Energy Saving with Thermal Insulation
Today, windows fulfill far more than just their original functions – daylight and fresh air. Increasingly, the focus is on energy efficiency and thermal insulation. An energy-efficient window has a clear goal: to permanently reduce energy and heating costs.
Since the Energy Saving Ordinance (EnEV) 2014, binding minimum energy requirements for windows and doors have applied to new construction and modernization measures. Since November 1, 2020, the Building Energy Act (GEG) is authoritative, combining EnEG, EnEV, and EEWärmeG into a modern set of regulations.
1. Why is thermal insulation needed?
Modern windows appear to be sealed from the outside – gaskets and frames reliably prevent drafts. What the eye does not see, however, is that up to 40% of a building's total heating energy loss occurs solely through the glass surfaces.
Modern energy-saving windows not only significantly reduce heating energy consumption – they also make an active contribution to sustainability. And the investment pays off: energy-saving windows pay for themselves significantly in the long term through the saved heating costs.
2. Energy Saving through Thermal Insulation
Many owners postpone replacing their windows – out of concern about high acquisition costs. However, an optically intact window is no guarantee of good thermal insulation. Windows are among the thermally weakest elements of the building envelope: Although they only account for about 8% of the building envelope, they are responsible for up to 40% of heat loss.
A thermal imaging camera can impressively show how much energy is lost through old single-pane windows. Replacing windows is economically sensible in most cases – and pays off faster than many think.
3. Advantages of Windows with Thermal Insulation
- Save energy sustainably
- Permanently reduce heating costs
- Utilize subsidies (e.g., KfW, BAFA) and reduce costs
- Increase property value
- Individual design: colors, muntins, surfaces
- Freely configure functions (opening types, security levels)
- Easily add accessories such as roller blinds, insect screens, and more.
- Combine matching patio doors and entrance doors
4. History of the Window with Thermal Insulation
The development of the window is impressive: While single glazing with a heat transfer coefficient of 5.5 W/(m²K) was standard until the 1970s, modern triple-pane insulating glass today achieves values of less than 1 W/(m²K).
Window TypePeriodCharacteristicsSingle-glazed windowsuntil approx. 1978One pane, high heat losses, ice flower formationBox windowsuntil approx. 1960Two separate sashes with a large gapCompound windowsuntil approx. 1980Two connected sashes, better than box windowsStandard insulating glasssince approx. 1975One sash, two panes – often foggedDouble-pane thermal insulation glasssince approx. 1995Inert gas filling, significantly better insulationTriple-pane thermal insulation glasssince approx. 2003Three panes, warm edge, optimal insulation
5. Key Figures: The U-Value
The U-value (heat transfer coefficient) is the most important key figure when buying windows. The lower the U-value, the better the insulation and the lower the energy costs.
- Ug (Glass): Insulation performance of the glazing – improved by multiple glazing, low-E coating, and inert gas filling.
- Uf (Frame): Insulation performance of the frame – dependent on material and chamber system.
- Uw (Overall window): The decisive overall value for the complete window as a unit.
6. Components of the Energy-Efficient Window
The Window Glazing (Ug)
Modern energy-saving windows rely on triple glazing with Low-E coating. The float glass is refined with a wafer-thin metal layer that minimizes heat loss. Single glazing: Ug over 4.0 W/(m²K) – modern triple Low-E glazing: Ug under 1.0 W/(m²K).
- g-value: Total energy transmittance – for double-pane: 0.65–0.72; for triple-pane: approx. 0.55.
- tL-value: Light transmittance – ensure a balanced ratio for optimal living comfort.
Spacer and Edge Seal
- Inert gas filling: Argon (standard) or Krypton prevents thermal bridges.
- Warm edge: Plastic spacer instead of aluminum reduces thermal bridges at the pane edge.
- Optimal pane spacing: Ideally 18 mm.
The Window Frame (Uf)
- Plastic: Inexpensive, easy to clean, good insulation values
- Wood: Excellent insulation, but high maintenance
- Aluminum: Modern and durable, requires a good chamber system
- Wood-aluminum: Premium combination with optimal properties
More chambers in the profile mean better insulation: 5 chambers (Iglo 5, Iglo Light) or 7 chambers (Iglo Energy) for the highest requirements. For seals: Triple sealing (3 levels) offers optimal thermal protection.
7. What else needs to be considered?
Ventilation Concept
Highly insulated windows are airtight – regular shock ventilation (3–4 times daily for 5–10 minutes) prevents mold growth. Modern smart home systems can automate the control of windows, heating, and roller blinds.
Wall Insulation
Ensure that the insulation quality of the new windows matches the insulation of the exterior walls. If the window is significantly better insulated than the wall, condensation can form at the wall edges.
