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Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
The importance of a common method and correct calculation of thermal bridges
Björn BerggrenLund University, Energy and Building Design
Founded by:
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Agenda
• Introduction• Theoretical background• State of knowledge• Example of consequence• Conclusions
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
INTRODUCTION
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Introduction
• Recast of the Energy Performance of Buildings Directive, EPBD– By the end of 2020 all new buildings shall be “nearly zero-
energy buildings”• Approach to design robust and energy efficient residential
building in a Nordic climate:
Source:SINTEF Byggforsk - Husbanken
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Introduction
• EPBD, recast:– The methodology for calculating energy performance should
be based not only on the season in which heating is required, but should cover the annual energy performance of a building. That methodology should take into account existing European standards.
EN ISO
13790
EN ISO 6946 EN
ISO 7345
EN ISO
10077
EN ISO
10211EN ISO
13370
EN ISO
14683
EN 15242
EN ISO
13789
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
THEORETICAL BACKGROUND
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
EN 13789 – Heat transfer coefficients
• Transmission heat transfer coefficient:
• Where– HD direct heat transfer coefficient– Hg ground heat transfer coefficient– HU heat transfer coefficient, unconditioned spaces– HA heat transfer coefficient to adjacent buildings
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
EN 13789 – Heat transfer coefficients
• Direct heat transfer coefficient:
• where– Ai area of element i of the building envelope [m2] – Ui thermal transmittance of element i [W/m2K]– lk length of linear thermal bridge k [m]– k linear thermal transmittance of thermal bridge k [W/mK]– j point thermal transmittance of point thermal bridge j [W/K]
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Application of EN ISO 13789
• Clearly divide the building envelopes into different elements:
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Application of EN ISO 13789
• EN ISO 13789 allows for measuring of elements according to one of the three methods
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
EN ISO 10211 – Thermal bridges in constructions
• Linear thermal bridges
• Point thermal bridges
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Important
• The sum of transmission losses through building elements, the term AiUi, will vary depending on the chosen measuring method
• Consequently, the thermal bridges, k-values and j-values will vary
• Subscripts to clarify used method for measuring
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
STATE OF KNOWLEDGE
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
The survey
• Sent to 100 engineers and architects with experience of building projects with focus on energy efficiency
• The survey– Four questions about measuring methods used to
define thermal transmitting area in energy calculations– Six questions, qualitative assessment of different
junctions– Eight questions about professional background
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Junctions
EXT. INT.
EXT. INT.
EXT.INT.
EXT.INT.
EXT.INT.
Insulation Concrete Wood
EXT.INT.
JUNCTION 1 – J1 JUNCTION 2 – J2 JUNCTION 3 – J3
JUNCTION 6 – J6JUNCTION 5 – J5JUNCTION 4 – J4
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Results
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Results
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
EXAMPLE OF CONSEQUENCE
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Heat transfer is not 1D
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Heat transfer is not 1D
0%
10%
20%
30%
40%
50%
60%
70%
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
3,00 5,00 7,00 9,00 11,00
Per
cent
diff
eren
ce [%
]
[W/m
·K]
Heat resistance in wall [m2·K/W]
J7-1D
J7-2D
J8-1D
J8-2D
%-Diff J7
%-Diff J8
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
CONCLUSIONS
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Conclusions
• State of knowledge is not satisfying• Important when we are increasing the use of BIM tools –
which may provide quantity take offs for energy simulations• Subscripts (i, oi, e) should always be used• It seems as consultants does not always understand that a
thermal bridge occur due to difference between internal and external areas
Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere
Thank you for the attention!
And thanks to everyone who took the time and answer the questionnaire.