1 PERM 07 Acoustic-permeability

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Building and Environment 57 (2012) 18e27

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Building and Environment journal homepage: www.elsevier.com/locate/buildenv

Acoustic approach for building air permeability estimation Vlad Iordache, Tiberiu Catalina* CAMBI Research Center, Technical University of Civil Engineering of Bucharest, Faculty of Building Services and Equipment, Bd. Pache Protopopescu 66, 021414 Bucharest, Romania

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 February 2012 Received in revised form 4 April 2012 Accepted 6 April 2012

Air infiltration represents an essential parameter for the building and for the HVAC design and thus its accurate estimation is very important. The classic approaches to estimate the leakage air flow for an existing building present a series of disadvantages. The mathematical prediction models present errors up to 100%, while the currently used experimental measuring approach is expensive and weather dependent. In this paper we analyzed the leakage air flow using a different approach: an acoustic method for building air permeability measurement. We aim to determine if the air and noise transfer phenomena through window joints are correlated and what is the relation between the two transfer phenomena. In order to analyze this relation we measured both the infiltration air flow transfer and the airborne noise transfer through window joints for the same building façade. Different joinery degradation cases were simulated by different fixed positions of the joinery. For each case, two measurements were performed: the airtightness measurements in order to determine the air change rate and the airborne noise transfer in order to determine the sound transmission loss. Finally, we found that the air change rate is inverse correlated to the sound transmission loss; the higher the sound transmission loss, the smaller the air infiltration rate. This acoustic estimation for the building air permeability presents multiple advantages compared to the two classic approaches: good precision because it is an experimental approach, no expensive measurement devices, free of climate changes and it also represents a fast tool for evaluating building air permeability. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Permeability law Airborne noise Facade transfer Experimental measurements

1. Introduction The air permeability of the envelope represents an important characteristic of the building which is significantly influencing different parameters of the indoor environment [1]: the heating load, the strategies of the ventilation system [2], the degree of the indoor air pollution [3,4], the indoor acoustic comfort [5] and the energy performance of the building [6]. Therefore, predicting the airtightness is very important for both the design and the rehabilitation stages of a building. Today, in the context of the thermal rehabilitation, in order to reduce energy consumptions, it appears a major need to acknowledge the processes of permeability and the leakage air flow corresponding to a building with damaged joinery. The leakage air flow rate might be evaluated using predictive models determined from experimental databases. In the specific literature, there are several databases for many countries such as:

* Corresponding author. Tel.: þ40 21 252 46 20, þ40 76 391 54 61; fax: þ40 21 252 68 80. E-mail addresses: [email protected], tiberiu.[email protected] (T. Catalina). 0360-1323/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2012.04.008

the United States [7], Greece [8], Finland [9], Spain [10], France [11,12], Italy [13], Australia [14], Canada [15] which are extensively used to deduce mathematical models for the infiltrated air change rate for different types of buildings. The air infiltration models can be classified into two major categories: singlezone models such as the Lawrence Berkeley Laboratory (LBL) model [16,17] or the AIM-2 model [18] and multi-zone models such as COMIS [19] and CONTAM [20]. Previous studies [21] present a mean error for the single zone LBL model of 26e46%, reaching up to 159% while the AIM-2 single zone model [22] presents mean errors around 19% reaching up to 87%. Similarly, high errors are obtained for multi-zone models [23]. Such errors, up to 100% in some cases, are unacceptable for the infiltration models given their importance in different studies. For example in the field of building energy performance, previous studies [24,25] proved that the air flow leakage may represent well over 50% of the heat consumption. Thus, the error of the air infiltration rate is further amplified and leads to high errors in the calculations of the heat consumption and wrong rehabilitation measures. The experimental evaluation of the infiltration rate replaces the lack of accuracy of the current prediction models. The fan pressurization method for measuring the transfer of air permeability

