Balancing of thermal and acoustic insulation

Balancing of thermal and acoustic insulation performances in building envelope design Antonino Di Bellaa) Nicola Granzottob) Hagar Elargac) Department of Industrial Engineering, University of Padova Via Venezia 1, 35131 Padova, Italy Giovanni Semprinid) Luca Barbaresie) Cosimo Marinoscif) Department of Industrial Engineering, University of Bologna Viale Risorgimento 2, 40136 Bologna, Italy Energy consumption of buildings depends significantly on the criteria used for design and operation. Therefore, thermal insulation of building structures is a topic of fundamental importance in building design and the acoustic protection of internal environment is often considered less important or feasible in a subsequent design time, as a consequence of thermal issue. The main problem about acoustic and thermal performances of building elements is that it's very hard to optimize both performances in the same time because of the different physics phenomena related to these two aspects. In this work several aspects of acoustic and thermal design are evaluated, with the aim of balancing overall performances. A comparative analysis was performed for the evaluation of the interaction between several approaches to building design. Different types of insulating materials and construction techniques have been used in the same case study and possible correlations between thermal and acoustic performances have been evaluated. Results show that thermal and acoustic design optimization have to take into account field transient conditions as well as structural joints, because the simple comparison between physical a)

email: [email protected] email: [email protected] c) email: [email protected] d) email: [email protected] e) email: [email protected] f) email: [email protected] b)

properties of heat and sound transmission of building materials in stationary conditions is not sufficient for establish clear correlation. 1

INTRODUCTION

The problem of balancing the performance of thermal and acoustic insulation of buildings must take into account many aspects that are derived, mainly, from the architectural design and the choice of building systems. As is known, the increase of the thermal insulating layers is not always beneficial for sound insulating performances of the building element itself or for the adjacent structures1,2. In this field, many physical phenomena that govern the behavior of the materials are in conflict with each other and also the trend over time of benefits must be carefully evaluated. This paper suggests a method of analysis for the optimization of the acoustic and thermal performance of buildings based on the comparison between different combinations of materials and building systems, both from the point of view of the sound insulation of the façade and from the analysis of differences in the thermal loads of rooms of a building that share the same façade characteristics of protection from external noise. 2

DESCRIPTORS OF ACOUSTIC AND THERMAL PERFORMANCES

In building design a number of physical parameters have to be compared and optimized to reach the goals of internal comfort and energy saving. In many cases these parameters are depending not only from the characteristics of building elements but also from building shape, orientation, external climatic conditions, shading, and so on. Limiting the analysis only to thermal and acoustic performance, it is possible to refer to some basic parameters to analyze the main aspects of protection against noise (external or internal) and thermal control. A widespread and consolidated model for prediction of sound transmission in buildings based on the performances of building elements is provided by EN 12354 series. This series of standards describes calculation models for the estimation of the airborne sound insulation and impact sound insulation between adjacent rooms in buildings, primarily on the bases of measured data which characterizes both direct and flanking transmission by the participating building elements and theoretically derived methods of sound propagation in structures. Focusing on protection from external noise and airborne sound propagation inside rooms, the main acoustic design parameters are the weighted apparent sound reduction index and weighted standardized level difference for façades. The weighted apparent sound reduction index is a single number quantity that describe the attitude of a separating building element to reduce noise transmission between adjacent rooms, taking into account the acoustic characteristics of side elements and their structural coupling, and can be calculated according to EN 12354-13: 12  −R 10  Rw′ = −10 lg 10− Rw 10 + ∑10 n ,w  , n =1  

(1)

where Rw is the weighted sound reduction index of the separating wall [dB] and Rn,w is the weighted flanking sound reduction index for the transmission paths [dB].

