Understanding and correctly determining window U-values
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What is the U-value of a window?
Would you like to enjoy cozy warmth in your home while reducing your heating costs? The U-value of your windows is crucial for minimizing heat loss and increasing energy efficiency.
What you should know about the U-value:
• Definition and unit: The U-value, also known as the heat transfer coefficient, describes how much heat per square meter and Kelvin temperature difference escapes through a window, measured in W/m²K, with a lower value indicating better insulation.
Influencing factors: Glazing, frames, edge seals, and professional installation determine the insulation performance and thus the U-value.
Calculation in practice: The U_w value combines the values of glass, frame, and edge. The exact calculation is based on standards and manufacturer specifications. For everyday use, a U-value calculator is sufficient to quickly find out how energy-efficient your window is.
Legal requirements in Germany: The Building Energy Act (GEG) stipulates a maximum U-value of 1.3 W/m²K for new buildings, while KfW or BAFA subsidies generally require a value of less than 0.95 W/m²K.
Relationship to other values: The U-value complements the g-value, which describes solar energy transmittance, and at the same time represents the reciprocal of the R-value, so that all three key figures together determine the energy balance of a house.
Components of the U-value.
The U_w value describes the total value of the window, which includes the glass, frame, and edge area (connection between the glass and frame).
The U_g value measures the thermal insulation of the glass, e.g., below 0.5 W/m²K for triple glazing.
The U_f value describes the frame; wood or plastic (approx. 1.0–1.3 W/m²K) insulate better than aluminum (1.6–2.0 W/m²K).
The psi value relates to the edge seal, i.e., the transition point between the glass and the frame.
Advantages of the U-value: why it is necessary
A low U-value has a positive effect on your home in several ways:
Lower heating costs: less heat loss permanently reduces energy costs
Greater living comfort: stable indoor temperatures in winter and summer
Improved sound insulation: modern multiple glazing reduces outside noise
Clear basis for comparison: the U-value enables objective evaluation of different windows
Legal certainty: meets renovation requirements such as the Building Energy Act (GEG)
Access to subsidies: government programs link subsidies to defined U-value limits
Contribution to climate protection: lower energy consumption reduces CO₂ emissions
What is the optimal U-value for windows?
Different guidelines apply to the U-value of windows depending on the type of building and requirements. Are you wondering what the optimal U-value should be to combine energy efficiency and living comfort? As a general rule, the lower the U-value, the better the insulation – however, as the value decreases, the requirements for glazing, frames, and professional installation also increase.
Recommended U-values for different areas of application:
Old buildings in need of renovation: Windows with U-values of 2.0 W/m²K or higher are often still found. Replacing them with modern models can result in particularly large savings.
Modernizations and new buildings: A U-value of no more than 1.3 W/m²K is required to meet the requirements of the Building Energy Act (GEG).
Energy-efficient new buildings: High-quality windows achieve values between 1.1 and 1.0 W/m²K. They offer a very good standard for long-term energy savings.
Passive houses: Peak values of 0.8 W/m²K or lower are necessary to meet the extremely high energy standards.
Calculating the U-value – how it works
The U-value describes how much heat escapes to the outside per square meter of window area. It is expressed in watts per square meter and Kelvin (W/m²K). Physically, the formula is:
U = 1 / R
Where R stands for the total thermal resistance. This is determined by the various components of the window: glazing, frame, and edge seal.
Factors influencing the U-value:
Glazing: Whether double or triple glazing – the number of layers has a significant influence on the insulation performance. While modern triple glazing achieves values of up to 0.7 W/m²K, double glazing usually ranges between 1.1 and 1.4 W/m²K.
Frame: Plastic multi-chamber profiles, wooden frames, or thermally separated aluminum profiles contribute differently to insulation. The better the frame insulates, the lower the overall value.
Edge seal: Spacers and gas fillings such as argon or krypton reduce heat loss at the edge of the glass, thus improving overall performance.
Practical tip: In reality, the U-value is not calculated manually, but determined on the basis of standards and measurements. For everyday use, online U-value calculators help by providing a reliable comparative value immediately after entering the type of glass, frame material, and construction.
If you want to calculate the U-value of a window, you can follow these steps:
Determine the total area: Calculate the area of the entire window, i.e., frame plus glass.
Calculate the area proportions of glass and frame: Divide the window area into glass area (A_g) and frame area (A_f). For standard windows, the glass area is usually 65–75%.
Determine individual values:
a. U_g = U-value of the glazing (depending on double or triple glazing, gas filling, coating).
b. U_f = U-value of the frame (varies depending on the material).
c. ψ value (Psi) = heat loss at the glass edge seal.
4. Measure the edge seal length: Measure the length of the glass edge where heat loss occurs via the spacer.
5. Apply the formula: Uw = (Ag Ug + Af Uf + lg * ψ) / (Ag + Af)
A_g = Glass area in m²
U_g = U-value of the glazing
A_f = Frame area in m²
U_f = U-value of the frame
L = Length of the edge seal (in meters)
Ψ (Psi) = Heat loss at the edge of the glass
This formula summarizes the heat losses of the glass, frame, and edge seal and divides them by the total window area. It is specified in the DIN EN 10077 standard. Manufacturers usually provide certified U_w values that you can use directly for planning and comparison.
6. Interpret the result: The lower the calculated U_w value, the better the insulation of the entire window.
Tip: With a U-value calculator, these steps can be carried out quickly and easily—so you can see immediately whether your windows meet current efficiency standards.
Avoid common mistakes when calculating the U-value
When calculating the U-value of your windows, it is easy to make mistakes. Pay particular attention to the following points:
• Only taking the glazing into account: If you only refer to the glass value (U_g), you are forgetting the frame and edge seal – this makes the U-value appear better than it actually is.
