Window-to-wall ratio

Window-to-wall ratio refers to the ratio of the transparent window area (glass area) to the total exterior wall area of a building and is expressed as a percentage. It is an important design parameter for energy efficiency, daylighting, summer heat protection, and building physics. The Building Energy Act (GEG) and DIN 4108 specify requirements for the window-to-wall ratio.

Significance for Energy Efficiency and Building Regulations

The window-to-wall ratio influences a building’s energy performance in both directions:

Advantages of high window-to-wall ratios:

• Good daylight supply, less artificial lighting
• Passive solar energy utilization (free heat in winter)
• High quality of stay, visual connection to the outside

Disadvantages of high window-to-wall ratios:

• Higher heat loss in winter (windows insulate less effectively than walls)
• Increased risk of overheating in summer (summer heat protection!)
• Higher costs for highly insulating glazing (triple-pane thermal insulation glass)

Requirements under the GEG: The Building Energy Act limits the window-to-wall ratio for non-residential buildings to a maximum of 35% of the exterior wall area (higher under certain conditions). For residential buildings, there is no explicit upper limit, but the total energy demand must comply with GEG limits.

Summer heat protection (DIN 4108-2): In Bavaria, where there is a high proportion of window area-especially facing south and west-shading measures (blinds, awnings, canopies) are mandatory to prevent overheating. This is documented in the GEG certification.

Typical Window-to-Wall Ratios and Their Effects

Building Type	Typical Ratio	Characteristics
Classic older building (pre-1960)	15-20%	Small, fragmented windows, little daylight
Modern single-family home	25-35%	Good compromise between energy efficiency and light
Passive house	35-50%	Large south-facing windows, optimized thermal insulation
Office building (glass facade)	60-80%	High cooling demand, sun protection essential

Window renovation: Impact on energy balance and property value

In existing buildings, window replacement is one of the most effective individual measures for improving the energy balance. Single-pane windows (U-value approx. 5.0 W/m²K) lose more than twenty times as much heat per square meter as a well-insulated exterior wall (U-value 0.24 W/m²K). Thermal insulation windows with triple glazing achieve U-values of 0.5 to 0.8 W/m²K, which is only slightly higher than that of high-performance insulation plaster.

Depending on size, material, and glazing, the cost of new windows ranges from 300 to 1,000 euros per window, including installation. For a typical existing apartment (90 m², 8 windows), this amounts to 3,000 to 8,000 euros. Window replacement measures are subsidized through the Federal Funding for Efficient Buildings (BEG) with a grant of 15% of eligible costs-provided that the U-value requirements are met and an energy efficiency expert is involved.

For real estate buyers and sellers, the condition of the windows is a relevant value factor: Old single- or double-pane windows from the 1970s and 1980s are considered a defect in a property valuation report, reducing the property’s market value. New insulated windows with high-quality glazing, on the other hand, contribute to the energy efficiency rating and thus to the property’s market appeal.

Design Optimization: Orientation and Shading

The energy-optimized window layout depends on the compass direction:

South-facing windows: Maximum passive solar gains in winter; shaded in summer with proper planning (roof overhang or external blinds). South-facing windows are the most important contributor to passive solar energy use in passive houses and nearly zero-energy buildings.

North-facing windows: Minimal solar gains, but even daylight without glare. Often preferred for workrooms and offices. Energetically, the “most expensive” window type.

West- and east-facing windows: Intense sunlight in the morning (east) or evening (west), especially in summer. Without external shading, significant overheating can occur.

As a rule of thumb: In an energy-optimized residential building, the south-facing portion of the window area should account for 50-60% of the total window area, while the north-facing portion should be no more than 15%.

Practical Tip for Homeowners in Nuremberg and Franconia

In our consultations, we repeatedly see that the proportion of window area is underestimated during the renovation or attic conversion of existing buildings in Nuremberg. For example, anyone wishing to install large skylights in the attic of a Wilhelminian-style house in Gostenhof or St. Johannis must provide proof of summer heat protection-and plan structural measures if the GEG limit values are exceeded.

We recommend clarifying this aspect early on with the planning architect or energy consultant before submitting the building application to the City of Nuremberg’s building authority. The best orientation for large window areas is south-in combination with external venetian blinds that minimize heat gain in the summer. For historic buildings, the Nuremberg Lower Monument Protection Authority (City Planning Office) often requires that the historic window divisions be preserved-even when new insulated glazing is installed.

Frequently Asked Questions

What percentage of window area do you recommend for a single-family home in Nuremberg?

As a rule of thumb, we recommend 25-35% of the exterior wall area for a balanced supply of daylight while keeping energy consumption reasonable. Large south-facing windows with external shading elements (external blinds) make good energy-saving sense. For rooms with high heat gain (west-facing facade, attic), external shading elements should be planned from the outset.

At what percentage of window area is sun protection mandatory in Bavaria?

DIN 4108-2 and the GEG do not require a fixed minimum value, but the summer heat protection certification must be met. For heavily glazed facades (40% or more), a functional shading system is generally required by calculation. Proof of summer heat protection must be provided in the building application-if it is missing, the authorities will not issue a building permit.

Can I install as many windows as I want when renovating my old building in Nuremberg?

In principle, yes, but with restrictions: Listed buildings are subject to strict requirements from the local historic preservation authority. In areas subject to preservation statutes (e.g., parts of Nuremberg’s Old Town), the permitting authority may also impose restrictions. Generally, proof of compliance with the GEG standard must be provided-anyone significantly enlarging the windows must verify that the building’s overall energy efficiency does not fall below the GEG limits as a result.

Are new windows eligible for subsidies?

Yes. The replacement of old windows with thermal insulation windows is subsidized through the Federal Subsidy for Efficient Buildings (BEG Individual Measures) with a grant of 15% of eligible costs. Prerequisites include a U-value of no more than 0.95 W/m²K for the new window and the involvement of an energy efficiency expert. The application must be submitted to the BAFA before the work begins.

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