V. Iordache, T. Catalina / Building and Environment 57 (2012) 18e27

of buildings [26] is more often used because it gives a characterization of the building in various states of indoor high-pressure/ low-pressure. The Blower Door system [3,18] built specifically for this type of measurements is easily exploitable and can be used for areas such as rooms, apartments or villas. The measurement protocol was further adapted to suit experimental measurements for higher volume buildings [27]. Thus, the experimental approach may reach errors under 5% while measuring the entire building, or under 15% while measuring one apartment of a building. This experimental acknowledgment process of the building permeability presents a higher precision, but it also has some disadvantages, such as: the high cost of the blower door itself, the climate sensitivity, the technical/scientific background of the user and the fact that it’s a relatively time consuming technique. The thermal inspections of the building are still based upon visual speculations (visible joints, light penetration through joints, damaged joinery) in order to approximate the infiltration rate. But these speculations lead to important errors due to both the lack of experience of the building thermal inspector performing the expertise and the variation of the joints length characteristics. Even technical staff with experience in permeability measuring cannot evaluate the permeability of a building based only on the visible aspects. In this study we wish to bring forward an alternative way to determine the permeability of a building façade using an analysis on the relation between the air infiltration through joints and the outdoor/indoor sound transmission. If such a relation exists, then it means we can determine the air infiltration rate by skipping over the expensive permeability measurements. This association between permeability and sound transmission was studied back in the eighties by means of laboratory experiments [28e30]. However, no useful correlation law was determined! In this study we will follow a different approach (experiments on real buildings) to further detail this phenomenon, in order to find a prediction model for the air leakage flow as a function of the sound attenuation loss. The aim of this study is to determine, for real buildings, the existence of a relationship between the air infiltration rate and the outdoor/indoor airborne noise transmission. We will investigate if there is any correlation between the two transfer phenomena (air and noise transfer) for real buildings, and, if there is the one, which is the mathematical relation that tides up the two phenomena? What is the form of this mathematical relation, is it a linear or not? Beside the scientific importance of this study, we are also attracted by its practical implications. If there is indeed a correlation between the two phenomena then the expensive experimental stand for permeability measurements, may be

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successfully replaced in certain cases by much cheaper measuring devices. Furthermore, the acoustic measurement of the permeability would have the advantage of being less conditioned by the weather conditions compared to the present blower door stand. As mentioned in EN 13829 [26] and [31] the needed conditions for measurement validation are: wind speed should be less than 2 m/s and standard deviation of the outdooreindoor pressure difference should be less than 2 Pa. This would be a simpler and less expensive experimental device for fast technical expertise that could be included in other studies like: building certification, indoor air quality, HVAC design for older existing buildings. An experimental approach is employed in order to determine the existence and the form of this relation between the two phenomena. This paper presents the experimental study of the air and noise transfer through the same façade followed by the correlation between the two phenomena. 2. Dual measurements experiment The measurement campaign was organized in order to point out the relation between the air and the noise transfer through the building façade and especially through window joints. Therefore, two types of experiments were conducted: facade airtightness measurements and sound level measurements, in accordance with the European norms EN13829 [26] and EN 717-1 [32], respectively. Both experiments were carried out in a test room situated in the building of the Faculty of Building Services, University of Civil Engineering of Bucharest. The room position in the building it is a convenient one for the experimental study as it is situated at the ground level (þ2.0 m) and it is surrounded on both sides by other identical rooms. It must be mentioned that the tested space facade is facing an inner courtyard, so that the outdoor noise sources are limited (Fig. 1). Moreover, the courtyard is surrounded by the building, which limits the impact of the wind pressure on the measurements. Consequently, the chosen test room is considered to be almost ideal for this kind of research study. The test room is a classroom of 3.95 m height and has a volume of 195 m3. This chamber is well illuminated through three identical large windows (2.1 m  0.7 m). The windows have a double-pane wooden frame and they are not air sealed. Each of the two glass layers thickness is of 3 mm. The floor surface material is made of wood while the rest of the walls and ceiling are covered by a thin concrete-cement layer. Also, inside the test room there are ten wooden chairs and tables. The permeability measurements are carried out by means of a classic blower door system consisting of the following equipment

Fig. 1. Location of the experimental site (a). Position of the experimental site, (b). Inner courtyard of the building.

