Evaluation of n-situ easurement ethods for ir ermeability of indows Christoph Geyer, Andreas Müller, Barbara Wehle, Martin Greiner, Pascal Urech Research and Development; Architecture, Wood and Civil Engineering Bern University of Applied Sciences, Switzerland, christoph.geyer@bfh.ch Abstract Switzerland possesses a large variety of historical buildings. Numerous of these buildings are located on the side of busy streets resulting in high traffic noise volumes. Therefore, improved sound insulation of these buildings' windows is necessary. Furthermore, regulations exist which demand a reduction of the thermal energy losses of these windows. To fulfill these requirements, the façades of these buildings have to be renovated. To prevent the loss of the historical essence of these windows and with it their cultural heritage, it is necessary to understand the parameters of historical windows. Beside the heat transmission coefficient, air permeability is one of the important parameters of historical windows, because it influences both, the sound insulation and the heat loss. To determine the potential for improvement of historical constructions, it is necessary to measure the air permeability of the windows before and after the refurbishment on-site. Today two in-situ measurement methods for air permeability of windows are available: the a-wert MessSystem and the Minneapolis Micro Leakage Meter (MLM) method, both distributed by the BlowerDoor GmbH in Germany. Questions exist as to whether the measurement values of the air permeability in-situ are comparable with measurement values determined in air permeability test suites according to EN 1026 in the laboratory. To evaluate the existing two in-situ measurement devices in the laboratory of Bern University of Applied Sciences, the air permeability of two windows were measured three times: with the two in-situ measurement methods and in the air permeability test stand according to EN 1026. Then, using statistical analysis, the measurement uncertainties were calculated. Furthermore the deviations between the measurement values of the in-situ methods on the one side and the values of the air permeability test suite were examined. The measurement values and the results of the statistical analysis are presented for both windows. Keywords: Air Permeability, historical Windows, In-situ Measurement 1. Introduction Most of the historical windows are constructed with wooden frames. The joints in these windows are without elastical sealing. Therefor one expects high air permeability in these windows. To quantify the numbers of the air permeability it is necessary to measure the air permeability in-situ. 2. Measurement Methods Today there are two in-situ measurement methods available: the a-wert MessSystem and the Minneapolis Micro Leakage Meter (MLM) method, both distributed by the BlowerDoor GmbH in Germany. 2.1 a-wert MessSystem One side of the specimen window is covered with a thin plastic foil for this measurement. The measurement system consists of a ventilator, which produces a negative pressure in the space between the specimen window and the foil. In this foil an aperture with a known diameter is mounted.
The ventilator produces varying pressure differences between the room and the outside during the measurements. The measurement result is the volume flow rate,, as a function of the pressure difference at the window,. For comparability reasons of the measurement results for all volume flow rates are calculated for standard conditions (pressure p 0 = 101 300 Pa and an air temperature of ). Figure 1 shows a sketch of the principal components of the a-wert MessSystem. Outside 2 Room 1 3 4 p ex p cav p in Figure 1: Sketch of principal of the measurement method: p ex denotes the external pressure outside, p in the internal pressure in the room and p cav the pressure in the cavity. The test specimen window is indicated with, the plastic foil with, the aperture in the plastic foil with, the ventilator with. The pressure difference over the construction element is given by Formula 1 The pressure difference at the hole in the aperture of the foil is calculated as follows Formula 2 The volume flow rate is calculated as follows [1] Formula 3 where volume flow rate in m 3 /h c d = 0,62 air resistance for sharp edge holes A ap area of the hole in the aperture in cm 2 Δp ap pressure difference in Pa ρ i,0 air density at standard conditions in kg/m 3 ADVANCED BUILDING SKINS 851
2.2 Micro Leakage Meter (MLM) A Micro Leakage Meter measurement set consists of the following devices: a plastic foil to cover the external side of the test specimen window, a ventilator to press air in the cavity between the foil and the window, a plastic tube with a volume flow meter to measure the volume flow rate. 2 3 1 4 p cav p ex p in Figure 2: Sketch of principal of the MLM measurement set: p ex denotes the external pressure outside, p in the internal pressure in the room and p cav the pressure in the cavity. The test specimen window is indicated with, the plastic foil with, the volume flow rate meter, MLM, with and the ventilator with. With this test method it is possible to measure the air permeability of the test window with an over-pressure in the cavity between the plastic foil and the window. The volume flow rate through the volume flow rate meter is given by: with C permeability coefficient at standard conditions n permeability exponent at standard conditions pressure difference in the volume flow rate meter Formula 4 There are different apertures which can be mounted in the MLM. For each aperture calibration values for the permeability coefficient and the permeability exponent at standard conditions are delivered by the manufacturer. 2.3 Test device for Air Permeability of windows according to EN 1026 In the laboratory of Bern University of Applied Sciences (BUAS) in Biel is a test device to measure the air permeability of windows and doors according to EN 1026 [2]. In this test device, the test specimen window is mounted on the front of the test chamber. By producing a negative and over-pressure compared with the barometric pressure in the laboratory and by measuring the volume air-flow rates to keep the air pressure in
