Heat Bridges in Building Constructions: Requirements for Sustainable Housing and Solutions

Transcription

1 Heat Bridges in Building Constructions: Requirements for Sustainable Housing and Solutions Gerhard Faninger Keywords: Cause and effect of heat bridges. Calculation of heat transmission losses in buildings. Heat bridges in building connections and windows. Measures to minimise heat bridges. Assessment of heat transmission losses through heat bridges. Methodology for calculation. Results of calculations and conclusions. 1. Introduction Heat Bridges in building constructions result from locally limited areas of a building envelope with raised heat transit. Examples for heat bridges in buildings are: Connection of building materials with different heat conductivity. Larger cooling outside area of a building compared with a smaller heat absorbing area of the inner edge; Figure 1. Heat bridges effect transmission losses as well as condensation problems. 2. Heat transmission losses The transmission losses through the building envelope include heat transfer through the external wall, the outside windows and doors, the highest floor level (roof) and the lowest floor level (basement, cellar), and additional the heat losses through heat bridges in the building envelope: corners, windows, highest floor (roof, attica) and lowest floor (basement, cellar), balconies and other building connections; Figure 2. The heat transmission losses in a building are calculated on the basis of the Thermal Conductance Methodology Leitwertberechnung /1, 2/. The calculation includes the building parts from heated rooms to outside, from heated rooms to non-heated rooms and from non-heated rooms to outside; Figure 3. The calculation from heated rooms to outside is illustrated in Figure 4, and through heat bridges in building connections in Figure 4. The calculation of heat losses through cellars different types and through the basement (non insulated and insulated) is more complex and includes also the heat losses through the different building connections in the cellar area. Figure 5 illustrates the numerical formulation; /2/. The heat losses through heat bridges in building connections are calculated on the basis of numerical formulation for transient and steady-state heat conductions in two and three dimensions, described in /3/. Computer programmes have been developed within the scope of international co-operation in EU-member countries: EUROKOBRA/AUSTROKOBRA; /4/. With this PC-tool, thermal bridges, heat transfer through corners of windows, heat loss from a house to the ground can be analysed, and it is quite easy to solve ordinary construction problems. The tool makes it simple to describe a large range of heat transfer problems. The user will be helped to design the building construction with minimised heat bridges. Figure 6 demonstrates the practical use of the PC-tool. The programme includes a database for typically building construction parts (outside walls, floors, window-frames, balconies, cellars, 1

2 basements, other building connections) in form of defaults for editing the structure (thickness of material layers, horizontal and vertical) and materials (characterised with the heat conductivity λ, W/(m, K) ). The results are temperature lines (isotherm lines)in the building construction with assessment of condensation problems and Ψ values for characterisation of the heat losses through heat bridges. The results of heat bridge analysis in building constructions are illustrated in Figure 7a and 7b (outside wall), Figure 8a and 8b (balconies) and Figure 9a to 9d (windows); /4/. 3. Heat bridges in windows The influence of heat bridges between glass and frame is described with the following equation (ÖNORM EN ): U w = (U g *A g + U f *A f + l fg *Ψ g ) / (A g + A f ) U w : equivalent thermal convective heat transfer coefficient, W/(m², K) U g : thermal convective heat transfer coefficient of glass, W/(m², K) A g : glass area, m² U f : thermal convective heat transfer coefficient of window frame, W/(m², K) A f : frame area, m² l fg : length of heat bridge, related to Ψ g, m Ψ g : correction factor (supplementary thermal conductance value) for 2D-heat bridge between window frame and window glass, W/(m, K). Guide numbers for Ψ g -values for heat bridges between window frame and window glass are shown in Table 1 /2/. The U-values of windows are increased through heat bridges between glass and frame by about 0,05 W/(m², K) (high-efficient windows) until 0,19 W/(m², K) (standard-window). 4. Minimizing of heat bridges in building connections Typically building connections are shown in Figure 10: correct and wrong solutions. 5. Calculation of heat transmission losses through heat bridges The basis for the calculation of heat transmission losses through heat bridges is the methodology of Thermal Conductance Analyses. The influence of heat bridges is described through characteristically supplementary heat conduction values Ψ for typically building constructions. Ψ can be calculated within the PC-tools for heat bridge analyses; /4/. Typically guide numbers are listed in Table 2. The thermal conductance value L HB is calculated with: L HB = Ψ * 1, W/K Ψ = l = supplementary thermal conductance value, related to length, W/m length of heat bridge, m Figure 11 shows an example for calculation. 2