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Fig. 2. Geometrical characteristics of the test room.

and measurement devices: extendable false door, radial fan with variable speed, variable voltage device, double differential micromanometer DG700, computer and software “Tectite”. We used this system in order to evaluate the air flow crossing the building façade and its variation as a function of the outdoor/indoor pressure difference. This system was mounted on the door of the test room and the measurements were controlled by the “Tectite” software. These permeability measurements are characteristic of the entire room. The method used for measuring the permeability of a room involves that the analyzed space to be put in over or low-pressure compared to outdoor, by means of the variable speed fan. Various pressure points, between 70 Pa and 20 Pa, with a 5 Pa step, were analyzed. For each pressure point two parameters were recorded simultaneously: the indooreoutdoor pressure difference Dp (Pa) and the air volumetric flow rate Q (m3/h). These values allow us to determine the two coefficients C and n of the permeability law (ex. Q ¼ C*Dpn). We acknowledged a couple of permeability laws (high/ low-pressure) for each experimental case. As the Fan Pressurization technique cannot directly measure the volumetric flow rates at low values of the pressure difference, it is necessary to extrapolate the measurable behavior of the analyzed façade for these values of the pressure differences. There are two main error sources associated to this technique: measurement errors and model specification errors. As proposed by Sherman and Palmiter [33] the uncertainty related to the volumetric flow rate estimation through the fan pressurization method could be expressed by:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d2 Qprecision þ d2 Qbias d2 Qmodel dQ ¼ þ measurement modelization

commercial one. If the first device allowed us to perform more precise measurements and discover more specific details on the noise level spectrum or room reverberation time, the second one was used to compare the results in order to validate its practical use. Because the experimentation is supposed to imply the use of two sonometers at the same time outdoor and indoor measurements, most of the engineers will prefer two commercial and less expensive versions of sonometers. The sound pressure level was recorded by means of Sonometer “2250 Investigator” from Bruel & Kjaer, which is a class 1 precision device. Several sonometer programs were used for this device (BZ 7223 - Frequency Analysis Software andBZ 7227- Reverberation Time Software) in order to record the sound pressure level for different frequencies or to estimate the reverberation time. Generally, for a building practical study it is commonly used the spectrum 125 Hze4000 Hz, but a more detailed analysis goes within the range of 63 Hze8000 Hz. For the A-weighted sound pressure level we also used two class 2 sonometers DT 8852 with a precision of 1.4 dB, which is an ideal instrument for noise monitoring in appartments, schools, office building and traffic areas due to its simplicity and fast operation. The signals data were collected from all sonometers by means of computer storage cards and later analyzed by using specialized data analysis software. The acoustic measurements were conducted on both sides of each window (see Figs. 2 and 3) with the purpose to analyze the

(1)

This relation has a confidence level of 95%. We used the method proposed by Sherman and Palmiter [33] in order to estimate an uncertainty range of the volumetric flow rate values obtained from the permeability laws. Initially only five pressure measurement points were used to estimate the permeability law. This choice conducted to an uncertainty range between 10% and 13%. By increasing the number of pressure points to 10, we managed to obtain a maximum uncertainty of 5.5%. Sound level measurements were performed in order to determine the degree of sound insulation of the room test facade. Measurements were carried out with and without weighting filter A using two types of sonometers. As it was mentioned in the article’s introduction, besides the scientific interest of this study, we were also concerned about the practical use of this method to evaluate the façade air change rate. This concern leads us to the use of two types of sonometers: a professional one and a common

Fig. 3. Acoustic measurement points measurement point.

C

outdoor measurement point;