In many practical cases, in any pair of adjacent rooms within a building, at least two side elements are referable to the external envelope. So, the building façade can have a direct effect on the internal noise propagation. The estimation of the reduction of outdoor sound by façades of buildings is based on the sound reduction index characteristics of the relevant elements. The weighted standardized level difference can be calculated according to EN 12354-34:

D2 m,nT ,w = R ′f , w + ∆L fs + 10 lg

V 6T0 S ,

(2)

where R'f,w is the weighted apparent sound reduction index of the façade [dB]; V is the volume of the receiving room, in cubic meters [m³]; T0 is the reference reverberation time [s]; S is the total area of the façade as seen from the inside of the room [m²]; ∆Lfs is the level difference due to façade shape [dB]. It is important to note that the effect of the shape of the façade and the room volume can be very significant for the protection from external noise and these aspects are also shared with the evaluation of the thermal dispersion of the building. The input data necessary for the acoustic calculations mainly derive from laboratory measurements of building elements. Both of these parameters can be easily verified in the field on the actual building. The thermal transmittance, U [W/(m²K)], is the rate of heat flow through a unit surface area of a component when a unit temperature difference is set between environments of the two sides of the component itself. It is the reciprocal of the sum of the thermal resistances of all the layers composing a building element plus the inside and outside surface resistances and it can be evaluated according to ISO 69465, that provides the method of calculation of the thermal resistance and thermal transmittance of building components and building elements, with the assumption of steady-state conditions:

U=

1 1 = RT Rsi + R1 + R2 + ... + Rn + Rse ,

(3)

where RT is the thermal resistance of the whole element [(m²K)/W]; Rsi and Rsi are, respectively, the conventional internal and external surface resistance, the value of which mainly depends on the direction of heat flow and Rn is the thermal resistance of each layer, obtained as the ratio between the thickness of the layer and its thermal conductivity λn [W/(mK)]. The evaluation of the energy needs of a building requires detailed calculations that take into account the heat loss through the envelope and the reference climatic conditions. The heat loss of the envelope depends, in turn, from the thermal transmittance of building elements and several other design parameters. 3

COMPARISON BETWEEN ACOUSTIC AND THERMAL PERFORMANCES IN A CASE STUDY

In order to compare the effects of the variation of the different parameters affecting the thermal and acoustic performance a simple case study is proposed. The reference model is a multi-storey building in which various construction systems have been applied, from time to time: reinforced concrete frame and panels made with different types

of hollow blocks; cross-laminated timber structure both for external panels and internal partitions; steel structures with double skin façade and lightweight partition walls. Only a small portion of the building is considered for thermal and acoustic modeling and simplified boundary conditions are fixed, in order to reduce the number of variables and complexity of calculations. Varying the building systems and materials considered, the interaction between acoustic and thermal aspects of the building envelope were assessed. 3.1 Description of reference rooms and building elements Two adjoining rooms belonging to different units were considered (Fig. 1), with the same size but with a different setup of the façade. For reinforced concrete frame and cross-laminated timber structure the “Room 1” has only one facing with a single window and the “Room 2” has a double facing with two windows on the same side. For double skin façade the entire surface of the building envelope is glazed.

Room dimension: - width, 4.00 m - depth, 3.50 m - height, 2.70 m - volume, 37.80 m³ - total surface, 68.500 m² - façade surface Room 1, 10.800 m² Room 2, 20.250 m² - window surface (each), 1.875 m² - separating wall surface, 9.450 m²

Fig. 1 – Dimensions of reference rooms for comparison of thermal and acoustic performance. The building elements arranged for the different combinations and their thermo-physical and acoustic properties (thickness, surface mass, thermal transmittance and sound reduction index) are reported in Table 1. The criteria for the choice of materials and construction systems are based on the possibility to create combinations in which, while maintaining constant one parameter at time (e.g. thermal transmittance) it is possible to have an appreciable variability of the other physical properties and performance. Four external walls or façade elements have been selected, with different constructive solutions, with similar thermal transmittance values (0.490 ≤ U ≤ 0.541 W/(m²K)) and a strong variation of sound reduction index (48 ≤ Rw ≤ 69 dB). Three of the walls are “heavy” (surface mass m' > 200 kg/m²) while one is “light”. In the same way, four internal separating walls have been selected, with thermal transmittance values between 0.280 W/(m²K) and 0.318 W/(m²K) and with sound reduction index value between 37 dB and 70 dB.