3 The transmission heat losses Q HB through heat bridges are calculated from the supplementary thermal conductance value L HB with /1, 2/: Q HB = (L HB * 3) * 0,024 * HGD/V g, kwh/(m², a) HGD: Heating degree days V g : Volume of heated building area, m³ (gross) 6. Calculation of total heat transmission losses of the building envelope The thermal conductance L e of the building envelope is the sum of all heat transfers through the building envelope: i j L e = A i * U i + Ψ j i=1 j=1 A i : area of outside building envelope, m² U i : U-value of A i, W/(m²,K) Ψ j : supplementary thermal conductance value for heat bridge, related to length j, W/K l j : length of heat bridge j, m The total transmission heat losses Q total through the building envelope are calculated from the thermal conductance value L e with /1, 2/: Q total = (L e * 3) * 0,024 * HGD/V g, kwh/(m², a) HGD: Heating degree days V g : Volume of heated building area, m³ (gross) 7. Results of calculations and summary The results of the calculation of heat transmission losses in housing with different envelope insulation standard are illustrated in Figure 11 (Apartment House), Figure 12 (Row House) and Figure 13 (Single-family Detached House). The contribution of heat bridges to the transmission heat losses are up to 25%. Figure 11c shows the share of the different building parts to the heat transmission for an Apartment House in Low-energy House- and Passive House-standard. The goal/requirement for the heat transmission through heat bridges in Passive Houses is below 4%; Figure 14a and 14b. The influence of heat losses through heat bridges may be high and has to be considered in the design phase. It is strong recommended to investigate some time to find solutions for minimized heat bridges in building connections. 3

4 Literature: /1/ ÖNORM B 8110, ÖNORM EN 1190: Wärmeschutz im Hochbau. Österreichisches Normungsinstitut. Wien.1999 /2/ Energieausweis für Gebäude: Rechenprogramm zur Abschätzung des Heizwärme- und Brennstoffbedarfes von Gebäuden. Ausgabe 8.0, März Gerhard Faninger. Institut für systemische Interventionsforschung und Weiterbildung, iff. Universität Klagenfurt. Sterneckstraße 15, A-9020-Klagenfurt Free download from: /3/ Heat Conduction in Two and Three Dimensions: Computer Modelling of Building Physics Applications Thomas Blomberg. Lund University, Lund Institute of Technology, Department of Building Technology, Building Physics. Report TVBH May 1996 /4/ EUROKOBRA / AUSTROKOBRA: Das EDV Programm für den Baupraktiker Dynamischer Wärmebrücken-Europa-Atlas im PC E. Panzhauser, K. Krek, J. Lechleitner Technische Universität Wien, Institut für Hochbau für Architekten Karlsplatz 13, A-1040 Wien /5/ Wärmebrücken, Luft- und Winddichte Erwin Schwarzmüller Energie Tirol, Adamgasse 4, A-6020 Innsbruck /6/ Wärmebrückenvermeidung Christian Astl, Christina Grembacher, Guido Wimmers, Helmut Crepaz, Karl Auer Energie Tirol, Adamgasse 4, A-6020 Innsbruck

5 Table 1 Ψ-Values for Heat Bridges in Building constructions Two dimension calculation Guide Numbers Building Envelope Standard- House Ψ-Values, W/(m, K) Low-Energy- House Passive- House Highest floor / roof / attic Outside wall / ceiling / floors Outside wall / balconies Outside wall / window frame Outside wall / basement Table 2 Ψ g -values for heat bridges between window frame and window glass Guide number (ÖNORM B ) Window frame Double-and triple glasses, without coating Ψ g -Werte Double-and triple glasses, evacuated, with coating Wood- and plastic frame 0,04 0,06 Metall-Frame 0,06 0,08 Metall-frame with insulation 0,00 0,02 5

