1、 Jin-Hee Song is a graduate student in the Department of Architectural Engineering, Ewha Womans University, Seoul, South Korea. Min-Joo Park is a researcher at the Civil the simulation model was determined based on the manner of actual construction with considering all the repeated thermal bridges.
2、By comparing the Ueff with the legally required thermal transmittance (Ucode) calculated by assuming one-dimensional heat transfer, the decrease in the thermal insulation performance due to thermal bridges was quantitatively evaluated. In addition, based on this comparison, the methods to minimize h
3、eat loss through the thermal bridges were examined. METHOD Types of Building Envelopes In this study, the concrete walls and steel truss curtain walls were considered. Steel truss curtain walls are non-bearing external walls which are installed by fixing grid-pattern steel trusses to floor slabs, an
4、d a stone or metal exterior are fixed to the trusses. In the case of the concrete walls, the following types were considered: (1) paint-finished (C-1), (2) stone exterior (C-2), and (3) metal-sheet exterior (C-3). In the case of the paint-finished concrete wall, the following types were further cons
5、idered: (1) the case with an internal insulation system (C-1a) and (2) the case with an external insulation system (C-1b). The steel truss curtain walls were of the following types: (1) stone (S-1), (2) metal-sheet (S-2a), and (3) metal-panel (S-3). In the case of the metal-sheet type, these walls w
6、ere further classified depending on the application of additional thermal insulation to the steel trusses (S-2b). The external wall types for evaluating the thermal insulation performance are described in Table 1. The paint-finished concrete wall is the commonly applied external wall type in apartme
7、nt buildings; in the case of the internal insulation system, the insulation layer cannot be continuous at the wall-to-floor, wall-to-roof, and wall-to-wall construction junctions. Thus, the additional heat loss occurs through these junctions and these junctions are considered as linear thermal bridg
8、es. In the case of the external insulation system, the insulations are mechanically fixed by fasteners, which can cause the additional heat loss as they penetrate the insulation. The fasteners are considered as point thermal bridges. The stone and metal-sheet concrete walls are widely used as the ex
9、ternal walls in apartment buildings or mid- and low-rise non-residential buildings; for these concrete walls, the thermal bridges occur at the metal fixings when they are used to fix exterior materials. Figure 1(a) shows an infrared thermal image of an apartment building showing the paint-finished c
10、oncrete wall with internal (right) and external (left) insulation systems; in the case of the internal insulation system, the additional heat loss is large due to the thermal bridges at the wall-to-floor junction (Song et al. 2009). Steel truss curtain walls are widely applied in mid- and low-rise b
11、uildings as well as high-rise buildings because steel truss curtain walls have the advantages of light in weights, dry construction system, prefabrication, and lower construction cost than aluminum curtain walls. Steel truss curtain walls are constructed by a method of fixing steel trusses on a slab
12、 (primary fixing) and installing exterior materials such as stone, metal-sheet and insulation or insulation-embedded metal-panels on the exterior of the trusses (secondary fixing). In this type of construction, all fixing or connecting parts becomes the thermal bridges; the trusses, the fastening un
13、its for fixing the trusses on a slab, the brackets and bolts for installing the exterior materials, and the vertical and horizontal joints between the exterior materials. Figures 1(b) and 1(c) show the infrared thermal images of stone and metal-sheet steel truss curtain walls in two buildings; the a
14、dditional heat loss occurs due to the vertical and horizontal joints between the external materials. Method of Evaluation To quantitatively evaluate the actual decrease in thermal insulation performance due to thermal bridges for each external wall type, the effective thermal transmittance (Ueff) in
15、cluding the heat loss through all the repeated thermal bridges was calculated and compared. Ueff was calculated using Equation (1): Ueff = Qtot / Ae (Ti- To) (1) Table 1. Evaluated Types of External Walls Concrete wall Paint (C-1) Stone (C-2) (External insulation system) Metal-sheet (C-3) (External
16、insulation system) Internal insulation system (C-1a) External insulation system (C-1b) Layer Concrete 150 mm (5.90)* Insulation 114 mm (4.49) Gypsum board 19 mm (0.75) Finish plaster 15 mm (0.59) Base coat 2.5 mm (0.10) Insulation 107.5 mm (4.23) Adhesive 4 mm (0.16) Concrete 150 mm (5.90) Cavity Gy
17、psum board 19 mm (0.75) Stone 30 mm (1.18) Insulation 110.5 mm (4.35) Concrete 150 mm (5.90) Cavity Gypsum board 19 mm (0.75) Metal sheet 0.8 mm (0.03) Insulation 111 mm (4.37) Concrete 150 mm (5.90) Cavity Gypsum board 19 mm (0.75) ImageDescription 1) The concrete external wall is finished with pai
