In Alsfeld, a historic building has been given a new lease on life: Villa Raab was renovated under the direction of the architectural firm Weppler-Jungermann. Building systems manufacturer SAKRET played a crucial role in the project.

Exemplary Renovation of Villa Raab in Alsfeld in the Spirit of Historic Preservation

The stately villa, built in the French palace style during the Gründerzeit era, was to undergo a comprehensive renovation in accordance with historic preservation guidelines after years of standing vacant and beginning to fall into disrepair—in retrospect, an extraordinarily successful project by the entrepreneurial couple Ralf and Tanja Bohn. Under the direction of the architectural firm Weppler-Jungermann, Sachs Baudekoration from Lauterbach, Hesse, and the historic preservation authorities, partners from a wide range of fields were coordinated: SAKRET as the system manufacturer of the necessary building materials and a partner for plastering, painting, and tiling work, Scheerschmidt and Schneider (Stadtallendorf) for the tiling work, and the specialty firms Golem (Sieversdorf) and Replicata from Freiburg recreated the historic décor and produced reproductions of historical originals.

The interplay of magnificent Neo-Baroque with the then-modern Art Nouveau style can be seen and felt everywhere, both inside and out. Photo: © sakret.de

To repair and replace historic tiles both indoors and outdoors, they had to be crafted using period-appropriate materials and expertly installed. These tiles had to be able to withstand extreme temperature fluctuations while also being waterproof, frost-resistant, and highly durable. Photo: © sakret.de

Valuable historic tiles, including some from Villeroy + Boch, in the hallways and stairwells were almost completely preserved. Photo: © sakret.de

The round arches of the vaulted ceiling (approx. 120 cm in span) were plastered with SAKRET MAP-SL+ machine-applied exterior plaster. This material could be applied and troweled to the required thickness of up to 5 cm in a single coat. In the basement rooms, SAKRET KIP // KPM I lime interior plaster was also used.

After standing vacant for years and beginning to fall into disrepair, this architectural gem was comprehensively restored true to its original form by a team of selected experts and specialists. Photo: © sakret.de

A Successful Team Effort

The renovation of Villa Raab can now be considered a prime example of the synergy between dedicated contractors and rigorous historic preservation standards. Lead architect Jochen Weppler also spoke highly of the success of this collaboration: “A truly successful team effort that allowed us to meet even the most complex requirements of historic preservation in the best possible way.”

Photo: © sakret.de

The shared goal: to restore the property to its original condition

The property’s special value lay in its numerous original interior and exterior details. These included, for example, the magnificent exterior facades with their rich ornamentation of decorative elements and moldings, as well as the stucco cornices and friezes in the main rooms. The valuable, historic tile flooring in the hallways and stairwells was also completely preserved, with only a few missing sections. To meet the requirements for materials in the areas of plastering and tile installation, SAKRET was able to offer system-based product solutions that—in accordance with historic preservation regulations—ensure a long service life. These included, for example, materials such as felt plaster, lime interior plaster, and marble-based fine-finish putty.

Fortunately, the villa remained largely in its original condition, enabling a faithful reconstruction of the house with its numerous moldings, stucco cornices, and friezes. Photo © sakret.de

The Exterior Work

The overall condition of the building required extensive preparatory work: Cracks running through parts of the masonry had to be grouted, and many of the stone balcony railings had to be removed for restoration. After cleaning the entire facade, it was plastered with SAKRET KAM adhesive and reinforcing mortar.

Next, SAKRET KAM felt plaster (0.8 mm grain size) was applied as a decorative topcoat. For smooth surfaces such as building corners, walls, window surrounds, etc., SAKRET MFS Fine Marble Filler was used. This made it possible to fill unevenness of up to 1 cm in a single pass.