6 Figure 1: Heat bridges in building constructions 6

7 Figure 2: Transmission heat losses in buildings Fig. 3: Heat flow through the building envelope 7

8 L e for Building Heated Area to Outside Building Part A B1 (m²) U (W/(m², K))A B1 *U (W/K) Upper Floor/Roof/Attic (1) 409,38 0,100 40,94 Upper Floor/Roof/Attic (2) Outside Wall (1) 833,86 0,100 83,39 Outside Wall (2) Outside Wall (3) Windows Frame 39,80 0,700 27,86 Glass 179,20 0, ,44 Outside Doors Outside Wall, soil-connected 409,38 0,100 8,19 Others (1) Others (2) A B1 1871,62 L e 285,81 L Ψ for two-dimension Heat Bridges Building Part l (m) Ψ (W/(m, K)) l*ψ (W/K) Upper Floor/Roof/Attic 90,50 0,01 0,72 Outside Wall/1 st Floor 271,50 0,01 2,17 Outside Wall/2 nd Floor Outside Wall/3 rd Floor Outside Wall/Balcony Outside Wall/Balcony Outside Wall/Windows 480,00 0,01 4,80 Outside Wall/Doors Basement 90,50 0,05 4,53 Others l 932,50 L Ψ 12,22 Figure 4: Calculation of thermal conductance of the building envelope /2/ 8

9 Figure 5: Heat flow through building basement and cellar 9

10 Figure 6: PC-tool for heat bridge analysis: input data and results 10

11 Figure 7a: Heat flow in outside wall; examples /5/ 11

12 Figure 7b : Heat flow in outside wall; examples /5/ 12

13 Figure 8a: Heat flow in building connections - balconies; examples /5/ 13

14 Figure 8b: Heat flow in building connections - balconies; examples /5/ 14

15 Figure 9a: Heat flow in window frames; examples /5/ 15

16 Figure 9b: Heat flow in window frames; examples /5/ 16

17 Figure 9c: Heat flow in window frames; examples /5/ 17

18 Figure 9d: Heat flow in window frames; examples /5/ 18

19 Figure 10: Minimizing of heat bridges in typically building connections /5/ 19

20 Transmissions Heat Losses Through Heat Bridges Reference Apartment House, 1872 m² /4070 m³ Location: Zurich Building Insulation Standard Standard-House Low-energy-House Passive-House A Passive-House B U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) Upper floor/roof/attic Outside wall Outside window Basement/ceiling Thermal conductance, W/K 195,55 95,06 83,20 12,22 Total transmission losses, kwh/(m², a) 74,46 36,11 23,20 17,93 Transmission losses through heat bridges, kwh/(m², a) 11,76 5,18 5,00 0,735 Q heat bridges / Q total, % 15,80 15,83 21,57 4,10 Figure 11a: Transmission heat losses in housing: Reference Apartment House, IEA-SHC-TASK 28 20

21 Figure 11b: Transmission heat losses in housing: Reference Apartment House, IEA-SHC-TASK 28 21

22 Figure 11c: Transmission heat losses in housing: Reference Apartment House, IEA-SHC-TASK 28 22

23 Transmissions Heat Losses Through Heat Bridges Reference Row House, 748 m² /1254 m³ Location: Zurich Building Insulation Standard Standard-House Low-energy-House Passive-House A Passive-House B U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) Upper floor/roof/attic Outside wall Outside window Basement/ceiling Thermal conductance, W/K 58,62 27,67 22,76 4,62 Total transmission losses, kwh/( m², a) 80,46 40,35 24,12 19,49 Transmission losses through heat bridges, kwh/(m², a) 11,44 5,40 4,44 0,90 Q heat bridges / Q total, % 14,22 13,39 18,42 4,62 Figure 12a: Transmission heat losses in housing: Reference Row House, IEA-SHC-TASK 28 23

24 Figure 12b: Transmission heat losses in housing: Reference Row House, IEA-SHC-TASK 28 24

25 Transmissions Heat Losses Through Heat Bridges Reference Detached House, 470 m² /625 m³ Location: Zurich Building Insulation Standard Standard-House Low-energy-House Passive-House A Passive-House B U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) U, W/(m², K) Ψ, W/(m, K) Upper floor/roof/attic Outside wall Outside window Basement/ceiling Thermal conductance, W/K 37,58 17,32 14,05 3,14 Total transmission losses, kwh/(m², a) 88,72 37,71 23,38 18,22 Transmission losses through heat bridges, kwh/(m², a) 14,72 6,78 5,50 1,23 Q heat bridges / Q total, % 16,59 18,00 23,54 6,75 Figure 13a: Transmission heat losses in housing: Reference Detached House, IEA-SHC-TASK 28 25

26 Figure 13b: Transmission heat losses in housing: Reference Detached House, IEA-SHC-TASK 28 26

27 Figure 14a: Goal / requirement for minimizing of heat bridges in Passive Housing 27

28 Figure 14b: Goal / requirement for minimizing of heat bridges in Passive Housing 28