indoor

V. Iordache, T. Catalina / Building and Environment 57 (2012) 18e27

widow characteristics. The measurements points are placed in the widow axis at 1 m height form the analyzed room floor. A distance of 0.5 m between the window and the outdoor measurement points was considered a good option to record the noise on the building façade, while inside the test room the measurements were carried out at 1 m. An external noise source was installed 3.5 m in front of the façade, so that all windows would be influenced similarly by the noise source (see Fig. 3). The type of external noise source is not relevant and less important because we are not interested in obtaining a certain outdoor noise level, but we are concerned about the difference between the indoor and outdoor sound pressure levels. However there are certain constraints that must be applied: the noise source should not be linear; its sound pressure level should overpass the background noise with at least 15 dB. In our case the noise source produced a SPL of more than 100 dB (at least 50 dB higher than the background noise). During the experiments the background noise might disturb the sound pressure level measurements, thus one should assure a low background noise during the experimental sequence [34,35]. In order to avoid undesired sources of noise, like occupant’s indoor activities or outdoor car circulation, the sound measurements were done during weekend and night periods, when outdoor noise is not influencing the results. Besides the actual analysis of the geometrical dimensions of the test room and of the site plan, a detailed look was directed toward the sound absorption materials found in the room. This estimation of the existing sound and vibration environment was necessary in order to predict correctly the sound attenuation. As previously stated, the background noise was not sufficiently strong to perturb the measurements and the data collected were reliable and meaningful for the research study. Before and after each series of measurements a sound calibrator with an accuracy of 0.3 dB (class 1 according to EN 60942 [36]) was applied to the sonometer in order to check the calibration of the entire measuring system at one or more frequencies over the frequency range of interest, which in our case it was of 125 Hz O 4000 Hz. In the vicinity of the sonometer, no obstacles disturbed the sound field. The experimental campaign was divided in two parts: permeability and sound measurements. The first analyzed case also called Reference Case (C0) represents the façade with an almost perfect air sealing. This was done using foam and adhesive tape all along the windows joints (Fig. 2). The second case considered (C1) was a medium sealed façade where a part of the previous adhesive tape was removed. The actual real case in the test room also named C2s is represented by the window frame in their original state without any air sealing measures. Cases C3 and C4 describe a façade with larger air infiltrations, which were manually introduced by larger window joints. A test was also done on a single wooden frame window and it will be mentioned of as C5. For each case both types of measurements were carried out: by using the blower door system the test room was depressurized and pressurized in order to obtain the room permeability laws and secondly, the sound level measurements were taken at three measurement points inside and outside the test room (see Fig. 3). The measurement campaign was organized in order to underline the relation between the air and the noise transfer through the building façade and especially through window joints. However, the sound wave transfer though window joints represents only one transfer path for the sound wave, the other transfer paths being through: the glazing, the façade wall, side walls and adjacent rooms. In order to unveil only the transfer through joints, we proposed a measurement protocol that would maintain the other transfer paths of the sound wave constant. Thus, we carried out experiments

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on the same room and the same façade, the only difference between the analyzed cases being the airtightness of the window. Next paragraphs will present the measurement data and their analysis so that the correlation study of the two phenomena can be achieved. 3. Permeability measurements results In this paragraph we will present the results of the permeability measurements for each of the studied cases and the further treatment in order to determine the façade permeability for each case. Room permeability measurements were carried out for both underpressure and overpressure scenarios. The measured values present, for each case, very close permeability curves for the two scenarios (Fig. 4). This is the expected result, given the widow type: double window where the indoor panel opens inside and the outdoor one opens outside. A visual comparison among the five different types of façade shows that the smallest values were recorder for the Reference Case (1200 m3/h at 100 Pa) while the highest values were recorder for Case 4 (2800 m3/h at 100 Pa). The permeability law of the room represents the average curve between the under-pressure and the overpressure curves, according to EN 13829 [26]. The average permeability curves of the room for each case were calculated (Fig. 4) and further used to determine the façade permeability curves. The façade permeability law of the non-sealed windows represents the difference between the room permeability (average value) with non-sealed windows and the room permeability with sealed windows, which is the Reference Case [27,37]. Thus, the façade permeability was calculated for cases 1e4 (Fig. 5). The comparison among the four different types of façade shows that the smallest values were obtained for the Reference Case (200 m3/h at 100 Pa) while the highest values was obtained for Case 4 (1650 m3/h at 100 Pa). While the façade permeability law show the variation of the air flow infiltration through window joints, the permeability is the air flow infiltration in accordance with building characteristics [26,38]. Further, we used the air change rate (ACH) as a permeability parameter, due to its calculation as the infiltration air flow divided by the room volume [27]. The most common ACH value is determined at 50 Pa [3] as this is a representative value for the pressure difference. However, during the operation, the building is exposed to an average value of 4 Pa pressure difference; this value corresponds to real buildings exposure in Romania. Thus the air change rate for real buildings exposure is calculated as the air flow at 4 Pa divided by the room volume. The transformation coefficient between the two air flows at 50 Pa and 4 Pa is calculated as F ¼ ACH50Pa/ACH4Pa. Fig. 6 presents the ACH calculation of the two pressure difference values, and the transformation factor. One can notice that the four cases are different according to the ACH4Pa. Case 4 is characterized by the highest value of air change rate while Case 1 is characterized by the smallest value. We can add to these four points of the hierarchy a fifth one, corresponding to an airtight façade, that is Case 0. Such airtight façades are characterized by an air change rate around 0e0.2 (1/h) [39e41]. Further in the paper we will present the hierarchy of the Cases 0e4 according to the noise transfer phenomenon, and finally correlate the two phenomena. 4. Acoustic measurements results The purpose of this paragraph is to present the acoustic measurements and their analysis in order to determine if there is an acoustic hierarchy of different cases of the façade airtightness.