Glazed surfaces considered both for windows and double skin façade show a small variation in thermal transmittance and a large variation in sound reduction index values due to the PVB plastic layer in laminated glass. In general, large performance differences can be found between double and triple glazing, with a variation of thermal transmittance, U, from 2.517÷2.608 W/(m²K) to 1.570÷1.580 W/(m²K) and a variation of sound reduction index values, Rw, from 35÷44 dB to 40÷45 dB respectively. Table 1 – Building elements utilized for acoustic and thermal simulations. Code EW1 EW2 EW3 EW4 GE1 GE2 GE3 GE4 IW1 IW2 IW3 IW4 IW5 IW6 FL1 FL2

Element type External light wall External heavy wall External heavy wall External heavy wall Double glazing Triple glazing Double glazing Triple glazing Internal light partition Internal light partition Internal light partition Internal heavy partition Internal heavy partition Internal heavy partition Light floor Heavy floor

s [m] 0.200 0.390 0.320 0.340 0.030 0.056 0.040 0.055 0.100 0.250 0.265 0.110 0.390 0.345 0.418 0.200

m' [kg/m²] 44 247 341 424 20 40 52 52 24 60 118 127 247 276 208 440

U [W/m²K] 0.510 0.501 0.541 0.490 2.608 1.580 2.517 1.570 -

Rw (C;Ctr) [dB] 48 (-2;-6) 55 (-2;-5) 52 (-2;-6) 69 (-1;-4) 35 (-2;-5) 40 (-2;-5) 44 (-1;-3) 45 (-1;-4) 46 (-4;-10) 63 (-3;-7) 56 (-7;-15) 37 (-1;-3) 55 (-2;-5) 70 (-5;-12) 56 (-3;-9) 60(-3;-6)

Note c,f b a,e a,f g i h j d d c a b a c a

Note: a) Hollow clay bricks/blocks wall/floor b) Hollow bricks cavity wall c) Cross-laminated timber wall/floor d) Lightweight gypsum-board wall e) Internal thermal insulation coating

f) External thermal insulation coating g) 4-22-4 mm glazing h) 6-18-6 mm glazing i) 6-20-6-20-6 mm glazing j) 6-18-5-15-6 mm glazing

3.2 Estimation of acoustic performance of building façades and partitions The prediction of the acoustic performance of separating walls and façades of the two rooms was carried out following the simplified model proposed, respectively, by EN 12354-1 and EN 12354-3 standards. The separating wall between the two rooms is joined to the adjacent structures with rigid cross-junction except for the façade connections, in which the “T” connection can change depending from the construction system of the façade itself and from the surface mass of the separating wall. The form factor of the façade, ∆Lfs, has been set equal to 0 (flat front). It was assumed a rigid connection between the façade and interior partitions when heavy walls were considered, whereas an elastic junction was applied in the case of lightweight internal walls and connection with a double skin façade. A reduction of 3 dB regarding the sound reduction index of the window was applied in case of double skin façade, due to the increased dimensions of the glazed surface, according to EN 14351-16.

An interesting result can be observed by combining the four types of exterior wall with the various glazing and comparing the data obtained for the double skin façade with the same glazing types (Fig. 2). The difference between the minimum and maximum values for each combination is from 6 dB to 10 dB, and the performances of protection from external noise of the two rooms are each time the same, even though the façade surface of the “Room 2” is double compared with “Room 1”. This result is due to a similar ratio between “opaque” and “transparent” façade elements for the two rooms. EW4

Max Min ∆

55

DSF 10 9

EW2

8 7

EW3

50

6

EW1 45

5 4

40

3 2

35

1 30

Façade Sound Insulation difference, ∆D2m,nT,w [dB]

Façade Sound Insulation, D2m,nT,w [dB]

60

0 EW1

EW2

EW3

EW4

DSF

Fig. 2 – Façade sound insulation of “Room 1” and “Room 2” due to the combination of several type of building elements and windows (EW) and different glazing for double skin façade (DSF). In any case, the effect of the acoustic performance of the glazed element on the final result is preponderant, as shown in Figure 3. Once have been exceeded the optimal surface ratio between the outer wall and the window, the trend of the overall acoustic performance remains almost constant. Façade Sound Insulation, D2m,nT,w [dB]

60

GE1 Rw=35 dB GE2 Rw=40 dB

55

GE3 Rw=44 dB GE4 Rw=45 dB

50

∆=1 dB ∆=4 dB

45 ∆=5 dB 40 35 30 30

40 50 60 70 Sound Reduction Index of building envelope, Rw [dB]

80

Fig. 3 – Values of Façade Sound Insulation, D2m,nT,w as a funcion of Sound Reduction Index, Rw, of building envelope for different combination of glazed elements.