The property’s special value lies in its numerous interior and exterior details that have been preserved in their original state. For example, the magnificent exterior facades with their rich ornamentation. Photo copyright: © sakret.de

The Renovation of the Interior

Inside the villa, the entire wall plaster had to be removed due to moisture that had penetrated the structure over decades.
SAKRET KIP lime interior plaster—awarded the Blue Angel label for its environmental compatibility—was used as the base coat. To plaster the old brick walls, SAKRET KIP was applied in two passes, wet-on-wet, using the G4 plastering machine, to a layer thickness of up to 7 cm.
After the drying period, the wall surfaces were completely finished with SAKRET KIP Multi Interior Lime Plaster KPM I, 6 mm thick, with an alkali-resistant SAKRET mesh embedded within to prevent cracking. The final coat was applied using SAKRET Edelfilz Plaster EFP 0.5 mm.

The interior spaces required extensive preparation and planning, as not only did the wall and ceiling designs need to be restored true to the original, but new installation and piping systems also had to be installed to meet modern standards. Photo: © sakret.de

The special value of the property also lay in the numerous design details on the walls and ceilings. These were mostly removed and then reinstalled after restoration. Photo: © sakret.de

Details on Tile Work

SAKRET Universal Primer UG and SAKRET Special Primer SG were used as a primer on mineral plasters, concrete, and cement screed, as well as for priming cement-based building boards. For waterproofing damp and wet areas, SAKRET Alternative Waterproofing AA was recommended as a tested system component.
SAKRET Flex Tile Adhesive FFKs and the multifunctional SAKRET Euroflex EF flexible mortar enabled efficient tile installation. Finally, the fast-setting SAKRET Flex Grout FFM was used to grout the tiles.

In Alsfeld, a historic building has been given a new lease on life: Under the direction of the architectural firm Weppler-Jungermann, Villa Raab was renovated. System manufacturer SAKRET played a decisive role in the project.

Today, triple-glazed windows are the technical standard for energy-efficient construction, and the U-value is the metric that quantifies this standard. Anyone who wants to understand why modern windows lose hardly any heat, why condensation on the inner pane is a thing of the past, and why triple-glazed windows are indispensable in passive houses and nearly zero-energy buildings must grasp the U-value in its full physical depth. The U-value for triple-pane windows is far more than just a number on a data sheet: it is the result of a complex interplay between the pane construction, gas filling, frame material, and installation conditions.

• What the U-value means in physical terms and how it differs from related parameters
• How the U-value for triple-pane windows is calculated and which component values interact in this process
• What role gas filling, spacers, and coatings play in heat transfer
• Why the Ug-value of the glass and the Uw-value of the window can differ significantly
• What values are achieved in practice and what standards require
• How triple-pane glazing compares to double-pane glazing and when each option is appropriate
• Which installation errors and thermal bridges negate the theoretical U-value in practice
• How triple glazing affects sound insulation, condensation behavior, and comfort

What the U-value is: definition, unit, and physical basis

The U-value, technically known as the heat transfer coefficient, describes how much heat flows per second through a building component with a surface area of one square meter when there is a temperature difference of one Kelvin between the two adjacent air layers. The unit is W/(m²K), or watts per square meter per Kelvin. The lower the U-value, the less heat is lost, and the better the insulation performance. A U-value of 0.5 W/(m²K) means that, with a temperature difference of ten Kelvin between the interior and exterior, only five watts per square meter of the building component’s surface area are lost. By comparison: A single-pane window from the 1960s had U-values of five to six W/(m²K)—ten times that of modern triple-pane glass.

The U-value is a steady-state parameter: it describes heat transfer in a steady state, i.e., when temperatures on both sides remain constant. In reality, outdoor temperatures fluctuate constantly, and the thermal mass of building components buffers short-term fluctuations. For glass—a thin, low-mass material—this thermal inertia plays hardly any role, which is why the U-value is particularly well-suited as a design parameter for glazing. The U-value must be strictly distinguished from the lambda value (thermal conductivity), which represents a material property, and from the R-value (thermal resistance), which describes the reciprocal of the U-value excluding surface resistances.