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Fig. 4. Room permeability laws.

important because typical outdoor sound sources may contain significant low-frequency sound energy. The first acoustic experiment consisted in measuring the sound pressure level (SPL) outdoor and indoor, while the difference DLp (dB) between the two SPL is further calculated. Fig. 7 illustrates the data of the three measurement points (O1, O2, O3 and I1, I2, I3) corresponding to outdoor and indoor sound pressure levels for Case 2. It can be noticed that on the 1000 Hz frequency the SPL reached its maximum value. The outdoor SPL has basically the same fluctuation for all the three measurements with a maximum value of 97.3 dB and a minimum one between 45.5 dB at 125 Hz. As for the maximum value of the difference outdooreindoor SPL, it is noticed that this value is reached on 1000 Hz frequency with a value of

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Windows are the critical weak component of the sound insulation of the building façade with lower sound transmission loss (STL) than the walls and other major façade components. Airborne sound transmission through windows is governed by the same physical principles that affect walls, but the noise control measures are influenced by the glass properties and the air leaks around the window frame. The aim of the acoustic measurements campaign is to analyze the STL for the different façade cases previously described. In this part there are presented the results of an extensive series of in-situ measurements on the sound insulation of the test room facade. These results include measurements of sound pressure levels on 1/3 octave and on reverberation time. The additional lower frequency measurements were particularly

V. Iordache, T. Catalina / Building and Environment 57 (2012) 18e27

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Fig. 6. Air changes per hour.

is the 60 dB decay time calculated by a line fit to the portion of the decay curve between 5 dB and 25 dB and T30 calculated between 5 and 35 dB. Further, we calculated the reverberation time of the test room as an average value of these two measured parameters (Fig. 8a). The reverberation time was found to be between 1.1 s for lower frequencies and 1.4 s for medium range frequencies. Next, we calculated the STL based on the outdooreindoor difference of the sound pressure level and the reverberation time (Eq. (1)). It is found that the STL has slightly higher values than the SPL difference.

33.9 dB. Despite the various differences at mid and higher frequencies (1000 Hze4000 Hz), the results in Fig. 7 show that we have essentially the same outdooreindoor SPL differences. A recent study [42] has shown, that at frequencies above 1 kHz, more sound energy is transmitted through the window frame than through glass; thus the frame joints represent acoustic bridges for the façade structure. This may be another contributing factor to the relatively lower sound transmission loss at higher frequencies (above 1 kHz). Knowing that the test room has three windows and on each of their sides were taken sound measurements, we have found that it is important to define the difference between outdooreindoor SPL as a single average value: DLp ¼ (DLp1 þ DLp2 þ DLp3)/3, where DLp1 ¼ Lp(O1)  Lp(I1), DLp2 ¼ Lp(O2)  Lp(I2) and DLp3 ¼ Lp(O3)  Lp(I3) are the differences corresponding to each window. Using this average value we can compare more easily the different façade cases that were studied in this article. The SPL differences represent the result of two superposed phenomena: the sound transmission through the building façade and the multiple reflections of sound inside the test room. We used the reverberation time measurement of the test room in order to separate these two phenomena. The reverberation time was measured for the test room by means of the “2250 investigator” sonometer, in the form of the two common parameters T20 which

The resulting values of the sound transmission loss and the outdooreindoor sound pressure level difference are compared in Fig. 8b. The maximum values of these differences are low and mostly less than 1 dB which leads us to the conclusion that the mean absorption value of the room is high. A comparison between the experimental STL and the theoretical value was also conducted from the need to understand the importance of other ways of outdooreindoor sound transmission that are superposed over the transmission through window joints.

