Gandhi-Sridharan-and-TsayJacobs-Andrew
ABSTRACT
Facades developed in response to climactic factors increase performance and human comfort while reducing energy loads. A single building envelope will perform differently in different climates. Different assembly types perform differently from one another in a given climate. However, the time consuming process of evaluating the performance of a range of envelope options using multiple software programs is a significant hurdle, resulting in projects defaulting to regional traditions. A simple process for determining the most energy efficient assembly in any given climate is lacking. However, this process can be achieved by using a generalized computational design workflow that is platform agnostic. This research presents a generalized workflow designed to make climate oriented façade selection simple. With ASHRAE 189.1 as a basis for selecting R values for climates and assembly types, the performance of five façade systems are compared against each other in eight Climate Zones of North America. Facade systems compared include: glass-fiber reinforced concrete on metal frame, metal panel rain screen over cross laminated timber, exposed precast, and metal rain screen over structural insulated panel. For consistency, the facades are deployed onto a prototypical classroom building called ‘Sprout Space’, designed by Perkins+Will. The results indicate those assembly types that have the highest performance in each Climate Zone. The workflow developed for modeling (Rhino and Grasshopper) and analyzing the energy performance of the façade (WUFI, THERM and Honeybee) assemblies is explained. The influence of the building geometry on results is discussed. The influence of ASHRAE 189.1’s baseline R values on results is discussed. The ability to identify the highest thermal performance façade system within each climate. The workflow can enable facade consultants, engineers, and designers to understand the behavior of the different envelope types in each climate, leading to the selection of higher-performance facades.
KEYWORDS
precast, CLT, SIP, GFRC, energy efficiency, condensation – moisture – rain – vapor, design processes, parametric workflows, computational design, academic/industry partnerships, pre-fabrication, climate, THERM, WUFI, Honeybee
INTRODUCTION
Hypothesis: Computational tools can assist designers to automate the identification, analysis, and selection of prefabricated building envelopes based on climate-specific requirements.
The building envelope is a protective layer that separates the outdoor environment and indoor built space (Aksamija, 2013). It is heavily responsible for the energy performance of the building and the aesthetic appearance of the building (Lovell, 2013). The building envelope is the outermost layer of a building and is subject to natural sources like sun, wind, and rainfall. The envelope should be designed in such a way that it responds to the natural sources, provides energy to the building and acts like one of the building services (Van, 2009). The configuration of the materials in the building envelope and their physical properties based on the climate factors helps increase the energy efficiency of buildings. For example, having the thermal mass of an envelope in the inner layer of a building envelope helps in retaining the heat in cold climates; having the thermal mass in an envelope in the outermost layer in climates with a huge temperature range in a single day can help offset the heat during the day and heat the space during the night when it is cold (Balaras, 1996). Another example is that using light weight envelopes in humid conditions can help improve the energy performance of buildings, illustrated brilliantly by bamboo houses in the Philippines (Wimmer, 2013). Additionally, an envelope designed for a certain climate should not be expected to perform the same way when used in another climate. However, to allow common envelope assemblies to be deployed in multiple Climate Zones, an understanding of the adjustments required to make them retain high performance combined with an understanding of alternate assemblies that may perform more efficiently is needed.
Pre-fabrication serves dual purposes within the climate-oriented prefabricated building envelopes (COPBE) method: 1) Reinforce the standardization of envelope assemblies that may require material swapping or thickness adjustments to address different climactic conditions without changing the construction mode or quality. 2) Help ensure the performance assumed during design is achieved in the construction phase, noting that envelopes manufactured off site are proving to have better performance due to construction within a monitored environment (Pang et al, 2005). The COPBE method’s focus on climate-oriented design of standardized but flexible prefabricated systems allows the evaluation and selection of high performance envelopes using digital tools with relatively high reliability.
Through an academic/industry partnership between the University of Southern California and Perkins+Will, the authors collaborated on the design of the methodology and its implementation on a prototypical school classroom to determine the optimal assembly types and configurations in multiple Climate Zones.
BACKGROUND
Various climate-specific design methods are being used by designers. CLIMATE ID (Van, 2009) and CROFT (Bilow, 2012) are concepts that suggest design solutions based on the climate. CROFT concepts are usually identified by analyzing the climate based on the weather data and arriving at a system and enclosure design based on the heating and cooling requirements of that climate. CLIMATE ID chooses the envelope design based on specific functions such as: energy generation, C02 reduction, adaptive, etc. These options vary and cannot be classified based on program. Thus, both CLIMATE ID and CROFT are not related to a specific program and cannot be repeated in a non-similar condition. Another method is explored in ‘Climate specific design envelope’ (Mitterer, 2011), a study that explained the requirements of envelopes inferred from various surveys in different locations. The study provides no specific design solutions, but gives principles that may be used to design a custom façade. Based on background research of these methods, COPBE introduces a new concept to integrate prefabricated envelopes with climate specific design principles. The standardized envelope design should be able to be deployed in similar climate conditions without refinement or redesign. The array of climate-zone-specific envelop assemblies analyzed and ranked in the COPBE method enables users to make informed decisions about assembly performance and selection to best serve their project.
ASHRAE 189.1 minimum R-value requirements for various structural framing options are fundamental assumptions of this research. ASHRAE defines R-values for mass walls, metal building walls, steel framed walls and wood framed walls. Different R-values are required for each of these framing types for each climate zone. In order to compare facades with different framing systems to one another, complete exterior wall assemblies for each system must be developed for a specific climate zone. It is important to note that the assemblies will not likely have the same R-value requirement per ASHRAE because it is assumed that some framing systems have better performance than others due to conductivity, diurnal effects of mass, etc. The COPBE method provides an opportunity to check whether the unique R-values assigned by ASHRAE for each framing type achieve similar performance across multiple framing types within each climate zone.
METHOD
The methodology uses Grasshopper to integrate multiple software programs to evaluate the performance of predesigned building envelopes in various climate contexts. The purpose of COPBE is to assist designers in the envelope selection and identification process. Using the COPBE methodology, a case study was done using the Perkins+Will designed Sprout Space as the prototype building. COPBE was used to rank the performance of four envelopes for eight Climate Zones in North America. The flow diagram in Figure 1 illustrates the steps involved in the COPBE method.
PRE-FABRICATED ASSEMBLY DESIGN
COBPE assumes that a project designer has identified baseline assemblies for evaluation by the COPBE method. Hence, prior to the application of the COPBE method, concept designs for four prefabricated envelope assemblies meeting several design objectives were prepared: Precast concrete envelope, CLT (cross laminated timber), SIP (structural insulated panel), and GFRC (glass fiber reinforced concrete). These assemblies correspond to specific ASHRAE structural types, which have corresponding thermal R-value requirements which differ for each climate zone. Subsequent to the development of the concept designs, a technical design team at Perkins+Will refined the design of each of them based on professional judgment. Finally, prefabricators evaluated the designs and provided comments that were used to improve the designs with respect to constructability, flexibility and cost. This design refinement process is illustrated in Figure 2. The concept design and the refined design for the CLT assembly can be seen in Figure 3. The refined designs for the all four assemblies were used as the baseline configurations for each assembly type that was introduced to the COPBE method. See Figures 4, 5 and 6 for the refined design of the GFRC, SIP and Precast facades.
