What is a U-value? Simply explained!
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Stefano Fonseca is an energy and environment engineer with over six years of experience in technical building equipment (TGA). He combines technical expertise with a passion for understandable communication. For more than five years, he has been writing as a freelance editor about renewable energy and sustainable living, in particular about photovoltaics and heat pumps.
How well a house keeps heat determines energy consumption and living comfort. An inconspicuous numeric value plays a central role in this: the U-value. In this article, you will find out what is behind it, why it is so important when it comes to construction and renovation and what is important when it comes to windows, walls and doors.
What is a U-value?
The U-value is a building physics parameter that indicates how much heat is lost through a component such as a wall, window or roof. The heat transfer coefficient is a key indicator for thermal insulation: The lower the U-value, the better the component retains heat in the building — this saves energy and heating costs.
U-value and K-value: What's the difference?
The K value is the outdated term for today's U value. Both describe the thermal transmittance of a component in W/m²K. Since the 1990s, only the U-value has been used; the physical meaning is identical.
What does the U-value say?
The U value indicates how much heat is lost to the outside through a component. It is a measure of heat transfer: The lower the value, the better the insulation effect. The U-value is measured in Watts per square meter and Kelvin (W/m²K). The unit describes the heat loss per square meter of area with a temperature difference of one Kelvin.
How do you calculate the U-value?
The calculation of the U-value is based on physical quantities that describe the transfer of heat through a component. The decisive factors are:
The basic formula is: U = 1/R
It says R for heat transfer resistance, which depends on the materials used and their structure.
Calculate U-value for homogeneous components
For components made from a single material (e.g. concrete wall without insulation), the calculation is easy. You share the Thermal conductivity (λ) through the Material thickness (d):
U = λ/d
example: A material with a thermal conductivity of 0.035 W/mK and a thickness of 20 cm gives: U = 0.035/0.20 = 0.175 W/m²K
Calculate the U-value for multi-layered components
For multi-layer structures (e.g. wall with plaster, masonry and insulation), the total U-value is calculated as follows:
Heat resistances of air layers or inner and outer surfaces (Rsi, Rse) can also be included — especially when calculating standards for Energy certificates.
Is a low or high U-value better?
A low U-value is better because it stands for low heat transmittance. The lower the U-value, the better the insulation effect of a component — and the less heat is lost to the outside.
What is a good U-value?
A good U-value is below the legal maximum values under the Building Energy Act (GEG) and ensures low heat loss. There are particularly low target values for energy-efficient buildings.
These values serve as guidelines for energy-efficient construction or renovation. For passive houses and efficiency house levels, the target values are in some cases significantly lower.
U-value for components in comparison
Not all components of a building lose the same amount of heat. Exterior walls, windows, roofs and doors differ significantly in their heat transmittance — i.e. in their U-value. While well-insulated roof elements allow particularly little energy to pass through, outdated windows and uninsulated front doors often cause the greatest heat losses. For the energy quality of a house, it is therefore not just a single U-value that is decisive, but the interplay of all components.
U value for windows
Windows consist of various components, each with their own U-values:
Depending on the glazing, the U-values differ significantly:
Tip for buying windows: Look for a low Uw value of a maximum of 1.3 W/m²K — better still 1.1 or less. Good windows also have a thermally separated frame, a high-quality seal and a warm edge (edge bond).
U-value for outer wall
The U-value of an outer wall depends heavily on the construction and the materials used. Old buildings often have values above 1.0 W/m²K, while modern new buildings must reach at least 0.24 W/m²K according to the Building Energy Act. Restructuring measures must also be based on GEG requirements when it comes to eligible measures.
Examples of typical wall structures:
U-value for the roof
The roof is a particularly sensitive area for heat loss because warm air rises upwards and escapes via the roof. Flat roofs and pitched roofs differ in construction and insulation strategy:
- pitched roof (intermediate rafter insulation): good insulation values when done correctly
- flat roof: usually requires rafter insulation with a high insulation standard
Typical U-values are:
A combination of surface and intermediate rafter insulation ensures particularly low U values and prevents thermal bridges.
info box: Thermal bridges are weak points in the building envelope where more heat escapes than in the surrounding areas. They occur, for example, at corners, window connections or due to interrupted insulation. This results in higher energy loss, possible mold formation and poorer living comfort.
U-value for front doors
Front doors are often weak points in the building envelope. In addition to the material, tightness also plays a decisive role. Leaky doors lead to convection and thus to high heat loss.
Typical U values depending on the material:
Tip: Pay attention to circumferential seals, thermally separated thresholds and a low U value ≤ 1.3 W/m²K. Modern front doors today offer thermal insulation at window level.
Why is the U-value important for renovation and new construction?
The U-value is crucial for meeting legal requirements and reducing a building's energy consumption. It not only influences thermal insulation, but also the eligibility and long-term efficiency of the building envelope.
The U value in the heat load calculation
The U-value is a key variable in Heat load calculation and thus decisively determines how much heating output a building requires. Without precise U values, the heat load can only be roughly estimated, which can result in over-dimensioned or undersized heating systems.
Typical applications of the U-value in heat load calculation:
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