(JBED) - Summer 2006 - The Whole Building Design Guide

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Feature Occupant Thermal Comfort and Curtain Wall Selection By Susan Ubbelohde, UC Berkeley IN SUSTAINABLE BUILDING DESIGN, ANNUAL energy use becomes a primary metric for curtain wall performance. To the extent that the curtain wall allows undesirable thermal transfer, whether through admitting summer solar radiation or through winter heat loss, energy is used to counteract this transfer and keep the occupants comfortable. Current rating systems for sustainable performance, such as LEED, substantially reward reduced annual energy use (USGBC 2002). State building codes similarly emphasize annual energy use as a primary metric of building performance. Data from the state of California, however, indicate that energy costs in state buildings are equal to only five per cent of labor costs for the same buildings. This means that a one per cent increase in productivity of state employees is equal to 100 per cent of the energy costs for the buildings that house those employees. If productivity can be related to occupant comfort, such comfort issues will challenge energy use as a primary measure of building performance. Sustainable buildings are delivering Figure 1 - View of the proposed building from the north indicating the northwest curtainwall and roof-top photovoltaic array (rendering courtesy of the architect). Figure 2 - (a) Full building section cut through NW and SE curtainwalls and (b) Plan for typical office floor. 32 Journal of Building Enclosure Design improved interior environments that result in increased occupant satisfaction, which is then reflected in documented increased productivity, recruitment and retention. A 2003 report on sustainable building (Kats et al. 2003) cites productivity increases of 5 per cent and absentee rates down 40 per cent after employees moved into the renovated VeriFone building in Costa Mesa, California. Similarly, the Firstside Center Building in Pittsburg, Pennsylvania which received LEED Silver Certification, reports increased productivity, reduced absenteeism, better recruitment and significantly lower turnover related to the improved interior environment. The Herman Miller SQA in Zeeland, Michigan, demonstrated increased productivity and significantly increased employee satisfaction over the previous facility (Herwagen 2000). More specifically, occupant thermal comfort is highly valued by office building occupants and research shows that many buildings fall short. Thermal comfort and acoustics are the only interior environmental factors that receive a less than satisfactory rating from 34,000 respondents in the Center for the Built Environment Web-based Occupant Survey (Huizenga 2003 and Abbaszadeh 2006). Thermal comfort and ability to control indoor air temperature has been ranked as the most important and least satisfactory of interior environmental factors in office buildings (BOMA 1999). Occupant thermal comfort became a critical issue in the recent design process for a sustainable San Francisco, California office building (Figure 1). During the schematic design phase, analysis identified cooling loads as a major factor in building performance. Specifications related to cooling loads and energy use (Solar Heat Gain Factor, SHGF, and Visual Transmission, Tvis) became a prime factor in glazing decisions. However, as the design team moved in the design development and contract document phases, occupant thermal comfort during winter months became a critical issue in the final curtain wall design and specification. SCHEMATIC DESIGN: COOLING LOADS AND GLAZING SPECIFICATIONS This sustainable office building of 267,000 square feet was designed for a height constrained downtown site. The 11- storey building fills out the volume of the site, with typical floor-plates that average 140 feet by 184 feet around a central core (Figure 2 a and b). This develops deep office bays of 63 feet on the northwest and southeast orientations, with narrow bays 34 feet deep on the northeast and southwest. The floor to ceiling height, and therefore the head height for the daylight glazing, is held to 9 feet 10 inches due to height restrictions on the site and the deep beams necessary for the long spans. The northwest curtain wall is canted and fully glazed, while the northeast and southeast corners are developed as punched openings to reflect the design of the 1977 building to the northwest owned by the client. With the project tightly bounded in terms of building massing and orientation, the design team focused quickly on curtain wall performance issues. To identify the relative factors in building energy use (for example, heating, cooling, lighting, solar radiation) DOE2.1e modeling software was used for parametric analysis of envelope thermal loads. The input model described the entire building and was well defined with architectural detail that included surrounding site shading. A California Energy Code Title 24 compliant envelope was developed for the base building. Hour-by-hour simulations of nine zones per floor and ten floors of offices were run with the San Francisco Typical Meteorological Year (TMY) climatic data (California 2005). While San Francisco is undeniably a temperate climate zone, local conditions of fog, winter rains, cooling winds off the Pacific Ocean, and three to five day periods of “heat storms” caused by winds