For windows, standards distinguish between several U-value components that together describe the thermal behavior of the entire building element. The Ug-value (g for glazing) refers to the heat transfer coefficient of the glazing alone—that is, the glass unit without the frame. The Uf value (f for Frame) describes the thermal insulation of the frame profile. The Psi value (Greek letter, symbol for the linear heat transfer coefficient) quantifies the thermal bridge effect of the spacer between the glass panes. Based on these three parameters, the Uw value—that is, the heat transfer coefficient of the entire window—is calculated using an area-weighted method in accordance with the European standard EN ISO 10077. This overall value is the key parameter for energy performance certificates, subsidy applications, and compliance with building codes.

Structure of Triple Glazing: Panes, Gases, and Coatings

Triple-pane glazing consists of three glass panes separated by two air spaces. Each air space is hermetically sealed and filled with a noble gas, typically argon or krypton. Argon is the more commonly used gas because it is readily available at a low cost and has significantly lower thermal conductivity than air. Krypton has even better insulating properties and allows for narrower air spaces, but it is more expensive. Depending on the design, the total thickness of a typical triple-pane unit ranges from 36 to 52 millimeters, with narrower units filled with krypton and wider ones with argon.

The thermal insulation layers on the glass surfaces are crucial for the U-value of triple-pane windows. Modern triple-pane windows feature a so-called Low-E coating (Low Emissivity) on at least two of the six glass surfaces. This wafer-thin metal oxide layer, usually made of silver or tin oxide, reflects long-wave thermal radiation back into the room instead of allowing it to pass through the glass. Without this coating, the glass surface would emit thermal radiation almost like a black body; with the coating, the emissivity drops from about 0.84 to values below 0.05. This effect is the single most important factor in improving the U-value.

The positioning of the coatings within the glazing unit follows a standard convention: The surfaces are numbered from the outside in, from Surface 1 (outside) to Surface 6 (inside). In triple-pane glazing, the Low-E coatings are typically located on surfaces 2 and 5—that is, on the side of the outer and middle panes facing the interior, respectively. This arrangement minimizes heat radiation through both air spaces between the panes. The spacer between the panes, known as the “spacer,” is made of warm-edge material—such as plastic, stainless steel, or thermally broken aluminum alloys—in high-quality triple-pane glazing to minimize the thermal bridge effect at the edge of the glass.

Calculation of the U-value for triple-pane windows: Ug, Uf, and Psi in combination

The calculation of the Ug value for triple-pane glazing follows the EN 673 standard. The total resistance of the glazing unit is composed of the resistances of the individual glass panes, the gas fillings, and the surface resistances. Since glass itself is a very good conductor of heat, the glass panes contribute very little to the total resistance. The main contribution comes from the gas fillings and the Low-E coatings. A well-designed triple-pane unit with two argon chambers and two Low-E coatings achieves Ug values between 0.5 and 0.7 W/(m²K). With krypton as the gas fill and optimized coatings, values below 0.5 W/(m²K) can be achieved.

The Uf-value of the frame depends heavily on the frame material and the profile design. Wooden frames achieve Uf-values between 1.0 and 1.4 W/(m²K) due to wood’s good thermal insulation properties. Plastic profiles with multiple chambers and embedded insulation materials achieve similar values, often between 1.0 and 1.3 W/(m²K). Aluminum frames inherently have poor insulation properties due to aluminum’s high thermal conductivity; Uf values below 1.5 W/(m²K) can only be achieved using thermally broken profiles with polyamide spacers; for passive house requirements, specially developed aluminum profiles with Uf values below 1.0 W/(m²K) are available, but they are complex and costly.

The linear heat transfer coefficient Psi of the spacer, often referred to in standards as Psi-g (g for glass edge), describes the heat loss along the entire glass edge assembly. Conventional aluminum spacers have Psi values around 0.08 W/(mK). Warm-edge spacers made of stainless steel or plastic reduce this value to 0.03 to 0.05 W/(mK). For large window areas with a comparatively short perimeter, this difference is less significant; for windows with small muntins or narrow window formats, heat loss through the glass edge can account for a significant portion of the total heat loss.