the test chamber constant, the air permeability of the window is measured. Figure 2 shows a photograph of the test device. Figure 3: Photograph of the test device in the laboratory of BUAS in Biel to measure the air permeability of windows and doors. 3. Measurements All three measurement methods determine the value of the volume air flow rate,, through the test element at discrete values of the pressure difference,, at the element. A regression function is fitted at these pairs of measurement values. As a result of this statistical analysis the air permeability of a construction element is defined by its permeability coefficient C and its permeability exponent n. The dependency of the volume air-flow rate is given by Formula 5 with C the permeability coefficient in m 3 /(h Pa n ) n the permeability exponent To determine the values of these numbers the pairs of measurement values are written in a diagram with logarithmic scales. Then a regression function is fitted to these values using the method of least squares. As a result of this statistical analysis, the permeability coefficient C and the permeability exponent n are calculated from the y-intercept respectively from the slope of the regression straight line. The joint permeability coefficient, a F, of the windows is defined as Formula 6 Where l F is the sum of the length of all joints of the test specimen window. 3.1 Results The air permeability of two different windows were tested. The frames of both windows are made of plastic. They are equipped with two casements. The air permeability of the first window is classified as class 2 according the European standard EN 12207 [3], the second window is classified to class 4. ADVANCED BUILDING SKINS 853
3.1.1 Window classified in permeability class 2 The measurement results for the joint permeability coefficient and the permeability exponent of the first window, determined with both in-situ measurement methods and with the air permeability test device according EN 1026 are summarized in Table 1. The sum of the lengths of the window joints is 7.734 m. Test method Scope a F da F n dn N m 3 /(h m Pa n ) 95% 95% a-wert MessSystem 8 0,26 ±0,028 0.61 ±0,031 MLM 8 0,260 ±0,0061 0,610 ±0,0061 (over-pressure) 8 0,255 ±0,0045 0,59 ±0,018 (negative pressure) 8 0,257 ±0.0063 0.59 ±0,025 Table 1: Summary of the test results for the permeability coefficient, a F, an the air permeability exponent, n, of the first window, determined by using the two in-situ measurement methods and the air permeability test device. Uncertainties of the permeability coefficient, da F, and of the permeability exponent, dn, are given as 95 % confidence interval 3.1.2 Window classified in permeability class 4 The measurement results for the joint permeability coefficient and the permeability exponent of the second window, determined with both in-situ measurement methods and with the air permeability test device according EN 1026 are summarized in Table 2. The sum of the lengths of the window joints is 8.376 m. Test method Scope a F da F n dn N m 3 /(h*m*pa n ) 95% 95% a-wert MessSystem 1 8 0.028 ±0.0046 0.54 ±0.046 a-wert Messsystem 2 8 0.031 ±0.0023 0.52 ±0.022 MLM 1 8 0.031 ±0.0020 0.62 ±0.018 MLM 2 8 0.034 ±0.0013 0.62 ±0.011 MLM 3 8 0.0294 ±0.00075 0.681 ±0.0072 MLM 4 8 0,0292 ±0.0011 0.69 ±0.011 (over-pressure) 8 0.024 ±0.0014 0.72 ±0.056 (negative pressure) 8 0.030 ±0.0018 0.62 ±0.059 Table 2: Summary of the test results for the permeability coefficient, a F, an the air permeability exponent, n, of the second window, determined by using the two in-situ measurement methods and the air permeability test device. Uncertainties of the permeability coefficient, da F, and of the permeability exponent, dn, are given as 95 % confidence interval.
4. Discussion To compare the measurement results of the different methods we calculate the volume flow rate at a pressure difference of 50 Pa at the test specimen window for both in-situ measurement methods and for the permeability test device, using the permeability coefficient C and the permeability exponent n and the following formula: Formula 7 with C the permeability coefficient in m 3 /(h Pa n ) C 0 = 1 m 3 /(h Pa n ) the reference value for the permeability coefficient n the permeability exponent the absolute value of the pressure difference at the test specimen window p 0 = 1 Pa the reference value of the pressure Table 3 summarizes the results for the volume flow rates for the three measurement methods and the two windows. Volume flow rates at pressure difference of 50 Pa Test method Window class 2 Window class 4 m 3 /h m 3 /h a-wert MessSystem 21.1 ± 0.46 1.93 ± 0.062 MLM 21.8 ± 0.27 MLM 1.1 2.93 ± 0.039 MLM 1.2 3.20 ± 0.024 (negative pressure) 20.1 ± 0.35 2.9 ± 0.12 (over-pressure) 19.8 ± 0.26 3.4 ± 0.13 MLM 2.1 3.54 ± 0.018 MLM 2.2 3.59 ± 0.028 a-wert MessSystem 1.96 ± 0.031 means not significant different on a 95 % uncertainty level from the measurement value of the air permeability device means significant different on a 95 % uncertainty level from the measurement value of the air permeability device Table 3: Summary oft the volume flow rate of the different measurement methods at a pressure difference at the window of 50 Pa. The result of the significant tests compared to the measurement value of the air permeability test device are given by the symbol which indicates not significant different and which indicates significant different. The errors are given as 1 sigma levels. To decide if the measurement results of the different measurement methods match with the results of the air permeability test device in the laboratory, we performed significance tests. The tests were done at the 95 % significance level. The results of these significance tests are marked in Table 3 with a green hook which means the measurement value of the in-situ method is not significantly different from the measurement value of the air permeability test device and a red cross stands for a significant difference at the 95 % of significance level. ADVANCED BUILDING SKINS 855
For both in-situ measurement methods it couldn t be shown at a 95 % uncertainty level, that all measurement values match the measurement values of the air permeability test device in the laboratory. We weren t able to identify the reasons for this difference, yet. More work has to be done by performing additional comparison measurements with the different methods to identify the reasons for this difference in order to increase the comparability of the measurement results of the in-situ methods with measurement results from the laboratory. 5. Acknowledgement We like to thank the Swiss Stiftung zur Förderung der Denkmalpflege for the financial support of the project. 6. References [1] W. Walther, Messung kleiner Volumenströme mit Hilfe von Lochblenden, Tagungsreader des 8. BlowerDoor Symposiums, Mai 2003. [2] SN EN 1026 Windows and Doors Air Permeability Test Method, Zuerich, June 2000 [3] EN 12207 Windows and Doors - Air permeability. Classification, Zuerich, January 2000