Insulated glass

Insulated glazing refers to window units consisting of at least two panes of glass with a gas-filled or evacuated air space (interpane space) between them. This design significantly reduces heat transfer (U-value) compared to single-pane glazing, thereby lowering heating costs and reducing emissions. Depending on the number of panes and the filling of the space between them, this is referred to as double-pane or triple-pane thermal insulation glazing.

Structure and Types of Insulated Glazing

The most important types of glazing in modern construction:

• Double-pane insulated glazing (2-pane thermal insulation glazing): Two glass panes with a 12-16 mm air gap, usually filled with an inert gas (argon) and featuring a thermal insulation coating. Typical U-value (Ug): approx. 1.0-1.1 W/(m²K). It is currently the standard for retrofitting existing buildings and meets the GEG minimum requirements for individual measures.
• Triple thermal insulation glazing (3-pane thermal insulation glazing): Three glass panes with two gas-filled chambers (krypton or argon), often with multiple thermal insulation coatings. Typical U-value: approx. 0.5-0.7 W/(m²K). It is a prerequisite for Passive House and KfW Efficiency House 40 standards and is now largely standard in new construction.
• Soundproof glass: Special laminated safety glass (LSG) with a soft interlayer that dampens sound waves. Important for properties in Nuremberg’s noise-polluted areas (train station area, main streets, near the airport).
• Solar control glazing: Glazing with a metallic coating that reduces solar radiation (g-value). Useful for glass surfaces and skylights facing predominantly south or west.
• Safety glazing (ESG/VSG): Single-pane safety glass (ESG) or laminated safety glass (VSG) is used in floor areas and for fall-protection glazing. VSG does not shatter into sharp pieces when broken and is preferred for skylights and sunrooms.

Importance of Insulated Glazing for Energy Efficiency and Property Value

The heat transfer coefficient (U-value) of windows has a significant impact on a building’s energy balance:

• Old single-pane glazing: U-value approx. 5.8 W/(m²K) - enormous heat loss; no longer permitted for installation today
• Old double glazing (pre-1995): U-value approx. 2.8-3.0 W/(m²K)
• Modern double-pane insulated glass: U-value approx. 1.0-1.1 W/(m²K)
• Modern triple-pane insulated glass: U-value approx. 0.5-0.7 W/(m²K)

The requirements of the Building Energy Act (GEG) now mandate at least double-pane glazing (total Uw: max. 1.3 W/(m²K)) for new buildings and subsidized renovations, and triple-pane glazing is mandatory for higher efficiency classes.

In a building’s energy performance certificate, the quality of the glazing is directly reflected in the final energy demand and thus influences the energy efficiency class-a factor of crucial importance for sales and rentals since the stricter labeling requirements took effect. A property with old single-pane or older double-pane glazing can quickly slip into classes F or G depending on other features, a situation increasingly viewed critically by buyers and tenants.

Consider the frame value: Uw instead of just Ug

A common mistake when planning window renovations: The U-value of the glass pane (Ug) is not the same as the U-value of the entire window unit (Uw). The frame-made of wood, plastic, or aluminum, depending on the material-has its own thermal transmittance properties and increases heat loss. The Uw value is the decisive factor for compliance with GEG requirements and for calculating subsidies. When planning, one should always compare total window values (Uw) and not just pane values.

Subsidies and Tax Deductibility

Replacing old windows with modern insulated glazing is eligible for subsidies in several ways:

• BAFA (BEG EM - Individual Measures): Window replacement under the federal subsidy program for energy-efficient buildings qualifies for a 15% base subsidy; with an individually determined renovation roadmap (iSFP), a 20% subsidy applies to eligible costs (max. 30,000 euros per residential unit per year).
• Tax Deductibility (§ 35c EStG): For owner-occupied properties, window replacements qualify as energy-efficiency renovation measures and are tax-deductible at 20% of the costs over three years (max. €40,000 total per property).
• Landlords: Costs for window replacement as a repair or modernization measure are deductible either immediately or via depreciation (AfA) as business expenses.

Practical Tip for Property Owners in Nuremberg and Franconia

In the Nuremberg metropolitan region, there are many older buildings from the 1950s to the 1980s that still have old double-pane glazing or even laminated windows (U-value approx. 2.5-3.0 W/(m²K)). We have observed that replacing windows with triple-pane units regularly has a positive effect on purchase price negotiations during sales-especially when combined with a current energy performance certificate showing a good efficiency class.

Specialized companies in Franconia currently have ample capacity for window renovations; upon request, we can provide contacts to tradespeople from our network. When planning their budget, owners should note that in addition to the pure window costs, ancillary work (sill plastering, exterior window sills, interior window sills, and possibly roller shutter boxes) may also be required, which can increase the total cost by 20-40%.

Frequently Asked Questions

When does replacing old windows with insulated glazing pay off?

As a rule of thumb: For single-pane glazing or old double-pane glazing (U-value > 2.0 W/(m²K)), the payback period-based on heating cost savings and moderate contractor costs-is usually less than 15 years, especially since BAFA subsidies and tax deductions significantly reduce the net investment.

As a landlord, am I required to modernize the windows?

There is no general obligation to modernize windows. However, the GEG stipulates that certain minimum U-values must be met when installing new windows. Landlords can classify window replacements as modernizations and pass on 8% of the annual costs to the rent, provided the legal requirements are met.

What is the difference between thermal insulation and soundproof glazing?

Thermal insulation glazing (WSV) is optimized for minimal heat transfer (low U-value). Soundproof glazing is designed for noise insulation (high Rw value in dB). Both properties can be combined in composite glazing, which is particularly useful for properties in noisy urban areas such as Nuremberg’s station district or along major highways.

Which glass is suitable for skylights?

For skylights (with a pitch of less than 75°), laminated safety glass (LSG) on the interior side is required by law, as broken glass must not allow shards to fall inward. For skylights optimized for maximum efficiency, triple-pane glazing with an LSG inner pane is used, which offers optimal performance in terms of both thermal insulation and safety.

Back to the Real Estate Glossary.