The German-Finnish collaborative project "Rapid-U", which aims to validate a measuring device for in-situ measurement of the heat transfer coefficient, was completed on schedule in 2020. Extensive laboratory tests were carried out on typical exterior walls of existing buildings for the project (03ET1564A), which was funded on the German side by the Federal Ministry of Economics and Technology through the project management organisation Jülich. Numerous buildings and building components were analysed through the cooperation with the German Energy Consultants' Network (DEN).

For economical and efficient restructuring measures its essential, to have ideally accurate knowledge for the actual energetic condition of a building. A calculation of the U-value is because of tabulated varlues often inaccurate and risk-avers. A measurement with etablated measurement methods is very time intensive and expensive. Measurements with short time measurement methods are on the other hand very inaccurate and error-prone. As in the private sector investments in energy efficiency measures

It is essential to have the most accurate knowledge possible about the true energy performance of a building in advance in order to carry out economical 
and efficient refurbishment measures. Calculating the U-value using tabulated values is often inaccurate and very risk-averse. Taking measurements with traditional methods is very time-consuming and expensive. Conversely, using short-term measurement methods is highly inaccurate and prone to error. Given that investments in energy efficiency measures are mainly triggered in the private sector when the measures are economical and the investors are presented with a vivid picture of the condition of the building components, measuring the actual quality of the building components on site is a very good way to stimulate energy efficiency measures in the building stock.

A fast, easy-to-use and non-damaging method for determining the U-value of a component was assessed in the course of the project. A large number of 
buildings can be surveyed quickly and inexpensively thanks to the simplicity of the measurement process. The resulting deeper knowledge of the building 
components helps with needs-based planning of any required renovation measures and acts as the basis for precise calculations of the expected savings. This also prevents any potential oversizing of measures because of safety margins in tabulated values, which in turn reduces investment costs and increases the attractiveness of energy renovation measures for investors and owners.

Im Vorhaben wurde eine schnelle, einfach anzuwendende und zerstörungsfreie Messmethode zur U-Wert Bestimmung eines Bauteils evaluiert. Durch das einfache Messverfahren kann eine große Zahl von Gebäuden schnell und kostengünstig vermessen werden. Die daraus resultierende bessere Kenntnis über die Bauteile hilft bei der bedarfsgerechten Planung der tatsächlich notwendigen Sanierungsmaßnahmen und ist die Basis für konkrete Berechnungen der zu erwartenden Einsparungen. Damit werden auch etwaige Überdimensionierungen von Maßnahmen aufgrund von Sicherheitszuschlägen in tabellierten Werten verhindert, was wiederum Investitionskosten senkt und die Attraktivität von energetischen Sanierungsmaßnahmen für Investoren und Eigentümer erhöht.

In addition to the potential savings and the improvements in demand-oriented planning, a U-value measurement system such as Rapid-U also allows for onsite monitoring of the success of insulation measures and quality control of individual building components. This increases the acceptance of and trust in energy-efficient renovation measures among the generalpublic. This creates particularly high potential when it comes to residential buildings, as a large proportion of the building stock that is in need of refurbishment is privately owned.

The primary objective of the laboratory and in-situ investigations was to validate the performance of the Rapid-U devices under different boundary conditions. 
In the course of the experiments, eight wall types typically found in residential buildings in Germany were subjected to detailed laboratory tests to determine 
their thermal properties. The reference values of the thermal properties were determined using established laboratory measurement methods.

A graded approach was used for the Rapid-U measurements. The actual measurement accuracy and the impact of mounting the units on a wall were 
verified in experiments at constant temperatures both indoors and outdoors. It was ascertained that the measurement method delivers results with good accuracy, despite the very short measurement times of between 45 and 90 minutes. The effects of mounting and handling were included in the subsequent tests, in which variable temperatures were applied to the outside of the components, as they are often unavoidable in reality. Extensive simulation calculations were also carried out for this purpose.

U-value measurement results under transient boundary conditions fluctuate depending on the thermalinertia of the components and the temperature 
curves. These influences are eliminated by averaging for transient long-term measurements, such as with heat flow measuring plates. This cannot be done with short-term measurements and the measured values always represent the prevailing heat flow during the measurement period. Finding time windows or even predicting when a short-term measurement will yield results that match the steady-state U-value is the real challenge of measuring U-values in a short period of time.

Using the Rapid-U measurement system correctly requires a high level of experience and knowledge of the measurement technology as well as the thermal 
behaviour of components under real and therefore fluctuating boundary conditions. The alignment and positioning of the devices on the component surfaces 
that are to be measured also have a significant influence on the measurement results. While the reference methods have an integrative character because of the larger measurement areas, the impacts of small-scale inconsistencies are directly reflected in the results of the Rapid-U instruments. The top priority during the project was therefore the training and continuous professional development of experts in how to apply the Rapid-U measurement technique in practice by means of a constant exchange of empirical values from the laboratory and the field. Partially valid results could therefore be obtained using the Rapid-U measurement technique, even under in-situ conditions. However, in Germany's temperate climate, only a few days a year are suitable for taking a quick U-value measurement with Rapid-U because it is critical to comply with certain boundary conditions. This puts further restrictions on the usability of Rapid-U.

Project 03ET1564A was funded by the Federal Ministry for Economic Affairs and Energy on the basis of a resolution of the German Bundestag.

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