Evaluation of alternatives for reducing thermal bridges in metal panel curtain wall systems

Significance Statement

Increased discharge of greenhouse gases is now a bigger menace to the environment especially in building sector and thermal bridges which causes adverse heat transfer leading to reduction in the insulation performance of the buildings. So, indispensable green building policies have started preferring zero energy utilization methodologies thereby resulting in improving insulation performance.

There have been lots of studies about the heat losses through different thermal bridges in combination with several walls to evaluate the energy utilization and the insulation performance.

Lately, curtain walls have admired as the envelope of high-rise buildings. For curtain walls, metals with a low thermal resistance are used for the main components. In many countries, including Korea, the non-vision parts of curtain walls are vital to meet a required U-factor (thermal transmittance) of walls, as spelled out in the building codes. Curtain walls for reducing thermal bridges are needed to deal with the enriched insulation performance design criteria to be realistic.

Henceforth researchers led by Professor Seung-Yeong Song from Ewha Womans University in South Korea suggested three possible cases that substitutes thermal bridges in steel truss metal panel curtain walls. In this metal panels are counterfeited by covering six faces of insulation with metal. Then they are fixed to the truss and they are compared for its economic feasibility and thermal functioning through three-dimensional steady-state heat transfer simulations.

 In prevailing system, Bracket-based fixing method is extensively preferred which makes construction more convenient. The metal panels can be fixed at only two points when the size of horizontal metal panel is less than two meters.  If it exceeds two meters, then the metal panel can be fixed at more than three points. For these types, both vertical and horizontal trusses are needed. For steel truss metal panel curtain wall systems, the main linear thermal bridges occur at the vertical and horizontal joints between the metal panels, where the insulation is non-continuous.

Slim metal panels (Vacuum insulation panels) used for designing purpose are susceptible by sharp objects. In order to protect and assuage linear thermal bridges at the joints, insulation is done by 2 layers by phenolic foam and vacuum insulation panel.  The front and rear surfaces of a metal panel are made of steel sheet. The upper, lower, right, and left edges of the metal panel are formed of aluminum moldings in which there is a groove for inserting the bracket. Polyurethane thermal breaker is amalgamated into the aluminum molding.

The researchers suggested the first possibility case which is described as follow: The length of the prevailing aluminum molding is reduced by one-third to lighten linear thermal bridges resulting from the aluminum moldings at the joints between metal panels. The remaining two-third of the metal panel thickness was fitted with a PVC-foam pad to ensure air-tightness between the metal panels and to prevent the exposure of insulation in the metal panel.

 In the second possibility case, a thermally broken bracket replaces the aluminum bracket of first case. The difference between the thermally broken bracket and the aluminum bracket is the polyurethane thermal breaker which is not inserted into the thermally broken bracket. It will reduce the trappings of point thermal bridges caused by brackets.

In the case of third possibility, its aluminum molding is same as to second possibility except that a polyurethane thermal breaker is inserted into the molding to further reduce the heat loss. They used Midas Information technology – a structural analysis program to investigate the structural performance of the thermally broken bracket. The insulation performance is evaluated using a three-dimensional steady-state heat transfer simulation program which is used to solve heat equation.

Finally, after analysis of annual energy costs, evaluation of economic feasibility the validity of the simulated results was investigated by comparing the authors’ alternatives and existed cases to the mock-up test results. In addition to this, heating and cooling energy uses were also calculated by considering the efficiency of the heating and cooling system. With all these analyses, this study finally brought up a beneficial and green zero energy building infrastructure designs that reduces linear bridges, which is feasible and cost effective.

    

About The Author

Seung-Yeong Song, Professor, Ewha University, Seoul, South Korea.

The corresponding author of the paper, Dr. Seung-Yeong Song, is currently a full professor in the Department of Architectural Engineering at the Ewha University, Seoul. Professor Song received his BS, MS and Ph.D degrees from Seoul National University and was a visiting scholar at the University of New South Wales, Sydney.

He has published more than 120 technical papers in peer-reviewed scientific journals and several research monographs. He is currently leading several research groups on the building energy, funded by the Korean Government, as a principal investigator.  

Journal Reference

Jin-Hee Song , Jae-Han Lim , Seung-Yeong Song. Evaluation of alternatives for reducing thermal bridges in metal panelcurtain wall systems. Energy and Buildings,Volume 127,  2016, Pages 138–158.
Department of Architectural Engineering, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, South Korea