More complex is the evaluation of the effects of change in building systems or materials of the façade on sound reduction performances of internal partitions. In case of separating walls with high acoustic performance but low surface mass, lower values of the apparent sound reduction index can be obtained compared with those obtainable with “heavy” walls. In general, for a fixed thermal transmittance of the building envelope and keeping constant acoustic boundary conditions, can be observed a large variation of the apparent sound reduction index between “Room 1” and “Room 2” (up to 6 dB), that depends on the acoustic properties of the separation wall itself. On the other hand, while maintaining constant the acoustic performance of the separating wall and varying the acoustic performance of the façade elements, the apparent sound reduction index has no significant variations (within 1 dB). From these two observations derives that the techniques of thermal insulation of the building envelope can have an effect on the performance of internal insulation only for certain combinations of materials and building systems. From acoustic point of view, the best performance of façade sound insulation and apparent sound reduction index of separating walls can be obtained with internal thermal insulation layers by the reduction of the flanking transmissions between façade and internal partitions. 3.3 Evaluation of thermal loads In order to evaluate thermal performance of different façades, heating and cooling loads of “Room 1” and “Room 2” were investigated dynamically on the bases of test reference year climatic data (TRY) in Venice, Italy. Design inside temperatures were assumed equal to 20 °C and 26 °C in heating and cooling seasons respectively. The following assumption are made: - non-residential use of the building (working time from 6:00 to 18:00), with two occupants per room; - internal loads for lighting 10 W/m²; - internal loads for equipments 160 W; - all internal walls were assumed adiabatic and ventilation loads were ignored; - no sunscreens are present for the natural lighting control. For thermal loads evaluation “TRNSYS”7 software was utilized. It simulates dynamically thermal behavior of buildings and their systems and relies on modular approach to solve equations described by FORTRAN subroutines, including the solar radiation affecting external walls and windows. To evaluate optical performance of glazed elements “WINDOW 6”8 was utilized, which is a software used in modeling windows and shading devices, and all optical and angular data of façade. Thermal performance in both rooms, including the losses due to external façades and internal loads, has varied considerably in the six studied cases. In Figure 4 a comparison between the differences of thermal loads between the two reference rooms for different combinations of building systems and building elements is shown. In Figure 5 a comparison in thermal performance using EW3 and GE1 building elements combination in “Room 2” is highlighted within a summer working week starting from 16th to 20th of July. Intervals shown the figure are due to control of air conditioning system in working hours. The increment reaches about 65% in thermal cooling requirements, this variation is due to the high value of thermal transmittance, U, of the glazed element compared to the other cases, in addition to the differences of materials thermal capacities and surfaces façade elements, which affect the thermal loads. On the other hand, differences in the other cases of façade elements combination based on EW1, EW2, EW3 and EW4 are moderate and thermal loads variations didn’t exceed a range of 2÷4% in summer and 3÷7% in heating season. This is due, mainly, to the closeness of the

characteristics of heat transfer coefficients of external walls. Figure 6 show thermal performance in a winter working week (from 16th to 20th December) in “Room 1” with façade elements combination EW1 and EW2. 500

500

0

-500

b) EW4+GE3

Therma loads difference [W]


a) EW3+GE3

0

-500

Annual variation on hourly basis

500


500

0

-500

d) EW1+GE3



c) EW2+GE3

0

-500


2000


2000 e) DSF GE1

f) DSF GE2 1500 Therma loads difference [W]


1500 1000 500 0 -500 -1000 -1500 -2000

1000 500 0 -500 -1000 -1500


-2000


Fig. 4 – Comparison between the differences of thermal loads of the two reference rooms (“Room 1”−“Room 2”) for different combinations of building systems and building elements, as described in Table 1. For each combination, the façade sound insulation of the two rooms is the same. Combinations “e” and “f” refer to double skin façade building system and the range the ordinate axis has increased fourfold compared to the other cases.

