The energy consumption in buildings contributes to nearly 40% of the primary energy consumption in Europe - around 3100TWh every year. However, a large amount of this energy is used for heating and cooling. This proves that better insulated buildings could have a large impact on our overall energy consumption.
Insulation in buildings can be improved by relatively low-cost retrofitting and renovation processes. The large saving potential of these processes also adds to the market value of the buildings.
However, these processes are not based on quantitative insulation data such as the U-value of the building structure. In order to achieve reliable and precise data on U-values at a specific location, it is necessary to empirically measure U-values of the building elements.
This article describes about the three different methods for assessing insulation quality of the buildings. Also, it explains in detail about the heat flux method and its application in a typical Swiss residential building constructed in 1948.
Methods for Assessing Insulation Quality
The following are the three different approaches used for measuring the insulation quality of building:
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Thermography (infrared imaging) – This method measures the thermal radiation of an object, and produces an image displaying areas of lower and higher radiation. It provides information on the overall quality of a building envelope and thermal bridges and sections with inhomogeneous insulation. The only drawback is that it does not produce quantitative data, such as U-value data, which is used to analyze the insulation quality.
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Multiple temperature measurements – This method is based on the temperature measurements within and outside the building element. The heat flux can be indirectly calculated by synchronizing these measurements such that the U-value can be calculated. Although it is possible to derive quantitative data, this method is difficult to use in most practical situations, as a minimum temperature difference of 10°C is required for reliable measurements, and this does not occur very often.
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Heat flux method – When there is a temperature difference existing between the opposite sides of the material, a heat flux is developed through the material. The heat flows from the warmer side to the colder side. Based on this principle, the heat flux method operates and measures the U-value of any building material in-situ. It is the only method that provides reliable quantitative information about the building envelope.
U-Value Measurement Using Heat Flux Method
This case study employs the gSKIN® U-Value Kit to carry out measurements. The measurements comply to ISO 9869, ASTM C1046 and ASTM C1155 standards.
The kit includes two temperature sensors, a heat flux sensor and a data logger with an adjustable measuring frequency. It facilitates automatic recording of heat flux through the building element and inside and outside temperatures. The software in the kit produces graphs of the heat flux and temperature measurements and the U-value of building element. Figure 1 shows the schematic of heat flux sensor, temperature sensors and data logger.
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Figure 1. Schematic of heat flux sensor, temperature sensors and data logger
The step by step procedure of the U-value measurement using the heat flux method is as follows:
- Install the heat flux sensor on the indoor surface such that the sensor is protected from solar radiation, convection and direct heating.
- Place two sensors at opposite sides of the wall at roughly the same position where the heat flux sensor is mounted indoors, and 2-10cm away from the wall for accurate U-value measurement.
- Record U-values at a frequency of 1 data point per 0.5 – 1h. The minimum measurement duration is 72h.
- Using gSKIN® U-Value Kit software, analyse the measurement data
- Calculate U-value using the mean values of the heat flux through the building element and ΔT
The building used in the case study was renovated on several occasions since its construction in 1948. The renovation includes changes to the ceiling in 1979, and the renovation of the ground floor in 1999. The insulation was renewed and adapted to the standards of the respective time periods.
Two different spots were selected for measurements - outer wall facing south-east (A) and the floor on the ground level (B). The spots were chosen such that undesirable influences from heaters, lateral convection and solar radiation are avoided. The sensors were mounted at a minimum distance of 1m from heating sources.
Results and Discussion
The measurement results of spot A is shown in the Figure 2. The outside temperature at this spot remained between -1 to 3°C, and the inside temperature fluctuated somewhat with the outside temperature, compensated for by the heating system. The largest heat flux is observed during the afternoons when the outside temperature falls and the inside temperature is heated based on the heating cycle.
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Figure 2. Graph showing temperature and heat flux of spot A
The heat flux and temperature measurements stayed stable in spot B for an extended period of time. It was observed that both temperatures start to decrease as the doors to the basement and ground floor are opened. The inside temperature, however, recovered to its initial level with several drops as the door is opened and closed again. Figure 3 shows the measurement results of spot B.
In order to measure U-value to ISO 9869 standard, the temperature difference should remain above 5°C throughout the measurement period. Therefore, in spot B, the temperature difference was not sufficient to measure an accurate U-value.
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Figure 3. Graph showing temperature and heat flux of spot B
The U-value of spot A is good compared to the standards at the time of construction. However, it is very poor when compared to today’s standards. Hence, the outer wall can be renovated to improve its insulation capability substantially.
Conclusion
In this case study, the measurements were recorded in two consecutive steps. Multiple U-Value Kits can also be used to make simultaneous measurements of multiple spots. The results prove that the U-value of a specific building element can be determined by measuring the insulation quality quantitatively. However, various spots should be measured to achieve a detailed overview of the building properties.
In this case, considering that the largest part of the outer wall gave the highest U-value, the overall insulation properties of the building are very poor. Hence, it can be concluded that retrofitting the building envelope to match modern insulation standards would lower the heating and cooling costs, and reduce the energy consumption of the building.
About greenTEG AG
greenTEG AG develops, manufactures and markets a new generation of heat flux and radiation sensors as well as thermoelectric generators. The company was founded as a spin-off from ETH Zurich in 2009. Since then greenTEG has won several awards for its technology and is currently supplying OEMs with customized sensor solutions and world-class researchers with measurement equipment for innovative experiments.
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This information has been sourced, reviewed and adapted from materials provided by greenTEG AG.
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