FAQ: U-Value Measurement

Norms and requirements:
What is the ISO 9869 norm?

The ISO norm is established to secure the reliability of a heat flux measurement for investigating the thermal transmission properties of plain building components . If a measurement is conducted in line with ISO 9869, the measurement results can be considered as accurate.

What are the ISO 9869 requirements?

About the measurement itself ISO 9869 states that a measurement of a wall should last at least  72 hours (+ integer multiple of 24h) and a maximum difference of 5% is tolerated between the last value and the value 24 hours before. Moreover, the R-value (adapted from the U-value) obtained during the first 2/3 of the overall measurement period does not deviate more than 5% from the values obtained during the last 2/3 of the overall measurement period. For further guidance please refer to our instruction manual or the readme of our software.

How is the U-Value calculated by greenTEG’s U-Value KIT software?

The U-value is calculated according to the moving average method as described in ISO 9869. Therefore, the measured heat flow rate is divided by the temperature difference for all data points. The average of these calculations (taken over a long time period) represents the actual U-value of a building envelope.

Why do I need to measure a wall at least 72h according to ISO 9869?

A wall has a thermal mass which enables it to store heat. Due to temperature fluctuations inside and outside the building, the amount of heat stored in the wall is fluctuating too. The influence of the thermal mass of the building envelope on the outcome of the U-value calculations is getting smaller over time as the average of more data points is taken. Under the right circumstances the calculated average is reaching the actual U-value of the building envelope.

Is it possible to get reliable measurement results for a wall in less than 72 hours?

Yes. However, the inside temperature should be kept as constant as possible during the measurement to diminish the effect of the thermal capacity of the wall. Moreover,  the measurement should preferably last a multiple of 24 hours.

Can I measure glass (windows) as well?

Yes, however, since glass has different thermal characteristics, the measurement procedure is a bit different. According to ISO 9869 three subsequent night measurements (e.g. of 7 hours) should be conducted. An example of such an assessment can be found in our application note.

Can I measure the whole year through?

When the difference between the inside temperature and the outside temperature is too small, the measurement results become unreliable. Therefore, we advise a minimum temperature difference of roughly 5° C degrees over the whole measurement time. This means usually that an assessment of a wall cannot be made during the summer months. As glass measurements are conducted over night, these measurement can usually be conducted during the whole year.

Can I measure during sunny days?

Yes, if certain aspects are taken into account, solar radiation will have no influence. You should try to avoid measuring the south wall of a building since the thermal storage capability of walls should not be underestimated.
Furthermore, always cover the temperature sensor to block any direct sunlight if necessary. In order to protect the sensor from the influences of the solar radiation you can cover it with aluminium foil.

Does the measured, dynamic U-value include the common heat transfer coefficients for inside and outside surface resistances?

Yes, it does. The device measures the u-Value of the whole building element. Therefore the λ-values of the single materials can’t be assessed.

 U-value measurement routine & analytics:

Does a measurement has any impact on the daily routines of building occupants?

The measurement set-up is rather small. If the sensor is located properly, it won’t bother any building occupant. To increase the accuracy of the obtained data building occupants might try to keep the inside temperature as constant as possible (e.g. by not switching the radiator off during the nights or limit the opening of the window).

I measured more than 72 hours but the measurement is not in line with ISO 9869. Why is this?

This means that the average U-value still fluctuates significantly and a stable value has not been reached yet. Unstable external conditions or a very small temperature difference (< 5°C) could be reasons why it takes longer to reach the actual U-value of the building envelope. You could decide to extend the measurement or start a new measurement when conditions are better.

My measurement set-up is not fully in line with ISO 9869, is the measured U-value unreliable?

Such a measurement could still be rather reliable. However, it requires some more in-depth interpretations of the measurement results. The length of the measurement and the volatility of the data should be considered. If the overall standard deviation and the deviation during the last 24 hours are not too high, the measurement results are likely to give a good representation of the measured results.

The measurement outcome seems to be unrealistic but in line with ISO 9869. Why?

First, you should make sure that the measurement set-up meets all requirements as described in our instruction manual. A reason for a unrealistic good U-value might be that the sensor does not have a good contact with the wall surface. If all criteria were taken into account, the U-value can be considered as correct. Values can be different than expected due to the heterogeneity of the walls or thermal bridges. Moreover, walls in historic buildings tend to have a better U-value than is expected by most models.

Is the software also available for Mac OS X?

Currently the software is only available for computer working with an operating system from Windows. However it is possible to run the software on a mac over a virtual machine with windows. We tested this with VMware Fusion with Windows 10.

How do I import the .csv-file into excel without having formatting issues (some numbers are shown as dates)?

Firstly, you have to open a new excel sheet and get the csv as an import from text. Now you set the delimiter semicolon (“;”).
Press „Next“ and select the format „Standard“. Additionally, you need to change the decimal seperator to dot („.“), the thousands seperator to nothing (“ „) and then press „Finish“.