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V. Iordache, T. Catalina / Building and Environment 57 (2012) 18e27

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This theoretical estimation of the STL is based on the theoretical calculation of the STL through walls, windows and air [1]. Finally, the façade STL is calculated as a-weighted mean of the three STL (wall, windows and air) as follows:

0

1 Pn

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B B ¼ 10log10 BP @ n

C

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(3)

The measured and calculated values of the sound transmission loss are plotted as a function of the frequency, on the entire range 31.5 Hz O 16,000 Hz, in Fig. 9. It can be noticed that the experimental values of this sound transmission loss are smaller than the theoretical values, which proves the existence of other ways of sound transmission between the two environments. The highest differences are noticed in the case of lower frequencies (>10 dB) meaning that in real situations the sound is also transmitted through the adjacent chambers or other noise bridges (e.g. electric sockets or wiring). This lack of precision on the theoretical modeling due to the existence of other transmission way for the sound wave, leads to the conclusion that we cannot use the comparison between the experimental and the theoretical values to estimate the leakage area. Thus, for now, a mathematical relation between the two transfer phenomena is the only way to estimate the leakage area and the leakage air flow. Even if the STL is the adequate scientific variable that describes the noise transfer though the building facade, it still presents many important inconveniences from a practical point of view:  its evaluation implies the use of a professional sonometer, which is also a more expensive one,

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40 30 20 10 0

Frequency (Hz) Fig. 9. Sound transmission loss comparison between the theoretical and the experimental data.

 the calculation through the entire frequency spectrum is needed,  a time reverberation measurement is needed for the STL evaluation. In conclusion it cannot be stated that the STL is a false estimator for permeability measurement, but the amount of work implied by its use is hardly achievable in a fast and non-expensive way. Consequently, we look forward toward a simpler estimator that can be easily used for the building thermal inspections. Next step during the acoustic experimental campaign was recording the A-weighting values of global sound pressure level. The A-weighting values follow the frequency sensitivity of the human ear at low levels as it predicts quite well the damage risk of the ear. The low-frequency noise is filtered with higher values similar to the response of the human ear. Another reason for using of this scale is that most of the less expensive sonometers record only this value and not the SPL by frequency. Table 1 summarizes the whole measurements campaign, where the maximum sound attenuation DLp of 33.3 dB(A) is found for the Reference Case 0 (air sealed façade), while a minimum value is recorded for Case 5 (nonsealed façade) with DLp ¼ 22,4 dB(A).

Table 1 A-weighting global sound pressure level. Measurement point Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case Case

0e1 0e2 0e3 0eAverage 1e1 1e2 1e3 1eAverage 2e1 2e2 2e3 2eAverage 3e1 3e2 3e3 3eAverage 4e1 4e2 4e3 4eAverage 5e1 5e2 5e3 5eAverage

Indoor SPL, dB(A)

DLp, dB(A)

92.30 90.36 89.20

56.475 58.57 56.96

100.20 94.00 92.00

65.965 62.6 60.695

35.83 31.79 32.24 33.29 34.24 31.40 31.31 32.31 30.88 31.90 31.67 31.48 28.88 26.05 34.01 29.65 22.97 30.56 28.07 27.20 20.28 23.71 23.22 22.40

Outdoor SPL, dB(A)

93.1 98.05 98.125

62.225 66.15 66.455

96.17 89.7 91.77

67.29 63.655 57.76

94.34 99.12 95.7

71.37 68.56 67.635

92.08 95.96 94.50

71.8 72.255 71.285

V. Iordache, T. Catalina / Building and Environment 57 (2012) 18e27

One can notice that the sound attenuation varies from Case 0 to Case 5, meaning that the air sealing is an important component in the outdooreindoor sound transfer phenomena. Thus, the existence and the quality of the air sealing can improve or degrade the overall sound performance of the windows. However, even the windows with better air sealing are poor barriers to airborne noise transfer because the sound wave transmission through the glass has a higher weight in the overall sound transfer. 5. Correlation between noise transfer and infiltration rate In the previous two paragraphs it was found that there is a clear variation of air leakage flow and noise transfer for the studied cases. In this chapter we will put together the two variations in order to study the simultaneity of the two transfer phenomena. To show this simultaneity, the SPL differences DLp and the air change rate are plotted on Fig. 10a. For higher DLp (about 30 O 35 dB) we found lower air change rates (0e0.2 h1) while for the lower values of DLp (about 22 O 27 dB) a maximum air change rate of about 2 h1 was found. The results display a drop in the acoustical performance of the building façade, simultaneously with the raise of the air infiltration and the room air change rate. Thus, the joinery degradation has two simultaneous consequences: the raise of the air flow leakage and the indoor noise transfer. The most common experimental measurements in acoustical analysis is the dB(A) level, therefore the final correlation graph (Fig. 10b) presents the variation of the air change rate as a function of the A-weighted values of sound attenuation and has the potential to be an useful tool for façade expertise. The mathematical relation between the two transfer phenomena was obtained in the form of a single variable second degree regression model using the least square estimator: ACH4Pa ¼ 0.0462DL2p þ 2.5413DLp  33.29. The regression analysis made possible a good correlation between sound attenuation and the airtightness as illustrated in Fig. 10. The proposed model can give a prediction of the air change rate as a function of sound attenuation level, where its values are within the range 27 dB(A) O 34 dB(A) which corresponds to most of the façade types in Romania for closed windows situation.

