Pressurization time for measuring very airtight buildings and buildings with flexible airtight layers

Purpose of the work 
When dealing with very airtight buildings and buildings with flexible airtight roofing felts it might take several minutes for the differential pressure and air flow to level out after having changed the fan speed. Taking premature readings of the values causes measuring errors. The lecture provides pointers for test teams to avoid such measuring errors. 

Method of approach 
The research project "Patience avoids measurement errors" derived algorithms for calculating the pressure curve. Differential pressure variations measured at the building envelope were compared against calculations. The project analyzed the dependence on various parameters such as the net air exchange rate at 50 Pa, the slope of the fan characteristic or the size of a bulge in flexible roofing felt. It was funded by the Federal Institute for Research on Building, Urban Affairs and Spatial Research on behalf of the Federal Ministry for Housing, Urban Development, using funds granted by the research initiative "Zukunft Bau" (Future Buildings). 

Content of the contribution 
A few examples of the key results of the research project: 

• Reasons why pressure changes sometimes take time to take effect after having adjusted the fan speed.
• The fan characteristic's gradient impact on latency and potential measuring errors.
• How to estimate additional latencies caused by flexible roofing felt.

Results and assessment of their significance 
After having adjusted the fan speed, operators will have to wait for a certain amount of time until they can read the results measured at each data point. This seven-second rule had already been described at the Buildair Symposium 2021: The numerator 7 divided by the denominator of the net air exchange rate at 50 Pa will produce the waiting time in seconds. When handling flexible roofing felt, the waiting time for “filling” or “emptying” the bulge in the felt is added: The number 20 is multiplied by the volume change between 0 Pa and 50 Pa stated in parts per thousand and divided by the numerical value of the net air exchange rate at 50 Pa to provide the additional waiting time in seconds. Adding up these two values yields the total latency. 

Conclusions 
The most important lesson learned in this research project for testing in the field is the formula for estimating the additional waiting time for buildings with flexible airtight roofing felts. Time will show whether the formulas will work in the field and to which extent it will be possible to estimate any changes in the position of the airtight roofing felt. The impact of the temperature differential (inside/outside) remains an open question. 

For further information please contact Joachim Zeller at: joachim.zeller@t-online.de