Heat transfer and energy use
In June, the American Society of Civil Engineers (ASCE) published a study that addresses methods for designing high-performance façades from the perspective of heat transfer and associated energy usage. (Aksamija and Peters published their study, “Heat Transfer in Façade Systems and Energy Use Comparative Study of Different Exterior Wall Types,” in the Journal of Architectural Engineering in June 2016.) It compares the thermal performance of four exterior façades:
- a brick cavity wall with metal framing;
- a rainscreen with terra cotta cladding;
- another rainscreen with glass fiber-reinforced concrete (GFRC) cladding; and
- a curtain wall assembly.
In addition to these basic systems, the study also evaluated three thermally improved systems, including curtain walls with thermal breaks, terra cotta and GFRC rainscreen façades with thermal spacers, and terra cotta rainscreen façades with thermal isolators. Using software that provided two-dimensional, steady-state heat-transfer simulation, the study assessed the systems using thermal and energy modeling. (The software used was THERM 6.3 by Lawrence Berkeley National Laboratory in Berkeley, California.)
The systems were studied in conditions that modeled different climate zones across the United States, including climate extremes, with representative exterior temperatures of 32 C (90 F), 16 C (60 F), –1 C (30 F), and –17 C (0 F), and with constant interior temperatures of 22 C (72 F). U-values (i.e. heat-transfer coefficients) were calculated for each system using heat-transfer simulation software. In all configurations, rainscreens using terra cotta cladding with thermal spacers and those using terra cotta cladding with thermal isolators exhibited the lowest U-values, outperforming all other façade systems in the study.
Energy use
The study also analyzed these systems for energy use. (To do this, the U.S. Department of Energy’s EnergyPlus 8.3 software was used.) It was designed to determine the best performance for whole-year total energy use for a defined office space, featuring examinations of five opaque walls. However, since typical office walls are seldom opaque, configurations of opaque walls with window-to-wall ratios of both 20 and 40 percent were modeled in each of the studied cladding systems. Parameters were evaluated for whole-year energy use and included heating, cooling, and lighting. The study also incorporated a range of building orientations, including 12 different orientations at 30-degree increments, using climate models for 15 different U.S. cities.
The study demonstrated wall types with the lowest U-values typically provided better whole-year energy performance in all climates and at all orientations. Again, terra cotta rainscreens with thermal spacers and those with thermal isolators—including both terra cotta and GRFC cladding systems—outperformed all wall assemblies studied, with the lowest heating energy demand and highest thermal resistance. Terra cotta and GRFC cladding systems also exceeded other systems’ performances when it came to cooling energy demand, likely because of terra cotta’s high thermal resistance. Curtain walls outpaced brick cavity walls in terms of energy demand, but performed worse than any system when evaluated for cooling energy consumption and whole-year energy performance.
Compared to other façade cladding solutions, the initial costs for terra cotta façades begin in the mid-range. This type of cladding outperforms other products in both lifetime expectancy and maintenance costs.