25

The short interval of the sound attenuation level reflects the low influence of the joints upon the sound transfer through the building façade for this closed window situation. Despite the reduced interval for the variation of the sound attenuation level, the mathematical model:  presents an accurate prediction characterized by high confidence, and  leads to a complete cover of the air change rate interval of values corresponding to the closed windows situations. Both these arguments testify the applicability of this model, while its simple form makes from it an ideal tool for helping the building thermal investigators. 6. Comparison between experiment and prediction models Apart from developing the prediction model, another aim of this paper is to compare different methods to predict the air infiltration. In the last few years different statistical models have been created, among which we mention McWilliams et al. (2006) [43], Montoya et al. (2009) [10] or Chan et al. [7]. In order to compare the results of different existing prediction models with our new model based on acoustic experimentation, we need some form of normalized description. The pressure chosen is conventionally 50 Pa and most of the published data quotes air flow at 50 Pa. The first analyzed statistical model [43] proposes the calculation of the normalized leakage NL () based on the age of the building, the number of stories, its age, its energy efficiency and the floor area: size1

Nstorey1

P

age

P

floor NL ¼ NLCZ $barea $bheight $b3 eff $bage $bfloor PLI  age size1 $ bLI;age $bLI;area $bLI

(4)

where NLCZ is based on the type of climate (eg. NLCZ ¼ 0.53 for cold climates), b are known as regression coefficients, Peff depends on the type of energy efficiency program (Peff ¼ 1 if building has an energy efficiency program and Peff ¼ 0 if there is no program), Pfloor (Pfloor ¼ 1 for low income households and Pfloor ¼ 0 otherwise), Age as the years since its construction and size calculated as the ratio between the floor area and a reference area (Aref ¼ 100 m2). Montoya et al. (2010) made a statistical model to predict the airtightness of Catalan dwellings. Their study resulted in an equation for C0 , which is a simplification of the leakage coefficient C in the air flow power law:

  C ’ ¼ exp a þ barea $Area þ bST $ST þ bage $Age þ bNS $NS q50 ¼

(5)

C ’ ðDpÞ2=3 Aenvelope

where ST is a factor that takes into account the type of building (ST ¼ 1 if the structure is light and ST ¼ 0 if it is a heavy structure) and NS for the number of stories (NS ¼ 1 if one storey and 2 otherwise), q50 the air permeability (l/s m2), Dp the pressure difference (Pa) and Aenvelope is the façade surface (m2). The last model used for comparison is the model proposed by Chan et al. [7] developed based on a high amount of air leakage measurements:

NL ¼ expðb0 þ b1 $Age þ b1 $Area þ b3 $IE þ b4 $IL Þ Fig. 10. Correlation between sound attenuation and air change rate (a). Simultaneous variation (b). Physical correlation between air infiltration and sound attenuation (valid range: 27e33.5 dB(A)).

(6)

where IL takes value of 1 if the house is occupied by a low income household and 0 otherwise and IE takes value of 1 if the house is energy efficient and 0 otherwise. Further, the total air flow through

26

V. Iordache, T. Catalina / Building and Environment 57 (2012) 18e27

1400 R² = 0.9917; R=0.9958

1000

Q (m /h)

6.0 5.0

C3

800

4.0

871 m3/h

Chan et al. McWilliams et al. 830 m /h

600

C2

Montoya et al. 811 m /h

Experimental value: 517m /h

400

3.0 2.0

C1

200

Air change rate (1/h)

7.0

C4

1200

1.0

Co

0 25

26

27

28

29

30

31

32

33

34

0.0 35

Sound attenuation ΔLp, dB(A)

Fig. 11. Comparison between statistical models and experimental data.

the envelope and the air change rate at 50 Pa will be calculated as follows:

