losses. Expect a drastic reduction: a modern window (Uw 1.4) cuts losses by 60–75% compared to old single-pane aluminium (Uw 5.0+). That improvement translates to tangible savings, but does not eliminate the need for heating.
Even if we discovered a magical, 100% insulating window, your home would still get cold. Why? Because windows are only one piece of the puzzle.
Heat rises. An uninsulated flat roof is the single biggest "culprit" in the Greek building stock. Roof insulation with XPS or EPS can reduce these losses by 70%.
Without external wall insulation (ETICS), walls lose huge amounts of heat. External insulation dramatically reduces wall losses but requires scaffolding, permits and significant cost.
Replacing windows alone "cures" roughly 25% of the problem superbly. But if the roof or walls are uninsulated, heat will find another way out.
Uninsulated pilotis (10–15%) and ventilation through chimneys, extractors etc. (10%) complete the "pie". Holistic energy renovation requires intervention on multiple fronts, not windows alone.
If new windows don't zero out the bill, is the investment worth it? The answer is a resounding YES, for entirely realistic reasons.
Replacing old wooden or single-pane aluminium windows with modern energy windows demonstrably cuts heating and cooling costs by 20–30%. Over 5–10 years, the investment pays back purely from the bills you've saved.
The biggest problem with old windows isn't the glass, but that they leak air! Modern windows feature multiple EPDM gaskets that create an airtight seal. Cold air stays out, and you stop feeling that chilling draught at your feet.
With single panes, the inner glass surface is freezing in winter. Sitting next to it, your body "radiates" heat towards the glass, making you feel cold even if the thermometer reads 22 °C! Energy double glazing keeps the inner surface warm, delivering incredible cosiness.
Unlike external wall insulation or roof insulation (scaffolding, weeks of work, weather-dependent), window replacement is done in 1–2 days, cleanly.
From day one you see the difference: zero draughts, drastic reduction in external noise (sound insulation), upgraded security (anti-burglary protection) and a visible aesthetic upgrade. No demolition, no scaffolding, no mess.
Step 1: Replace windows (quick, clean, big improvement).
Step 2: Roof insulation (25–30% of losses).
Step 3: External wall insulation (scaffolding, high cost).
Step 4: Heat pump / photovoltaics (for Class A+).
Modern energy windows are technological marvels, but they do not override the laws of thermodynamics. If someone promises you'll "never turn on the heating again", they are simply lying. If, however, you want a drastic 20–30% cost reduction, the elimination of draughts and complete peace and quiet inside your home, it is the best investment you can make.
Return to category.
Go to categoryReturn to the central guide.
Go to guide
