Domestic and global demands for energy are on the rise as the population and the global economy continue to grow. As a significant portion of the US energy demand is used to heat, cool and light buildings, the International Code Committee (ICC) and ASHRAE have been increasing the requirements for minimum energy efficiencies over the past several decades as a way to curb energy demand in this sector.
Stivers School for the Arts using Canyon Blend Velour and Beacon Gray Velour brick
Upon reading the article you will be able to:
- Quantify the effects of building envelope design on overall energy conservation in masonry-walled school buildings
- Quantify the effects of other systems such as lighting and HVAC system design on the overall energy efficiency and cost effectiveness of design
- Demonstrate energy modeling path to energy code compliance as an alternative to R-value table prescriptive method
As example, the sustainable and energy efficient design of school buildings has been a significant focus of the design community in the past few years. This effort has culminated in a number of design guidelines such as the USGBC’s Leadership in Energy and Environmental Design (LEED) for Schools-New Construction and Major Renovations. There have also been significant efforts directed at designing Zero Net Schools [DOE, 2017]. Interviews with architects, engineers, school facility managers and contractors have identified concerns related to the higher first costs of energy efficient school designs and as well as questions related to potential higher maintenance costs, fire resistance and indoor environmental impact of newer materials and systems.
Masonry Prototype Serves as Baseline Energy Design
In response to this, a study on Cost Effective Energy Efficient Design of Schools in the Kentucky climate was conducted [McGinley, 2012]. The goal was to develop a list of low life-cycle cost systems that can be used for energy efficient school designs in Kentucky. Specifically, the study focused on building envelope systems, day-lighting, and heating and cooling system configurations that have been, or could be, incorporated into school designs. To answer which systems have the greatest effect on building energy use, a prototype school design was developed. This prototype was developed based on a design published on the School Design Clearinghouse website developed by the North Carolina School Planning Section of the North Carolina Department of Public Instruction [NC CLEARINGHOUSE, 2008]. Figure 1 shows the first floor plan of the school, including the gym, auditorium and cafeteria.
The prototype was selected so that it had both single-story and two-story sections and incorporated aspects typical of both high school and elementary schools. The 158,000 sf prototype design was then modified to serve as the baseline school design.
Typical of school construction in Kentucky and throughout much of the country, an exterior brick and block masonry cavity wall system was used for the baseline prototype school design.
Typical of school construction in Kentucky and throughout much of the country, an exterior brick and block masonry cavity wall system was used for the baseline prototype school design. These systems have been used extensively in school design as they are durable, aesthetically pleasing, have low sound transmission, have a high resistance to fire, are low maintenance, and are quite cost effective, especially if designed as load-bearing masonry. To provide the minimum base line allowed by code at the time of investigation, the baseline building was designed to prescriptive requirements described in ASHRAE 90.1 ANSI/ASHRAE/IESNA Standard 90.1-2004 Energy Standards for Buildings; Except Low- Rise Residential Buildings [ASHREA 90.1, 2004].
Brick veneer steel stud wall system needed much higher R-values to give comparable performance to thermal mass from CMU backup
To meet the prescriptive thermal properties listed in the ASHRAE standard, the exterior wall construction was assumed to consist of 4" red clay masonry brick, a 1" air space, 1 ¼" polystyrene rigid insulation, and an 8" concrete masonry unit backing wall (CMU). It was also assumed that all the interior walls were 8" hollow CMU, since this is quite common for school design.
Sloped roof construction was assumed to consist of a standing seam metal roof system, asphalt impregnated building paper, 3" polyisocyanurate insulation and steel framing at 2' spacing. Flat roofing construction was assumed to be a white single ply roofing material, 3" polyisocyanurate insulation, and steel framing at 2' spacing. Ceilings were assumed to consist of lay-in acoustic tile with no batt insulation. It was also assumed that there are two types of doors, steel urethane foam core and single pane glass doors. Windows are assumed to be clear double pane operable windows with the code minimum allowed thermal transmittance (U) values.
Variation of each of the building systems were incorporated into a typical prototype middle school configuration, the effect each system has on the overall energy used over the life cycle of the building was determined using the eQuest (DOE2) analysis program, for five typical Kentucky climates. Specifically, a variety of alternate wall systems considered for the study included ICF walls with brick and gypsum board finishes, steel stud walls with brick and gypsum board finishes, each with varying types and thicknesses of insulation. Designers have multiple insulating strategy options available for cavity wall designs that will achieve optimum thermal performance results, beyond those evaluated, including systems using injection foam or polyurethane spray foam insulations. Conventional materials and construction practices were used where feasible and differential costs were developed for each system variation. Additionally, for all climate zones, an incremental construction cost estimate was conducted on the building designs. All costs were obtained using RSMeans construction data base (2011). Incremental construction costs for these alternative building systems relative to the code prescriptive baseline configurations were calculated with simple payback (capital cost/yearly energy cost savings) and self-funding analyses performed.
