energies Article Insulation Performance Comparison of Curtain Wall Systems with Existing Pipe Frames and Truss-Shaped Insulation Frames Bo-Hye Choi and Seung-Yeong Song *   Citation: Choi, B.-H.; Song, S.-Y. Insulation Performance Comparison of Curtain Wall Systems with Existing Pipe Frames and Truss-Shaped Insulation Frames. Energies 2021, 14, 4682. https://doi.org/10.3390/ en14154682 Academic Editors: Monica Siroux and Francesco Nocera Received: 25 May 2021 Accepted: 26 July 2021 Published: 2 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Architectural & Urban Systems Engineering, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Korea; bhchoi@ewha.ac.kr * Correspondence: archssy@ewha.ac.kr; Tel.: +82-2-3277-3913 Abstract: The purpose of this study was to compare insulation performance between a base case that applied the existing steel pipe frame and an alternative case that reduced thermal bridging by apply- ing a truss-shaped insulation frame (TIF) to a back frame type curtain wall. Insulation performance was compared by obtaining the effective U-factor and the lowest indoor surface temperature through a three-dimensional steady-state heat transfer simulation. In addition, mock-up tests were performed to compare the U-factors of the base case and alternative case. The simulation results showed that the effective U-factor of the alternative case was 36% lower than in the base case, a significant heat loss reduction. The lowest indoor surface temperature of the alternative case was 0.5 ◦ C higher than in the base case, showing that the surface condensation risk also decreased. In the mock-up test results, the alternative case U-factor was 33% lower than in the base case, confirming the associated large heat loss reduction. For the base case, both the effective U-factor by simulation and the U-factor by the mock-up test were much higher than the design U-factor according to the Korean Design Standard, which neglects thermal bridging, indicating a significantly increased heat loss caused by this factor. For the alternative case, however, both U-factors were similar to the design U-factor. Keywords: thermal insulation; thermal bridge; curtain wall; truss-shaped insulation frame 1. Introduction 1.1. Background and Objective Reducing greenhouse gas (GHG) emissions is a global issue, and the international com- munity has made efforts to reduce GHG emissions at the global level. The Paris Agreement signed in 2015 dealt with measures to reduce GHG emissions after 2020, and the South Korean government also established and announced the “Revision of the basic roadmap for achieving the national greenhouse gas reduction target in 2030 [1]” by reflecting the Paris Agreement. In the case of the building sector, which represents 22% of GHG emissions from South Korea as of 2017, a reduction target of 32.7% compared to the expected GHG emissions by 2030 was set. In addition, the reinforcement of building permit standards for new buildings, improvement of the energy performance of the existing buildings, im- provement of equipment efficiency, and expansion of the distribution of renewable energy systems were presented as the main measures to reduce GHG emissions. Accordingly, zero energy building certification has become mandatory for new public buildings with a total floor area of 1000 m 2 or larger since 2020, and the certification will be mandatory for all new buildings with a total floor area of 500 m 2 or larger after 2030 [2]. Thermal insulation of the building envelope is a very important element for zero energy buildings. The building envelope that requires thermal insulation can be mainly classified into the external wall, roof and floor. Since the external wall generally represents the largest area in buildings, the insulation performance of the external wall is particularly important. Even though there are some differences depending on the construction method, most external walls have thermal bridges in which insulation is partially discontinued. The Energies 2021, 14, 4682. https://doi.org/10.3390/en14154682 https://www.mdpi.com/journal/energies