Determination of the thermal transmittance of insulating glass (Ug value)

One of the most important parameters in building construction, both during the planning phase and in the development of energy-efficient restoration plans for old buildings, is the U-value (thermal transmittance) of the building materials used. The U-value is the measure for the heat transfer from one medium through a solid body into a second medium. It indicates the thermal energy per unit of time for a given surface area and for a given Kelvin temperature difference between the two fluids. For solid building elements, the U-value can be calculated from the thermal conductivity with U=λ/d.

Thermal transmittance through a glazing is disproportionately more complex than the U-value of solid building elements since it occurs through multiple layers (2-layer and 3-layer insulating glass) and since – along with heat transfer and thermal radiation – convection in the inter-pane cavity must also be taken into consideration. Corresponding standardized measuring techniques are described in DIN EN 674 and DIN EN 675. These techniques are suitable for the laboratory, but have their limits as pertains to glazing parameters and are stationary methods.

DIN EN 673 is a standardized method for mathematical calculation of the Ug value under the assumption of theoretical benchmark data (glass emissivity, glass thickness, inter-pane cavity dimensions, gas filling level, etc.).

In our “Uglass” product, NETZSCH has developed a transient thermal measuring technique allowing for mobile measurement of the Ug value by means of two heated sensors, specially conceived software and a one-dimensional finite difference model exclusively developed for insulating glazes. The Ug value of a glaze can thus be easily measured by a manufacturer for quality assurance in the factory or by a processer for incoming goods control. Existing inventories can also be evaluated directly on-site. The state of a glazing can be reliably assessed within a few minutes.  

The mobile Ug value measurement device

Straightforward measurement of Ug values on insulating glass for the assessment of existing glaze inventories and for quality assurance.