3.5 Room 2 EW3+GE3

Room 2 DSF GE1

3.0

Thermal Energy [kWh]

2.5 2.0 1.5 1.0 0.5 0.0 16 Jul

17 Jul

18 Jul

19 Jul

20 Jul

Fig. 5 – Comparison of thermal energy demand for a summer week in “Room 2” between a combination of external walls and windows and a double skin façade. 0.50 Room 1 EW1+GE3

0.45

Room 2 EW2+GE3

Thermal Energy [kWh]

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 16 Dec

17 Dec

18 Dec

19 Dec

20 Dec

Fig. 6 – Comparison of thermal energy demand for a winter week in “Room 1” between two different combinations of external walls and windows. 4

DISCUSSION AND CONCLUSIONS

The optimization of thermal and acoustic properties of building envelope is a complex problem that cannot be solved simply by comparison and matching of best values of a selection of parameters chosen on basis of material properties. The analysis of a case study with extremely simplified boundary conditions showed a large variation of total thermal load differences between two rooms with a building envelope characterized by the same façade sound insulation but a different facing and façade surface (Fig. 7, a-b-c-d). For the façades composed by combination of external walls and windows the best acoustic performance correspond to the greater differences in thermal loads of the rooms. In this case, the position of thermal insulating layer in the external wall can significantly affect not only

the acoustic insulation of the façade but also the apparent sound reduction on the internal separating walls. Acoustic and thermal behavior of double skin façade is more critical (Fig. 7, e-f) and a deep integration of sunscreens and ventilation systems, together with a specific design of the form factor of the façade, can be suggested in order to achieve satisfactory results for both parameters. 50 48 46

2000

44

1500

42 ∆TLD [W] D2m,nT,w [dB]

40

1000

38 36 34

500

32 0

Façade weighted standardized level difference, D2m,nT,w [dB]

Variation of Thermal Loads Difference, ∆TLD [W]

2500

30 a)

b)

c)

d)

e)

f)

Fig. 7 – Comparison between total thermal load differences and façade weighted standardized level difference for different combinations of external walls and windows as showed in Figure 5. This study can be developed by considering a large set of case study for the evaluation of a number of elements and variables that are not considered now, as the combined effect of thermal bridges and acoustic leakages, the relation between thermal inertia and sound insulation due to the surface mass of the building elements, the effectiveness of sound absorbing surfaces and sunscreen to reduce the propagation of external noise inside buildings and so on. 5

REFERENCES

1. G. Semprini, A. Cocchi and C. Marinosci, “Possible correlation between acoustic and thermal performances of building structures”, Proceedings of EAA-ASA-SFA Acoustics’08 Conference, Paris, (2008). 2. A. Di Bella, N. Granzotto and C. Pavarin, “Comparative analysis of thermal and acoustic performance of building elements”, Proceedings of EAA-Forum Acusticum, Krakow, (2014) 3. EN 12354-1 (2000): Building Acoustics - Estimation of acoustic performance of buildings from the performance of elements - Part 1: Airborne sound insulation between rooms. 4. EN 12354-3 (2000): Building acoustics - Estimation of acoustic performance of buildings from the performance of elements - Part 3: Airborne sound insulation against outdoor sound. 5. ISO 6946 (2007): Building components and building elements - Thermal resistance and thermal transmittance - Calculation method.

6. EN 14351-1:2006+A1 (2010): Windows and doors - Product standard, performance characteristics - Part 1: Windows and external pedestrian doorsets without resistance to fire and/or smoke leakage characteristics. 7. S. Klein, W. Beckman, J. Mitchell, J. Duffie, N. Duffie, T. Freeman et al., “TRNSYS 16 - A transient system simulation program. User manual”, University of Wisconsin, Madison, Wisconsin, USA, (2004). 8. R. Mitchell, C. Kohler, D. Curcija, L. Zhu, S. Vidanovic, S. Czarnecki, D. Arasteh et al., “THERM 6.3/WINDOW 6.3 NFRC Simulation Manual”, University of California, Berkeley, California, USA, (2011).