The calculation of the Uw-value according to EN ISO 10077-1 weights the three component values according to their respective area and length proportions. A simplified example illustrates the order of magnitude: For a window with a glass area of 70 percent and a frame area of 30 percent, the Uw value is calculated as the area-weighted sum of Ug and Uf plus the edge heat loss. A window with an Ug of 0.6 W/(m²K), an Uf of 1.1 W/(m²K), and a warm-edge spacer achieves a Uw value of approximately 0.8 to 0.9 W/(m²K). This value is significantly lower than what can be achieved with double glazing and meets the requirements for passive houses and subsidy programs.

Regulatory Requirements, Comparative Values, and Practical Application

The Building Energy Act (GEG), which regulates the energy performance standards for buildings in Germany, stipulates maximum Uw values for the replacement of windows in existing buildings. In new construction, the requirements are governed by the primary energy demand of the entire building, so no fixed individual value applies to windows; however, practical experience shows that triple glazing is required in nearly all new construction designs to meet the KfW Efficiency House standard. For the Passive House, as defined by the Passive House Institute in Darmstadt, a maximum Uw-value of 0.8 W/(m²K) applies as a guideline for the entire window, including installation.

By comparison: Double glazing with insulating glass (so-called thermal insulation glazing) achieves Ug values between 1.0 and 1.3 W/(m²K), which results in Uw values between 1.2 and 1.5 W/(m²K) for window combinations with typical frames. Older double-pane glazing without a Low-E coating has Ug values around 2.8 W/(m²K). The difference between triple-pane glazing with an Ug of 0.6 W/(m²K) and an old double-pane unit with an Ug of 2.8 W/(m²K) corresponds to a difference of several hundred kilowatt-hours of heating energy per year for a typical single-family home with 20 square meters of window area and a heating season of 200 days. This savings is supported by building physics and explains the economic appeal of the replacement.

The question of whether triple-pane glazing is always the right choice cannot be answered with a blanket “yes.” In buildings with excellent solar orientation and large south-facing windows, a g-value that is too low (total energy transmittance, i.e., the proportion of solar energy that enters the building through the glass) can become problematic. Triple-pane windows typically have g-values between 0.45 and 0.55, while double-pane windows reach values around 0.60 to 0.65. In passive solar designs, where solar gains are intended to cover a significant portion of the heating demand, the U-value of triple-pane windows must be optimized in conjunction with the g-value. Triple-pane glazing with a very good U-value but a low g-value may save less energy in such a design than double-pane glazing with a higher g-value.

Installation, thermal bridges, and the difference between theory and practice

The U-value for triple-pane windows listed on the data sheet is a laboratory value measured under defined conditions. In practice, the installation determines whether this value is actually achieved in the building. The critical point is the connection zone between the window frame and the exterior wall. Thermal bridges form here if the window is not seated deep enough within the insulation layer, if the connection is not airtight, or if the insulation in the soffit area is missing or interrupted. These installation-related thermal bridges can increase the effective heat transfer in the area of the window connection to many times the Uw-value of the window itself.

The EN ISO 10211 standard describes the calculation of thermal bridges, and the design recommendation is to position the window in the exterior wall so that the frame extends at least to the insulation layer—ideally, so that it is overlapped by the insulation layer. In the case of exterior insulation (ETICS), this means placing the window as far outward as possible and carefully connecting the soffit insulation. Different geometric constraints apply for interior insulation or core insulation. If you install a window with a U-value of 0.8 W/(m²K) into a poorly constructed reveal, the benefit of triple glazing can be virtually negated by the thermal bridge loss at the installation edge.

The airtightness of the installation is another factor that influences effective thermal insulation. Leaky connection joints not only allow warm air to escape—which increases heat loss through ventilation—but also allow moisture to enter the connection joint, where it can condense if the temperature drops sufficiently. The principle of “tighter on the inside than on the outside” applies to window installation just as much as it does to the entire building envelope: The inner seal (vapor barrier) must be more vapor-tight than the outer seal (rain screen) so that moisture penetrating the joint can diffuse outward.

Roller shutter boxes are a frequently underestimated weak point in the system. Many older roller shutter boxes have virtually no thermal separation and form a significant thermal bridge directly above the window. Even if the window itself has an excellent U-value, an uninsulated roller shutter box can cause the surface temperature on the ceiling of the roller shutter box area to drop below the dew point of the indoor air, leading to condensation or mold growth. Modern roller shutter boxes with integrated thermal insulation and thermal separation are therefore essential complements to triple-pane glazing.

