U-value
The U-value is the ratio of the heat flux density flowing through the component at steady state to the difference between the two adjacent ambient temperatures. The heat transfer coefficient of a component is the reciprocal of the total resistance to heat transfer.
U-Values Catalog: Easy determination of the U-value of building components
The physical unit of U-value is watts per square meter and kelvin: W/(m2-K).
The following processes influence the U-value of a building component:
- Heat transfer from the indoor air to the building component.
- Heat conduction inside a building component.
- Heat transfer from the component to the outside air.
The following rule applies: the smaller the U-value, the better the thermal insulation.
In addition to a proportionally decreasing heating energy demand, a decreasing U-value brings rising surface temperatures. This increases the comfort for the residents. Also, the risk of surface condensation, as a result of which gray spots, mold and musty odors, is reduced.
The standard SIA 180 "Thermal insulation, moisture protection and indoor climate in buildings" has maximum heat transfer coefficients for comfort and moisture protection.