Insulation - Comparing Measured and Calculated U-Values

In this case study, the theoretical U-value based on a calculation model is compared with a U-value based on a heat flux measurement with greenTEG’s gSKIN sensor. An office wall at Technopark, a business and meeting center in Zürich, has been measured.

The building was built in 1990 according to the highest energy standards and thus reached exceptional thermal characteristics for that time. The aim of this case study is to find out to what extent the U-value is calculated when the construction data deviates from currently measured value.

Buildings have become more and more energy efficient over time and also the documentation of data on the construction details and materials has enhanced. For a number of buildings constructed in the last decades, this data is available which allows a precise theoretical calculation of the thermal properties such as the U-value.

The problem with these calculations is that the value in practice might be extremely different. Specific kinds of foam insulation, extensively applied in the first insulated buildings, tend to deteriorate because of infiltration of moisture or displacement of gas. Also, the U-value of newer buildings insulated with mineral wool could be worse in reality because of damage, poor installation methods, or increased moisture.

Assessment of the Building Envelope

A wall of an office within the Technopark has been evaluated. Since its erection in 1990, the building has not been renovated. The wall is a multi-layered structure (Figure 1) built up from inside to outside of concrete (180 mm; 1), vented cavity (40 mm; 3), mineral wool insulation (100 mm; 2), and a gypsum fiberboard (10 mm; 4).

Wall structure, including all layers.

Figure 1. Wall structure, including all layers.

Calculation Method

The theoretical U-value is calculated with the online model of u-wert.net using the materials and thicknesses of the wall as described above. This results in a theoretical U-value of 0.31 W/m2K.

Measurement Characteristics

The wall faces the West and is blocked from solar radiation by nearby buildings. The room in which the measurement is carried out is occupied during office hours. The measurement period has been roughly 72 hours to be in compliance with ISO 9869 and to decrease the effect of a daily temperature pattern within the room.

The heat flux sensor has been fixed to the inside of the wall. The inside temperature sensor is placed next to it, almost 3-4 cm from the wall. The outside temperature sensor is also fixed 3-4 cm away from the wall (and not influenced by direct sunlight).

Measurement set-up

Figure 2. Above: Measurement set-up; heat flux sensor with inside temperature sensor, Bottom right: outside temperature sensor.

Results of Measurement

Results of the U-value measurement are presented in the following figure. The graph includes the inside temperature, the heat flux, the U-value, and the outside temperature.

Results of U-value measurement

Figure 3. Results of U-value measurement in line with ISO 9869. (Report on basis of greenTEG Software v1.00.03, 2015)

In Figure 3, the heat flux clearly indicates a daily pattern because of the temperature changes within the room and the thermal capacity of the wall. Every morning, the heating system is turned on and the temperature in the room marginally rises, causing an increase in the measured heat flux. During the night, the heat flux is moderately low as a reversed effect. The measurement demonstrates that it is vital for measuring roughly 72 hours or another multiple of 24 hours (ISO conform) when evaluating a wall with a large thermal capacity and daily (inside) temperature patterns in order to get an accurate U-value.

Besides these fluctuations, it can be seen that every day a window was opened for a few minutes to ventilate the room and the sensor was placed close to this window. The standard deviation of the last 24 hours was just 2.65% which makes this measurement in line with ISO 9869. The U-value of this wall according to the heat flux measurement is 0.63 W/m2K.

Comparison

The table below compares the measured U-value, the calculated U-value, and the minimum required U-value for a Minergie refurbishment. Besides these U-values, it demonstrates the heating costs per m2 wall surface, the corresponding heat loss per m2, and the total heating costs associated with the wall. In order to calculate the heat losses, the average number of heat days of the last two years in the north of Switzerland are considered. A heating price of € 0,10/kWh is used for calculating the heating costs. A wall surface of 400 m2 is assumed for calculating the total heating costs associated with the walls for an average Technopark company.

  U-value
(W/m2K)
Heat loss1
(kWh/m2yr)
Heating costs
(€/m2yr)
Total heating costs
(€/year)
Calculation 0.31 25.69 ~ € 2.60 € 1,030.-
Measurement 0.63 53.20 ~ € 5.30 € 2,090.-
Minergie Ref. 0.25 20.72 ~ € 2.10 € 830.-

 

1Data degree days: Location: Zürich-Kloten; Base temperature: 19 °C; www.degreedays.net

Analysis

The in-situ measured U-value is more than twice as high as the calculated U-value. The insulation of this wall was a lot worse than could be expected from the construction data. This results in a difference in the energy costs of € 2,70 per m2 each year. If it is assumed that the measured value is representative for all walls of the office, the energy bill of the office tenant is roughly €1000 - greater than it is supposed to be. A Minergie refurbishment would, in this case, save the office tenant around €1200 - every year in heating costs.

In order to assess the overall insulation quality of all offices, it should be examined if the measured spot is indeed representative of the entire wall surface. The homogeneousness of the wall can either be determined by additional heat flux measurements or by a thermographic survey. An insulation company should then be capable of evaluating if improving or replacing the insulation would be possible and economically viable.

It is hard to define why the U-value is so much greater than expected. The insulation quality of mineral glass wool is barely influenced by aging effects. However, an increase of the moisture content could lead to a major deterioration of the thermal performance. Another explanation could be that a few mistakes have been made in the initial installation of the insulation.

Lastly, the data employed in the calculations is based on hand-drawings from the facility manager of the Technopark. The material stated by the facility manager had to be matched to the materials listed in the U.wert.net database. During these processes, inaccuracies or errors might also have occurred. An in-depth analysis is needed to find out the exact reason for the higher than estimated U-value.

Conclusion

A successful measurement has been performed in line with the ISO 9869. The outcome of the measurement can thus be considered as reliable. Even though material characteristics of the wall were available, the measured in-situ value appeared to be twice as high as the calculated value. It specifies that by just depending on building characteristics, it could be possible for one to get a wrong understanding of the thermal performance of a building and the heating costs the office tenant is experiencing. Additional investigations are needed for reviewing if a refurbishment could be a motivating option for the Technopark.

greenTEG AG.

This information has been sourced, reviewed and adapted from materials provided by greenTEG AG.

For more information on this source, please visit greenTEG AG.

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