Comfort, Condensation Behavior, and Sound Insulation with Triple Glazing

An often underestimated advantage of triple-pane glazing lies not in energy efficiency but in thermal comfort. Under normal winter conditions, the inner surface of a triple-pane window is significantly warmer than that of a double-pane window. While double-pane glazing has an inner pane temperature of about eight to twelve degrees when the outside temperature is minus ten degrees and the inside temperature is twenty degrees, triple-pane glazing reaches temperatures of seventeen to eighteen degrees under the same conditions. This higher surface temperature reduces radiative asymmetry: people near the window no longer perceive a cold source of radiation, which significantly increases comfort without requiring the heating to be turned up.

Condensation behavior is a direct result of these surface temperatures. Water condenses on a cold window pane as soon as the pane’s temperature drops below the dew point of the indoor air. In a typical living space with a temperature of twenty degrees and fifty percent relative humidity, the dew point is approximately nine degrees. Double-pane windows regularly drop below this temperature in cold outdoor conditions, whereas triple-pane windows rarely do. Fogged-up window panes, water beading on the glass surface, and damp window sills are a thing of the past with triple-pane glazing, provided the installation was done correctly and there are no extreme humidity levels in the room.

When it comes to sound insulation, triple-pane glazing does not automatically perform better than double-pane glazing. The sound insulation of glazing depends on the mass of the panes, the spacing between them, and the damping properties of the gas filling. Although triple-pane glazing generally has good potential due to its greater total mass and the presence of two air spaces between the panes, symmetrical pane thicknesses can lead to coincidence effects, in which certain frequencies are transmitted particularly well. For high sound insulation, asymmetrical pane thicknesses are therefore often used, such as a combination of six, four, and eight millimeters of glass thickness. Anyone choosing triple glazing primarily for sound insulation should explicitly check the sound reduction index (Rw) and the rated sound level difference, rather than relying solely on the U-value.

Triple Glazing in the Context of the Overall Energy Concept

The U-value of triple-pane windows is a key factor in a building’s overall energy efficiency concept, but it is effective only in conjunction with the entire thermal envelope. A building with exceptionally well-insulated walls, an airtight envelope, and controlled residential ventilation, but which has poorly installed windows with thermal bridges at the joints, loses a significant portion of its theoretical efficiency in practice. Conversely, excellent windows with U-values below 0.8 W/(m²K) can only reduce the total energy demand to a limited extent in a poorly insulated older building envelope, because heat losses through the walls and roof dominate.

The cost-effectiveness of using triple-pane glazing depends on several factors: the initial condition of the existing glazing, energy prices, building use, climate zone, and the availability of subsidies. Specialist planners calculate the payback period based on the heating energy saved and the additional costs compared to double glazing. In well-insulated new buildings and in energy-efficient retrofits to “Efficiency House” standards, triple glazing is now the superior choice in terms of both cost-effectiveness and building physics. In simple retrofit measures without an overall concept, high-quality double glazing with a good U-value may offer a better cost-benefit ratio.

Planning windows with triple glazing also requires careful coordination with the ventilation system. Since triple glazing allows virtually no infiltration through joints and the building envelope becomes more airtight overall, the need for controlled ventilation increases. Without sufficient air exchange, moisture from cooking, showering, and breathing accumulates in the indoor air, which—despite high pane temperatures—can lead to increased relative humidity and, in the long term, to mold risks at thermal bridges. The U-value of triple-pane windows is thus not merely an energy efficiency metric but part of a comprehensive building physics system that jointly optimizes thermal insulation, moisture protection, and indoor air quality.

Architects and specialist planners who consistently use triple-pane glazing while coordinating installation details, thermal bridges, ventilation concepts, and solar gains create buildings that are not only energy-efficient on paper but actually remain so in operation. The U-value is the foundation of this understanding, but only its expert application within the overall context makes it an effective planning tool.