Old windows are often the biggest heat leaks in the house. Insulating film with Velcro solves the problem in minutes – without a handyman, without drilling and for a few euros per window.
Cover image: Thermal image of an old window without film (left) and with insulating film (right). Illustrative image.
How much heat is lost through old windows?
The proportion depends heavily on the building – expert sources cite a wide range:
- According to a study by the German Federal Environmental Foundation (DBU), up to 15% of heating energy is lost through outdated windows.
- For old, single-glazed windows, it is up to 20% of the room heat, according to Vattenfall.
- For old windows with double glazing, according to ingenieur.de, often over 40% of the room heat is lost.
In the worst case, up to 40% of the heat – and thus a large part of what you pay for heating – simply escapes through the pane.
How much does an old window cost per year?
The Institute for Housing and the Environment (IWU) calculated how much heat is lost per square meter of window area per year. Converted to a standard window measuring 100 × 120 cm (1.2 m²) and a gas price of €0.12/kWh, the result is:
| Glazing | Loss per m² per year | Window 100 × 120 cm | Cost per year |
|---|---|---|---|
| Single glazing | approx. 43 l of heating oil (≈ 430 kWh) | approx. 516 kWh | approx. €62 |
| Old insulating glass (1975–1994) | approx. 22 l of heating oil (≈ 220 kWh) | approx. 264 kWh | approx. €32 |
| Modern heat-insulating glazing | approx. 9 l of heating oil (≈ 90 kWh) | approx. 108 kWh | approx. €13 |
In an old building apartment with eight single-glazed windows, around €500 per year literally goes through the panes.
The quick solution: an additional layer of still air
New windows are the permanent solution – but quickly cost several hundred euros each, require a handyman and are often not possible at all in rented apartments. A window insulating film is much faster: It is attached all around the window frame with Velcro. A layer of still air is created between the glass and the film – the same principle that also provides insulation for multi-pane windows. At the same time, cold air from leaky joints no longer enters the room.
How much the film saves
For our sample calculation, we assume that the film halves the heat loss in single glazing and reduces it by around 30% in old insulating glass:
- 1 window, single glazing: approx. €31 savings per winter – the film (100 × 120 cm, €43.50) has paid for itself after around 1.5 winters.
- 1 window, old insulating glass: approx. €10 per winter – paid for in about 4–5 winters, with a volume discount in around 3.
- Old building with 8 single-glazed windows: approx. €250 per winter. 8 films cost around €244 with a 30% volume discount – paid for in the first winter.
Drafts are not included: With leaky windows, the savings can be higher, and because it immediately feels warmer, many people turn down the thermostat. As a rule of thumb: Every degree less saves about 6% of heating energy.
Attached in 5 minutes
- Measure the window: visible glass area and outer edge of the frame. The film size lies somewhere in between.
- Clean the frame and let it dry.
- Stick the Velcro all around the frame (preferably on the window sash) and press it down.
- Press the film all around the edge – done. Remove it again in spring and reuse it in autumn.
You can find detailed instructions under Measuring windows correctly.
Further tips against heat loss at the window
- Close roller shutters and curtains at night – this forms an additional layer of air.
- Check window seals and replace brittle seals.
- Shock ventilation instead of continuous tilting: open wide 5–10 minutes several times a day.
- Do not cover radiators under the window with long curtains.
Conclusion
Old windows cost money every winter. Those who cannot or are not allowed to install new windows immediately can quickly and affordably get a large part of it back with an insulating film.
→ To the window insulating film with Velcro – 21 sizes, white or black border, from 2 films up to 30% volume discount.
All values are sample calculations. The actual savings depend on the window, heating, energy price and usage.