Last update: 15.10.2020
Thermal insulation

U value - heat transmission coefficient

This value characterises the heat loss through a component.

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This value characterises the heat loss through a component. It indicates how much heat passes through 1 m² of a component when there is a temperature difference of 1 K between the two adjacent sides – for example, between a room and an outside wall. The smaller this value, the better the heat insulation. 

The U value is given in Watts per square metre Kelvin [W/(m²K)].

According to EN 673, the spectral range of the concerned heat radiation is long wave infrared between 5.000 and 50.000 nm (5 ... 50 µm).

Please note that the European U values are different from American values. This must be taken into account when making international comparisons.

Ug value

The Ug value is the heat transmission coefficient for glazing. According to the European standard EN 673, this characterises the heat transfer through the central part of the glazing, i.e. without edge effects, and states the steady-state density of heat transfer rate per temperature difference between the environmental temperatures on each side.
It can be calculated according to EN 673 or determined by measurement according to EN 674 (guarded hot plate method) under standardised boundary conditions. Four factors determine this value: the emissivity of the coating, which is determined and published by the producer of the coated glass; the distance of the gas space between the panes; the filling gas and the filling rate when using inert gases. 

(To find the rated value for real life usage, national aggregates need to be considered – DIN 41408-4 applies in Germany).

Ug value for inclined glass surfaces

The Ug value that is most often defined and published refers to glazing that is vertically (90°) installed. Glazing installed at an angle changes the convection in the interspace and deteriorates the Ug value. The smaller the angle of the glazing, the more rapid the circulation in the interspace and the higher the heat transmission from the inner to the outer pane. This can worsen the Ug value by up to 0.6 W/m²K for double insulating glass.

Ug value for inclined glass surfaces


Uf value

The Uf value is the heat transmission coefficient of the frame, the nominal value of which can be determined in three different ways:

•    measuring according to EN ISO 12412-2.
•    calculating according to EN ISO 10077-2.
•    using the EN ISO 10077-1 definition, appendix D.

The nominal value, as well as additions due to national regulations determine the rated value for real-life usage.

Ψ value

The Ψ value (Psi value) is the linear thermal bridge loss coefficient for a component. With regard to windows, it describes the interaction of insulating glass, dimensions, spacer and frame material and defines the component’s thermal bridges. The Ψ value can be influenced by the spacer bar material. So-called warm edge technologies using stainless steel, metal-plastic combinations or thermoplastic systems can reduce Ψ significantly. Insulating glass itself has no Ψ value, as this only applies to the construction element into which it is integrated.

Uw value

Insulating glass is normally used in windows. The Uw value describes the heat conductivity of the construction element window. Based on the Ug value, it can be determined using three different methods:

•    using a given value in EN ISO 10077-1, Tab. F1.
•    by measuring in accordance with EN ISO 12567-1.
•    by calculating in accordance with EN ISO 10077-1 as per the following formula:

Uw value formula

Uw: Thermal transmittance of the window.
Uf: Thermal transmittance of the frame (assessment value).
Ug: Thermal transmittance of the glazing (nominal value).
Af:  Frame surface.
Ag: Glass surface.
lg: Periphery of the glazing.
Ψ: Linear thermal transmittance of the glass edge.

The heat loss in the edge zone is more important than the middle of the glazing, which is why thermally improved spacers are becoming increasingly important.

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