18、nt. 2) The insulation is installed on the interior of the concrete wall. 1) The concrete external wall is finished with paint. 2) The insulation is installed with adhesive and fastener on the concrete wall. 1) The stone is installed on the bracket fixed with an anchor bolt on the concrete wall. 2) T
19、he insulation is installed between the concrete wall and the stone. 1) The metal sheet is installed on the bracket fixed with an anchor bolt on the concrete wall. 2) The insulation is installed between the concrete wall and the metal sheet. Steel truss curtain wall Stone (S-1) Metal-sheet (S-2) Meta
20、l-panel (S-3) Without additional insulation of steel trusses (S-2a) With additional insulation of steel trusses (S-2b) Layer Stone 30 mm (1.18) Insulation 112.6 mm (4.43) Cavity Gypsum board 25 mm (0.98) Metal sheet 3 mm (0.12) Insulation 113 mm (4.45) Cavity Gypsum board 25 mm (0.98) Metal sheet 3
21、mm (0.12) Insulation 113 mm (4.45) Cavity Gypsum board 25 mm (0.98) Metal sheet 0.8 mm (0.03) Insulation 113 mm (4.45) Metal sheet 0.5 mm (0.02) Cavity Gypsum board 25 mm (0.98) ImageDescription 1) The stone is installed using the bracket on the steel trusses fixed on the slab. 2) The insulation is
22、installed in the empty space between the vertical and horizontal trusses. 1) The metal sheet is installed using the screws on the steel trusses fixed on the slab. 2) The insulation is installed in the empty space between the vertical and horizontal trusses. 1) The insulation is additionally installe
23、d on the interior of the vertical and horizontal trusses in the S-2a method. 1) The insulation-embedded metal panel is installed using the bracket on the steel trusses fixed on the slab. 2) The aluminum mold is installed at the side of the metal panel where the bracket is inserted. * The numbers in
24、( ) are in IP unit; mm is converted to inch. (a) (b) (c) Figure 1 Infrared thermal images of buildings in winter; (a) Internal insulation (right) and external insulation (left) concrete wall apartment, (b) Stone curtain wall office, (c) Metal curtain wall multi-purpose building To include all the re
25、peated thermal bridges, the common dimensions of exterior materials were considered, and to conform to the thermal bridge modeling principle of ISO 10211 (ISO 2011, see Figure 2(i), simulation models were established by external wall type, as shown in Figure 2. In addition, the indoor floor and ceil
26、ing structures were simplified and made identical, and the internal/external surface areas were set identically as well. Furthermore, the thermal transmittance (Ucode), calculated assuming one-dimensional heat transfer without considering thermal bridges, was set to 0.27 W/m2K, which is the minimum
27、thermal insulation performance required by law. To obtain Qtot, a three-dimensional steady state heat transfer simulation was performed. The indoor and outdoor boundary conditions and material properties are listed in Table 2. In the case of the air cavity included in the Ucode calculation in the si
28、mulation models, the effective thermal conductivity satisfying a thermal resistance of 0.086 m2K/W, as provided by the Code for Building Insulation Design (MOLIT Annex 5 2013), was obtained using Equation (2), and this value was applied. In the case of other air cavities, the effective thermal condu
29、ctivity depending on the type of air cavity was set to be automatically calculated by the program: eff = Rcavity / Dcavity (2) THERMAL INSUALTION PERFORMANCE OF VARIOUS OPAQUE BUILDING ENVELOPES Results The heat loss (Qtot) and the effective thermal transmittance (Ueff) by external wall type are lis
30、ted in Table 3. Ucode was set at 0.27 W/m2K for all cases. However, when including all repeated thermal bridges as in actual construction, the Ueff by each external wall type was 0.290.86 W/m2K, which was 1.063.20 times Ucode. In other words, the decrease in thermal insulation performance due to the
31、rmal bridges was quite large, and the deviation was also large depending on the external wall type. The concrete wall showing the largest decrease in thermal insulation performance was the paint-finished concrete wall with an internal insulation system (C-1a). The Ueff was 0.74 W/m2K , which was 2.7
32、3 times Ucode. Conversely, the paint-finished concrete wall with an external insulation system (C-1b) showed the best thermal insulation performance. However, the Ueff was 0.29 W/m2K, which was slightly higher than the Ucode because additional heat loss occurred through the fasteners used for mechan
33、ical fixing of the insulation. Even when the external insulation system was applied, in the case of the stone and metal-sheet concrete walls (C-2 and C-3), the Ueff were 0.43 and 0.33, which are 1.59 and 1.23 times the Ucode, because of the heat loss through the metallic brackets and bolts. In the c
