You've decided to rent or sell your old apartment. The engineer carries out the inspection, issues the certificate, and you see your home classified as Class Z or H (marked in the characteristic red colour). Today, with electricity and gas costs concerning every household, a red "Z" on the EPC is a red flag for prospective tenants or buyers.
The most direct, clean and effective intervention to escape the lowest energy classes is replacing your old windows. Let us look at the real numbers behind how this move transforms the engineer's report and, above all, your property's market value.
Why does the Energy Inspector place so much emphasis on windows? The answer lies in building physics. In a typical old apartment (built before 1990), wooden windows with single panes or old sliding aluminium frames without a thermal break are responsible for 25% to 30% of total thermal losses.
Old windows literally function as a hole in the wall, through which heating escapes in winter and air conditioning in summer. A single pane has Ug ≈ 5.8 W/m²K, while the surrounding basically insulated wall may sit at 1.5 or lower. This dramatic difference means heat "runs" toward the window and is lost like water through a sieve.
When the engineer enters old window data (Uw > 4.5 W/m²K) into the TEE software, the system "penalises" the building, dropping it into the lowest energy categories. Conversely, replacing those windows with modern thermally broken ones (Uw 1.4–1.8) lets the algorithm recognise the improvement, reducing calculated losses at openings by 60–70%.
Honesty is needed here. Many promise that simply changing the windows will turn your apartment into an "A+". That is not true. Reaching Class A or A+ requires wall insulation, heat pumps, solar water heaters and possibly photovoltaics.
Replacing windows in a 1970s or 1980s apartment can upgrade the property by 1 to 2 full energy classes (e.g. from Z up to E or even D). This is because windows are the biggest "weak point" in the building envelope, and replacing them dramatically improves the weighted average thermal transmittance coefficient.
New windows don't just stop heat transfer. They seal the home with perimeter EPDM gaskets, eliminating draught losses. This improvement in airtightness is explicitly recognised by the TEE software, drastically improving the engineer's calculation as ventilation losses virtually disappear.
Consider a typical example: an 85 m² apartment in Athens (Zone B), built in 1978, with 4 old sliding aluminium windows without a thermal break and single glazing. Initial EPC rating: Class H.
Before: Uw > 5.0 W/m²K, Class H, high heating consumption
(~1,800 kWh/m²/year).
After: Replacement with thermally broken aluminium Uw
1.4 + Low-E Argon double glazing. New rating: Class E or even D (depending on wall insulation). Annual energy
consumption reduction:
20–30%.
Saving approximately €300–500/year on heating and cooling bills, the initial investment pays back in 5–8 years. From that point on, savings are pure profit for the remaining 20+ years of the windows' life. If you add the "Exoikonomo" subsidy, payback can shrink to just 2–4 years.
Investing in new windows should not be treated as a maintenance expense, but as a tool for increasing your property's value. In today's property market this is called the "Green Premium".
A tenant is willing to pay €50–100 more per month for a home with new energy windows, knowing they will save double that on electricity and gas bills. That translates to €600–1,200 in additional annual revenue for you as the owner.
Renovated windows (especially in modern colours) radically transform the home's visual appeal. They are the #1 "selling point" in listings. The buyer sees a "move-in ready" property that won't require immediate capital for renovation.
The European Union is preparing directives (EPBD) that will progressively prohibit renting or selling properties in the lowest energy classes (Z and H). Owners who fail to upgrade in time may face inability to let their property or financial penalties.
The EPC is your home's "mirror". Replacing old windows with new, certified energy systems is the fastest and least "invasive" method (no scaffolding or massive building work) to improve that image. A 1–2 class upgrade means realistic 20–30% annual savings, faster sale or rental, and protection against future regulations.
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