from the California Central Valley provide a varied set of conditions and extremes for building performance. The initial runs, without lighting and mechanical systems, isolated and quantified heating and cooling loads attributable to the building envelope. Heating loads were relatively low, with the northeast and northwest zones demonstrating the highest demand. The southeast curtain wall developed the highest cooling loads due to solar radiation. The southwest façade benefited from some site shading conditions and the other orientations were less challenged by solar exposure (Figure 3). The overall annual cooling loads led the design team to place a high priority on daylighting and shading in the curtain wall design. A series of DOE2.1e parametric simulations for each orientation were run, using solar loads to compare and evaluate glazing types and shading strategies: spectrally selective glazing versus standard, exterior overhangs, interior light shelves, horizontal louvers, mullion caps, canted glazing, etc. Additional physical modeling using a mirrorbox artificial sky quantified the performance of two types of light-redirecting glass: insulated glass with specular internal louvers for the northwest clerestory glazing and lasercut prismatic glazing for the southeast clerestories (Figure 4). By the end of Schematic Design, the high performance spectrally selective glazing with redirecting clerestory glazing was selected. Shading devices for all orientations were designed to optimize daylighting performance and minimize summer cooling loads. With the cooling loads optimized, annual energy use simulations were developed to compare the performance of mechanical system options. A chilled water system with thermal storage was selected as it delivered the least annual operational cost and required the least kWh. DESIGN DEVELOPMENT: WINTER THERMAL COMFORT As the design moved into the DD phase, the design team and the owner were interested in eliminating the perimeter heating system. This would require maintaining the interior surface temperature of the curtain wall close to air temperature so that occupants near the window wall could remain comfortable, including on cold winter mornings during building start-up hours. By decreasing the overall U-value and thermally breaking the frame of the curtain wall assembly, the interior surface temperatures of the curtain wall could be controlled and the mechanical system would not be required to make up for radiant loss by the occupants to the building envelope. A full team including the architects, curtain wall contractors, glazing manufacturers, the construction administrator, the general contractor, daylighting and energy consultants, and a cost consultant developed a range of curtain wall options for consideration. The glazing specifications that impact cooling performance and daylighting (SHGF and Tvis) were consistent for all options, while the glass configuration, frame design, overall U-value and costs varied across the options. The U-values of the curtain wall assemblies are described in Figure 5. The final seven options were then evaluated by the metric that is well known and understood—annual energy use. The results (Figure 6) were convincing in terms of the contribution that daylighting would make to energy savings and the performance of the light redirecting glass. However, annual energy use did not make a clear case for any one of the curtain wall options, except to identify the worst performing (and least expensive) which had no thermal break in the frame and the best performing (and most expensive) option. Since these options had developed in large part due to concerns about occupant thermal comfort, it made sense to try to quantify the comfort performance of the seven curtain wall assemblies. Comfort is a much more difficult performance characteristic to quantify than energy use. Comfort is most evident when it is absent; discomfort causes complaints, reduced performance and local or individual modifications to a workspace such as sweaters, space heaters, desk fans and aluminum foil over windows. ASHRAE Standard 55 is the accepted standard for occupant thermal comfort in nonresidential buildings. Comfort is quantified by a calculated Predicted Mean Vote (PMV), an index that predicts the mean value of the votes of a large group of people on a sevenpoint thermal sensation scale. Additionally, the Predicted Percentage of Dissatisfied (PPD) quantifies the percentage of thermally dissatisfied people (ASHRAE 1992). The analytical process developed to predict winter comfort conditions throughout MBTU 2.50 2.00 1.50 2.00 .50 25 20 15 10 5 0 0 Visionwall Cooling Loads Typical Floor 1 2 3 4 5 6 7 8 9 10 11 12 Visionwall Kawneer 7500 w/ 1.25" Alpen Product Data Building Geometry Fishey LOISOS + UBBELOHDE WW Improved w/ 1.25" Alpen Month Figure 3 - Typical floor cooling loads. Figure 4 – Test cell of southeast façade mounted in artificial sky for tests. 1. Front Silvered Mirror 2. Redirecting Film 3. Building façade mounted in the side of Artificial Sky 4. Light Meter Array. 71 71 69 68 67 66 65 64 63 62 61 60 59 58 Kawneer 750 w/ 1.25" Alpen WW Improved 1.25" Alpen U-Values WW Improved w/ 1" Alpen Kawneer 7500 w/ VEI-2M WW Improved w/ VEI-2M ANALYTICAL PROCESS Window 5 LBNL Therm LBNL + Partners Surface Temperatures Energy Use Glazing Performance Mean Radiant Temp WW Improved w/ 1" Alpen Kawneer 750 w/ VEI-2M WW Improved w/ VEI-2M UCB Comfort UC Berkeley- ASHRAE PMV - PPD Radiant Asymmetry 6000 5000 4000 3000 2000 1000 0 WW Standard w/ VEI-2M Figure 5 - U-values of the seven curtain wall options. KBTU/Sq Ft/Year WWStandard w/VEI-2M kCal Summer 2006 33 South East (8) West Corner (1) South West (5) North West (2) North east (4) South Corner (7) East Corner (6) Window-Assembly U-val Frame U-val Glass U-val No Okasolas Serraglaze Okasolar Serraglaze Included Figure 6 - Annual energy used of the seven curtain wall options, with and without daylighting contributions and light redirecting glazing. Weather Data (San Francisco) DOE2 LBNL + Partners Air Temp Humidity Air Motion Metabolic Rate Clothing Figure 7 - Analytical process used for thermal comfort modeling of selected office bays.
Page 1: JBED Summer 2006 Journal of Buildin
Page 6: Message from NIBS David A. Harris,
Page 10 and 11: Feature Dynamic, Integrated Façade
Page 12 and 13: 1. COMMERCIALLY AVAILABLE SOLUTIONS
Page 14 and 15: procure them for the final building
Page 16 and 17: obstruction and/or daylight reflect
Page 19 and 20: Feature All That Glass Is there an
Page 21 and 22: façade at the street corner. It is
Page 23 and 24: cess to the perimeter, concrete str
Page 26 and 27: Feature Architectural Glazing for S
Page 28 and 29: could consist of lites that have di
Page 30: esidents. Another concern from the
Page 37 and 38: Feature Window Comfort & Energy Cod
Page 39 and 40: By Valerie L. Block and Tammy Amos,
Page 41 and 42: Feature How Does Fenestration Fit I
Page 43: Council (ICC), one might expect tha
Page 46 and 47: Industry Update By James C. Benney,
Page 48 and 49: BEC Corner HOUSTON-BEC By Andy MacP
Page 50: Buyer’s Guide AIR BARRIERS Dupont

(JBED) - Summer 2006 - The Whole Building Design Guide

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