Using incorrect surface areas: If you underestimate the frame area, the total value will be significantly different. As a rule, the glass area accounts for 65–75%.
Neglecting the edge seal: The ψ value (Psi) at the edge of the glass often accounts for 10–15% of heat loss. If you leave it out, your calculation will be too optimistic.
Using non-standard values: Manufacturer specifications usually apply to standard window sizes. The actual U-value may differ for very large or small windows.
Using outdated tables: Old data often does not include modern gas fillings, coatings, or “warm edges.”
Errors in conversion units: If you use the wrong area or temperature difference, you will distort the result.
The safest option is to use a U-value calculator or check the manufacturer's specifications. This will ensure that your calculation is correct. Use the myWindow_C3 configurator to calculate your individual window quote and compare directly which U-value is possible for your project.
U-value: Comparison of 1.5-pane and triple glazing
The differences between outdated 1.5-pane glazing and modern triple glazing become particularly clear when their typical U-values and advantages and disadvantages are directly compared:
1.5-pane glazing:
Windows with 1.5-pane glazing generally have a U-value of around 1.6 to 1.8 W/m²K. Although they are cheaper to purchase and insulate slightly better than single-glazed windows, they are now considered outdated in terms of energy efficiency. Heat loss is high, which leads to significantly higher heating costs. In addition, they no longer meet legal requirements and are hardly available in stores.
Triple glazing:
Modern triple-glazed windows achieve U-values of around 0.7 to 0.9 W/m²K. They provide excellent thermal insulation and are ideal for new buildings and renovations. In addition to noticeably higher living comfort, they often qualify for government subsidy programs and contribute to significant energy savings in the long term. However, the higher purchase costs and the greater weight of the window construction must be taken into account. Incorrect installation can also lead to condensation damage.
Our practical tip:
While windows with 1.5-pane glazing usually need to be replaced urgently in order to reduce heating costs and energy losses, modern triple-glazed windows achieve almost passive house standards. They offer both financial benefits through lower energy costs and a significantly more pleasant indoor climate.
Legal U-value requirements for windows in Germany
The U-value of windows is crucial for your energy costs and is also regulated by law. Since 2020, this has been based on the Building Energy Act (GEG), which replaced the previous Energy Saving Ordinance (EnEV). By improving the heat transfer coefficient of your windows, you will meet the legal requirements, gain access to attractive subsidies, and reduce your energy costs at the same time.
An overview of the key legal requirements:
New buildings: A maximum U-value of 1.3 W/m²K is prescribed for new windows, while the limit for roof windows is 1.4 W/m²K.
Renovations: When replacing existing windows, the glazing must achieve a U-value of at least 1.1 W/m²K, provided that the existing frame allows this technically.
Subsidies: Stricter requirements apply to KfW or BAFA subsidy programs. A U-value of ≤ 0.95 W/m²K is often required in order to receive subsidies or low-interest loans.
Retrofitting requirements: If more than 10% of a building's exterior components are replaced, the windows used must also comply with the legal limits.
Optimize window U-values with Salamander myWindow
With Salamander myWindow, you can achieve window solutions that are designed for low U-values—for maximum energy efficiency, lower heating costs, and greater living comfort. Whether for new construction, renovation, or passive houses: our systems combine high-quality insulation technology with sustainable materials.
Your advantages with Salamander myWindow:
• Optimal insulation values: Our window systems achieve U-values up to certified passive house standards.
Individual selection: With the myWindow C3 tool, you can find exactly the right solution for your location and building type.
Multi-chamber profiles and thermal separation: These designs reduce heat loss and improve the overall U-value of the window.
Sustainability: Recycled materials and evoCore technology ensure stable, durable, and environmentally friendly insulation values.
Eligibility for subsidies: Many systems fall below the legally required U-value limits and are therefore eligible for KfW or BAFA subsidies.
Comfort and security: Windows with low U-values ensure consistent room temperatures, protect against noise, and increase burglary resistance.
Start optimizing your U-value now with Salamander myWindow! Use the window configurator to see directly which systems achieve the best U-value for your home, or contact us for a personal consultation with our specialist partners.
FAQ: Frequently asked questions about window insulation
Are you unsure which U-value is right for your windows or what you should look out for when insulating? Our answers to the most frequently asked questions will quickly guide you so that you can make the right decision for your home.
How can I have the U-value of my windows measured?
The U-value is usually determined by manufacturer specifications or standard calculations in accordance with DIN EN ISO 10077. For existing windows, you can use thermal imaging or measurements taken by specialist companies. Online U-value calculators also provide a rough guide.
What are some common mistakes when estimating the U-value?
A common mistake is to only consider the U_g value of the glazing and neglect the frame or edge seal. Incorrect assumptions about surface areas or transferring manufacturer values without taking the actual window size into account can also lead to incorrect results.
When is it worth replacing old windows because of the U-value?
It is usually worth replacing windows if they have a U-value of over 2.0 W/m²K – this is often the case with old single or 1.5-pane glazing. Modern triple-glazed windows achieve values of 0.7–0.9 W/m²K and significantly reduce heating costs. The worse the previous value, the greater the savings after replacement.
How is the U-value related to mold or condensation?
A high U-value means that a lot of heat is lost and the inside of the window remains cold. This can cause condensation to form on the edges of the glass, which increases the risk of mold in the long term. Windows with a low U-value keep the surface temperatures inside significantly higher, thus contributing to a healthier living environment.
Why is a low U-value worthwhile?
A low U-value reduces heating costs, ensures pleasant room temperatures, meets legal requirements, and makes you eligible for subsidies. At the same time, you are actively contributing to climate protection through lower energy consumption.