Thermal insulation - Ug value of windows are relevant

Author: Dr. Helmut Weinläder, Sven Hippeli, ZAE Bayern, Alexander Frenzl, NETZSCH-Gerätebau GmbH (As of: January 2017) The newly developed mobile Ug-value measuring instrument can measure the Ug-value (thermal insulation value) of glazing simply and reliably within a few minutes. For energy consultants, building physicists, customer advisors, etc., the new mobile measuring device thus opens up for the first time the possibility of determining the actual thermal insulation value of installed glazing with high accuracy on site. The measurement results obtained provide a sound basis for the thermal evaluation of existing glazing. This makes it possible to visualize potential energy savings in the building sector and make a significant contribution to climate protection.

If there was uncertainty about the insulation value of existing glazing, this had to be estimated from the glazing data - if available - or the glazing had to be removed at great expense and measured in the laboratory. Only in the case of older glazing with air filling and without a low-e layer can the Ug value be determined quite accurately from the pane structure. However, as soon as inert gas fillings or low-e layers are involved, such an estimate is questionable, since the actual gas filling level of the glazing and the exact emissivity of the low-e layer are usually not known.

"Fenstercheck" project

In order to provide more certainty in this respect, the mobile measuring device Uglass for determining the Ug value of insulating glazing was developed as part of the "Fenstercheck" project funded by the German Federal Ministry for Economic Affairs and Energy on the basis of a resolution of the German Bundestag (Fig.1). The actual thermal insulation value of the glazing can now be determined easily, quickly and reliably on site.

The two halves of the device are equipped with heating foils on the active measuring surface. An internal sensor logic checks the temperature gradient applied to the glazing. If there is no sufficient temperature gradient, the device applies an additional one-sided heating pulse to the glazing via the heating foil. If the existing temperature gradient is sufficient, there is no heating. In both cases, the change in temperature over time after the device halves are placed on the glass is used for the evaluation (transient measurement method). The numerical analysis and evaluation of the measurement data is already performed continuously in parallel during the measurement. Control and evaluation are fully automatic via user-friendly software. The two halves of the device itself are connected to the PC via WLAN.

Numerous validation measurements were carried out with the developed measuring device during the project period. Fig. 2 shows the Ug values determined with the measuring device for different types of glazing above the theoretically calculated values. A good correlation along the angle bisector can be seen. The measurement range extends from well-insulating triple insulating glass to double insulating glass to air-filled double glazing without low-e layer and noble gas filling.

The relative measurement uncertainty of the measurement method determined experimentally with the measuring device is ±10% for Ug values above 1.0 W/(m²K). For Ug values below 1.0 W/(m²K), it is ±0.1 W/(m²K). The measuring device is suitable for double and triple insulating glass as well as roof and facade glazing and completely covers the relevant measuring range of Ug values between 0.5 W/(m²K) and 4 W/(m²K) with a measuring time of only a few minutes.

Very reliable in practical use

In practical use, it is shown that the measuring device allows very reliable statements to be made about the condition of windows in existing buildings. Likewise, the method proves to be a helpful tool in the context of inspection during production. The first insulating glass manufacturers are also measuring the Ug value of the finished insulating glass unit during production, in addition to the prescribed measurements of the gas filling ratio. This enables the manufacturer to optimize its production quality and creates confidence among customers and consumers.

In practice, deviations from the presumed estimated value could be determined when evaluating existing glazing in the context of energy-related renovations. On the one hand one is confronted with estimations, which would entail the receipt of the glazings in the course of the reorganization with an Ug-estimate value of 1,3 W/(m²K) - one determines during the field measurement however clearly worse values and the entire energetic condition after the reorganization must question. On the other hand, glazing was measured that was initially estimated to be significantly worse at 2.3 W/(m²K) and was scheduled for replacement. By means of a field measurement, significantly better values could be measured, which made the preservation of the glazings worthwhile and significantly reduced the refurbishment expenses. In both cases, it is clear that the Ug-value should be taken into account by means of measurement as a fundamental basis for the development of a refurbishment concept. The estimation of the Ug-value on the basis of the age and the total condition is from there inadequate.

It is likewise conceivable to use the equipment in the incoming goods inspection with the window and facade farmer randomly. Thus it can be guaranteed that in time possible quality lacks are discovered, before it comes to the installation and to the causation of further costs. In any case, it is helpful to carry out a mobile Ug-value measurement in case of doubt before removing existing windows, installing emergency glazing and taking the glazing for laboratory testing, which can be destructive. This often entails legal disputes and further costs, which can easily be avoided.

The Institute for Window Technology (ift) in Rosenheim has carried out comparative measurements between Uglass and the standardized laboratory methods and reported on the good comparability at the Rosenheim Window Days. Glazing with Uglass was measured non-destructively and compared with the calculation according to DIN EN 673. In order to obtain well-founded calculation values, the glazing was destroyed after measurement and values of the gas filling ratio and emissivity were determined, which were then used in the calculation. For standard double and triple glazing, for which Uglass was primarily developed, the results are comparable with the calculation according to DIN EN 673 within the scope of the measurement accuracy of Uglass. For laminated safety glass and here in particular for structures with high pane thickness or asymmetry, the measured values deviate in part and require further tests or recommendations for action. An overview of the comparative measurements is shown in the following tables. The mobile Ug-value measuring device was awarded the Bavarian Energy Prize in the category "Products and Applications" in 2016.