NL ¼

 0:3 ELA H $ Afloor 2:5

V50 ELA ¼ sffiffiffiffiffiffiffiffiffi 2Dp

(7)

r ACH50 ¼

V50 V

where n50 is the air change rate at 50 Pa (1/h), V50 is the air flow through the envelope at 50 Pa pressure difference (m3/s), V the building volume (m3) and r the air density (kg/m3). This validation procedure will be carried out for the non-sealed façade of the building; this situation corresponds to Case 2. The measured permeability is compared with the predicted value according to the acoustic measurements and the three mathematical models proposed in literature (Fig. 11). It is noticed that the prediction based on acoustic measurements is almost equal to the measured value, while all three statistical models show a weak correlation compared with the measured air change rate with more than 30% errors. This result sustains the practical use of this method. The air changes rate can indeed be determined based on a very simple acoustic measurement of the outdoor/indoor sound pressure difference and on the regression model from Fig. 10b. Finally, we can conclude that this acoustic approach for building air permeability measurement is distinguished by precision, simplicity, and non-expensive measurement devices.

experience and give a better estimation on the airtightness of the building. Moreover, a few aspects should be detailed. Firstly, the air change rate of 0 (1/h) corresponds to an STL of 32 dB; this value depends on the room geometry, wall structure and window type (double glazing in our study). One might expect that, for a different type of window, the STL for an airtight façade might present slightly different values, say: 38 dB for a triple glazing or 25 dB for simple glazing. Secondly, we recorded a STL of 5 dB for an opened window, this being the case where the air change rate varies between 6 and 10 (1/h). Thirdly, the in-between situations, ACH between 2 and 6 (1/h), correspond to the partially opened windows and the STL may vary considerably according to the type of window and its openings directions (inside or outside). A comparison with other prediction models was carried out for the non-sealed façade. It was found that the new acoustic protocol leads to a far more accurate prediction than the current prediction models. Beside its scientific novelty, this relation between the two phenomena represents a powerful tool to be used in order to estimate the air change rate of a building façade. This model can successfully replace the expensive, and weather dependent permeability measurement with a much more simple acoustical system (one loudspeaker/sound source and two commercial sonometers). This second system is by far less expensive and the measurement can be done in just a few minutes. The error of this type of measurement is around 5%, thus in the range of a usual standardized permeability measurement. The positive results obtained in this study for double-pane wood windows show that this approach might be successfully applied to other window types, like metal or plastic frame windows with simple or triple glazing. We conclude by recommending this procedure for fast evaluation of the air change rate for any study (indoor environment quality, building certification, rehabilitation measures) that requires an air infiltration estimation. Acknowledgment This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS e UEFISCDI, project number PN-II-RU-TE-2011-3-0209. We also would like to thank engineers Adrien Fauquin and Florent Lenoir for their contribution to this study. Nomenclature

7. Conclusions The measurement campaign was organized in order to underline the relation between air and noise transfer through the building façade and especially through window joints. The permeability was measured by means of standardized pressurization technique using a blower door system. The sound transmission loss between the outdoor and the indoor environment was recorded by means of two sonometers and a noise source; the indoor and outdoor measurements were carried out simultaneously. We can answer at our initial concerns about the existence of a relation between the two phenomena and its form. We found indeed that the two transfer phenomena are inverse correlated; the air tighter of the building façade the smaller infiltration air flow rate and the higher the sound transmission loss. We recorded sound transmission losses between 27 and 33 dB. This relation between the two phenomena has the form of a second degree polynomial function with a correlation coefficient R ¼ 0.9958. Thus, for buildings without any requirements on the use of a blower door test, the developed regression model could replace field

Dp Q ACH4Pa ACH50Pa STL SPL DLp Tr V S NL b Peff Age Size ST NS V50

indooreoutdoor pressure difference (Pa) volumetric air flow rate (m3/h) air change rate at 4 Pa difference (1/h) air change rate at 50 Pa difference (1/h) sound transmission loss (dB) sound pressure level (dB) difference outdooreindoor sound pressure level (dB) reverberation time (s) volume (m3) sum of absorbent indoor surfaces (m2) normalized leakage () regression coefficients () energy efficiency program () years since building construction (years) ratio between floor area and reference area () building structure factor () number of stories () air flow through the envelope at 50 Pa pressure difference (m3/s)

V. Iordache, T. Catalina / Building and Environment 57 (2012) 18e27

r IL IE Afloor q50 ELA

air density (kg/m3) factor for household income () factor for energy efficiency () floor surface (m2) air permeability (l/s m2) Effective Leakage Area (m2)

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