Figure 2 shows reduction in yearly energy used based on building area Energy Used Index (EUI) for selected building configurations. This plot clearly shows that increasing thermal resistance (R) of mass masonry and concrete walls much beyond the code minimum values did not significantly decrease the yearly energy use in typical school buildings. The same can be said for the effects of air barriers (beyond the code minimums) and high thermal resistance for roofs, windows and doors. It should be noted that performance of the ICF wall system and masonry cavity wall systems with comparable insulation is quite similar. Furthermore, the low mass, brick veneer steel stud wall system needed much higher R-values to give comparable performance to the CMU backup, when thermal mass was accounted for.
Simple payback periods for increases in thermal resistance of walls, roof, windows and doors are typically more than 100 years. The payback period is determined by dividing incremental capital costs by the yearly energy cost savings, but does not include maintenance, interest and energy price increases. (Figure 3). For Kentucky, southern Indiana and Ohio climates (Zone 4), optimum R-values for masonry cavity wall systems appear to be in the R12 to R14 range.
As shown in Figure 4, more efficient mechanical equipment provide significant yearly energy savings. Savings of 60% – 70 % can be obtained using conventional VAV systems with chillers and boilers, coupled with aggressive controls. Note that conventional HVAC systems and controls have simple payback periods less than 2.5 years compared to 23 years for ground source heat pumps. Aggressive control systems also significantly impact energy use and typically have low payback periods (less than 2 years), as do more energy efficient lighting systems. However, the electrical energy savings proved by more efficient lighting is partially offset by the increase in heating energy needed.
To examine whether the insensitivity of yearly energy use to envelope improvements was specific to the Kentucky climate (ASHREA Climate Zone 4), additional analyses were conducted. The base building model was evaluated using hourly weather data from Dallas TX (hot), Miami FL (hot and humid) and Madison WI (cold). Select envelope improvements Energy Conservation Measures (ECM) were also evaluated. Yearly energy savings based on the baseline building are presented in Figure 5 for each ECM. For comparison purposes the yearly energy savings for Covington KY and Paducah KY are also shown in Figure 5.
** Potential lower initial cost ignores structural steel frame costs and probable condensation and maintenance issues.
Evaluation of the results in Figure 5 shows that increases in envelope thermal resistances can actually increase the energy used in hot climates (Dallas and Miami). In colder climates, the quantity of energy saved on a yearly basis increases modestly compared to Kentucky climates. Changes in MEP systems gave similar results to that of the Kentucky study, with increases in energy efficiency of the HVAC and light producing higher reductions in energy use and lower payback periods.
Results of the study illustrate that traditional, reasonably well insulated, masonry cavity wall systems can be used in cost effective energy efficient school designs.
Based on investigation, the following conclusions can be made:
- Analyses (and experience) show that majority of the energy used in school facilities is associated with the heating, cooling and lighting system. Optimum building system design must address the holistic behavior of the facility.
- Envelope improvements beyond code minimums in the prototype school design typically reduced yearly energy used by less than 1.0%.
- For configurations studied, simple payback periods for envelope improvements are typically in excess of 100 years.
- Large decreases in yearly energy use are produced by changes in the HVAC systems.
- For configurations studied, simple payback periods of HVAC changes were generally far less than those of the envelope improvements.
- For hot climates, envelope improvements in prototype school design typically increased the yearly energy used.
- For colder climates, envelope improvements in the prototype school design have typically decreased yearly energy by a larger percentage than that shown for typical Kentucky climates, but always less than 3% for the configurations evaluated by the study.
Results of the study illustrate that traditional, reasonably well insulated, masonry cavity wall systems can be used in cost effective energy efficient school designs. Little change in energy performance will be seen by increasing insulation levels past code minimums. For the best bang for the buck, energy efficient school designs need to focus on holistic design. The building systems must be designed holistically to accurately predict building performance, and optimize the system’ s design. The efficiency of the HVAC, lighting and control systems most dramatically impact overall building performance.
Primary funding for this investigation was provided by the Kentucky Renewable Energy Consortium. Some of the follow-up work was also funded by the Indiana-Kentucky Structural Masonry Coalition.
BONUS Full report can be read at dynamicsofmasonry.com
Dr W Mark McGinley, PhD, PE, FASTM, professor and Endowed Chair for Infrastructure Research in the Civil and Environmental Engineering Department at the JB Speed School of Engineering, University of Louisville, is a structural engineer and building scientist with more than 25 years of research and forensic engineering practice in building systems. He is a recognized expert in masonry building systems and envelopes. More than 120 publications have resulted from his research.
McGinley is active in The Masonry Society and currently chairs the Flexure, Axial & Shear Subcommittee of TMS 402/TMS 602 Building Code Requirements and Specification for Masonry Committee. He has been a primary author of the TMS Masonry Designers Guide and has served on several committees. He is actively involved with ASTM committees C12 and C15 and currently chairs the Subcommittee on Lab Accreditation and task groups on Bond Wrench Testing Apparatus and Field Evaluation of Mortars. McGinley received his PhD, MSc and BSc in Civil Engineering from the University of Alberta. firstname.lastname@example.org | 502.852.4068
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