## U<sub>g</sub>-value The U-value of a glazing, called Ug value, indicates the heat transfer through the undisturbed, central area of ​​the glazing and is based on DIN EN 673.

## DIN EN 410

DIN EN 410 – Glass in construction – Determination of photometric and radiation physical parameters of glazing: specifies the calculation method for determining the photometric and radiation physical parameters of glazing.

## DIN EN 673

DIN EN 673 – Glass in construction – Determination of the heat transfer coefficient specifies the method of calculation for determining the U value of a glazing.

The heat transfer coefficient depends on:

• Structure
• Gas filling
• Coating
• Mounting position

This takes into account:

• Convection
• Heat conduction

## U-value

The U-value indicates the heat transfer through a homogeneous solid. It depends on the boundary conditions of the thickness and the conductivity of the material. With the layer thickness (d) and the conductivity (l) the heat transfer resistance Ri = d / l is calculated for each individual layer. The sum of the internal resistances (Rsi), outside (Rse) gives the total thermal resistance (RT). The U-value is now calculated from the reciprocal of RT = 1 / RT and is expressed in watts per square meter and Kelvin (W/m2*K).

The most important parameters are:

• Transmission τ (tau) • Absorption α (alpha)
• Reflection ρ (rho)

τ + α + ρ = 1 or 100%