34、ase of the stone concrete wall, in which the external material size is relatively smaller than that of the metal-sheet concrete wall, the heat loss increased because the number of fixed brackets per unit area increased. The steel truss curtain wall showing the largest decrease in the thermal insulat
35、ion performance was the metal-sheet curtain wall (S-2a). The Ueff was 0.86 W/m2K, which was 3.20 times the Ucode. In the cases of the stone curtain wall and the metal-panel curtain wall (S-1 and S-3), the Ueff were 0.80 (2.98 times Ucode) and 0.62 (2.31 times Ucode) W/m2K, respectively. For the meta
36、l-sheet and stone curtain walls, the insulation was not continuous on the interior of the trusses because insulation was installed in the empty spaces between the vertical and horizontal trusses, and very large heat loss occurred here. In the case of the metal-panel curtain wall, additional heat los
37、s occurred due to thermal bridges at the vertical and horizontal joints between panels. When additional insulation was installed on the interior of the vertical and (a) (C-1a) Paint-finished concrete wall (Internal insulation system) (b) (C-1b) Paint-finished concrete wall (External insulation syste
38、m) (c) (C-2) Stone concrete wall (d) (C-3) Metal-sheet concrete wall (e) (S-1) Stone steel truss curtain wall (f) (S-2a) Metal-sheet steel truss curtain wall (g) (S-2b) Metal-sheet steel truss curtain wall (with additional insulation of steel trusses) (h) (S-3) Metal-panel steel truss curtain wall (
39、i) Cutting planes according to ISO 10211 (A: from a steel truss to the horizontal middle point of a panel, B: from a slab to the vertical middle point of a panel (more than 1.0 m), C: from a slab to the vertical middle point of a panel, including the ceiling plenum (more than the depth of the plenum
40、 + 1.0 m), D: from the inside surface to the cutting plane (more than 1.0 m) Figure 2 Elevation, section, and plan of each external wall Table 2. Material Properties and Boundary Conditions Material properties Material Thermal conductivity (W/mK) Material Thermal conductivity (W/mK) Concrete 1.600 (
41、0.924)* Polyurethane 0.120 (0.069) Gypsum board 0.180 (0.104) EPDM gasket 0.250 (0.144) Insulation 0.034 (glass wool) (0.020) Silicone sealant 0.350 (0.202) Steel 45.0 (26.0) Finish plaster 0.196 (0.113) Aluminum 200.0 (115.6) Adhesive 0.353 (0.204) Stone 3.200 (1.849) Base coat 0.181 (0.105) Backup
42、 rod 0.050 (polyurethane sponge) (0.029) * The numbers in ( ) are in IP unit, Btu/hftF. Boundary conditions Set-point temperature Surface heat transfer rate Outdoor -11.3 C (11.7 F) 23.25 W/m2K (4.09 Btu/hft2F) Indoor 20.0 C (68.0 F) 9.09 W/m2K (1.60 Btu/hft2F) horizontal trusses of the metal-sheet
43、curtain wall (S-2b, as shown in Figure 3(g), the Ueff was 0.49 W/m2K, showing that the heat loss was reduced by approximately 42.8% after installation of the additional insulation. However, heat loss still occurred through the vertical truss, and the Ueff was 1.83 times the Ucode because the thickne
44、ss of the insulation added on the interior of the vertical truss was not sufficient. Discussion As seen from the simulation results, when the thermal insulation performances of the building envelopes are regulated using the thermal transmittance calculated assuming one-dimensional heat transfer, it
45、is impossible to ensure the designed thermal insulation performance in the actual construction. Therefore, it is necessary to develop a method to eliminate thermal bridges depending on the type of envelope through accurate evaluation. In the case of the paint-finished concrete wall with an internal
46、insulation system, linear thermal bridges occur at the construction junctions such as wall-to-floor, wall-to-wall, and wall-to-roof junctions, and the occurrence of such thermal bridges can be prevented by applying an external insulation system in which the insulation is continuous on the exterior o
47、f the concrete wall. However, even if an external insulation system is applied, it is important to minimize the additional heat losses through the metallic fixings used to fix the insulation or exterior materials. In the case of the steel truss curtain wall, the required thermal insulation performan
48、ce is relatively difficult to achieve in actual construction because numerous thermal bridges occur at various fixing and connecting parts. Therefore, to achieve a high thermal insulation performance, a method to eliminate thermal bridges must be taken into consideration. Similar to the stone and me
49、tal-sheet curtain walls, when insulation is installed in the empty spaces between the vertical and horizontal trusses, installing additional insulation on the interior of the trusses is essential; in this case, the thickness of the additional insulation should be sufficient. In the case of the metal-panel curtain wall, further research on methods of minimizing heat loss due to the vertical and horizontal joints between panels is required. For example, Koo et al. (2007) and Song et al. (2008) proposed
