How to get Shading Right
Recent improvements to window technology make substantial air conditioning energy savings possible. However, shading remains a time-tested method to accomplish the same end.
Architectural and site shading can have an even greater impact on reducing daily cooling use than upgrading windows. Using both a prototype building and modeling simulations, a study by our team at the National Renewable Energy Laboratory (NREL) evaluated the relative impacts of different solar load control strategies. The study points the way toward optimizing the interaction of various methods for reducing solar heat gain. This is helpful,as the wide variety of shading options currently available can make choosing an effective solar load control strategy confusing (see Snapshots of Shading Options,HE Sept/Oct '00,p.20).
The prototype house was built in 1998 in Tucson, Arizona, as part of the Department of Energy's Building America program.Its integrated package of energy-saving features includes structural insulated panels (SIPs) for the wall and roof construction, white coating on the roof, spectrally selective windows, architectural shading, an interior location for the air handler and ducts, highefficiency air conditioning equipment, and solar water heating.
Building America (see Clean Breathing in Tract Homes,p. 29) works with five building industry teams to produce advanced residential buildings on a community scale. Systems incorporated into these houses are evaluated by conducting successive design, test, redesign,and retest iterations until cost and performance trade-offs yield innovations that can be cost effectively implemented in production-scale housing.RGC CourtHomes,Inc., built the prototype, with input from the IBACOS Building America Consortium.
Building performance was modeled using a detailed hourly energy simulation tool and was measured while the building was unoccupied for a period of 12 days. Model inputs included direct measurements of the net air exchange rate, surface reflectance,and window transmittance. Model results, after calibration,showed good agreement with the direct measurements of cooling loads and air conditioning energy use. Analyzing the interactions between building performance and solar load control strategies in a prototype house can facilitate the optimization of cost and performance trade-offs in large-scale production.
Typical new productionscale houses in the Tucson market are framed on a slabon- grade foundation with stucco exterior finish and a sloped concrete tile or flat built-up bituminous roof. These standard-practice houses are built using standard construction materials and techniques, including nominal 2 x 4 wood framing,fiberglass batt insulation,1-inch polystyrene sheathing,and double- pane,clear-glass,aluminum frame windows. The slab foundation has no insulation,and the attic is usually vented. A forcedair distribution system provides space heating and cooling,with the air handler located in the garage and the flex duct in the attic. This system is typically supplied by a 10-SEER air conditioner and an 80% annual fuel utilization efficiency (AFUE) gas furnace.
The prototype house differs from these houses in several respects. It has a well-insulated airtight envelope, with minimized air distribution losses. The house's energy-saving features, plus the large ratio of window to floor area in the design, render window contributions more important than they are in conventional housing, particularly in the Tucson climate.
The prototype incorporates several reengineered features into its structural and mechanical equipment systems (see Table 1). Envelope changes include a sealed, insulated, and conditioned crawlspace foundation (a shallow basement) and SIPs (see SIPs Face the Skeptics,HE,Mar/Apr '98). The foundation stem walls are 6-inchthick reinforced concrete, insulated on the interior with a 2-inch-thick rigid foam board (R-10) that serves as the concrete form. Each wall and roof panel consists of a polyurethane foam core sandwiched between 7/16- inch thick oriented strand board (OSB) sheathing. SIPs 41/2 -inches thick are used for the walls, and SIPs 61/2 -inches thick form the flat lowslope ceiling/roof assembly. The walls are finished with synthetic stucco on the exterior. The roof panels are finished with a white singleply rubberized fabric coating on the exterior (the inside is cathedralized, with no attic space). The windows have vinyl frames with a thermal break,double panes, and spectrally selective coatings on the inside of the outer pane (surface two) of the tinted glazing.
Mechanical system features include putting the air handler in an interior chase, locating all ductwork within the conditioned space,and installing a 12-SEER (seasonal energy efficiency ratio) air conditioner. A batch-type solar water heater, with an integral collector storage unit,preheats domestic hot water. The gasfired water heater is coupled with an integrated hydronic space-heating coil in the air handler. The house has a controlled-ventilation system consisting of a separate,single-speed and manual-switch fan that supplies fresh air on demand from the outside to the air handler return plenum.
This ranch-style house is located in a high-density, single-family residential development (see cover photo). The architectural plan has approximately 1,170 ft2 of floor area, including two bedrooms and two bathrooms. The house has a relatively large window area with 272 ft2; four sliding glass doors facing a patio make up about 80% of this window area. The sliding glass doors are partially shaded by the patio cover,which is 24 ft long, 6 ft wide, and 10 ft above ground level (see photograph on p. 24). The front entrance is a solid wood door. Another overhang on the front elevation of the house is an open horizontal trellis made of nominal 2 x 6 lumber; vegetation has been planted and is intended to grow over it. Prior to testing, the interior of the house was fully finished and landscaping was complete. No interior window coverings were installed during the test period.
Building performance measurements included environmental conditions, net air exchange rate,and electric power use during normal operation of the building (see Measuring Thermal Performance). In addition, cooling loads were measured using a co-cooling test protocol in which a six-zone portable air conditioning unit was substituted for the building’s air conditioner. The modeled and measured results were compared; the results showed good agreement for cooling loads and air conditioning energy use. The model was then used to evaluate annual energy use and the impacts of alternative solar load control strategies over a broader range of conditions than could be measured in the field.
The simulation model includes accurate building geometry to account for the effects of shading on windows and walls. The windows are all generally well shaded, either by overhangs or by the adjacent houses. The simulation model was imported into a three-dimensional graphic representation program that has rotational view capabilities to check building geometry.Figure 1 shows the location of exterior walls, windows,doors, and overhangs. Crawlspace walls are evident in this view. Shading from adjacent houses is significant at this site. The geometry of houses and fences to the east and west are modeled as measured at the site; these shading surfaces are also shown in the figure.
How the Shading Options Added Up
For the determination of annual heating and cooling energy,occupied building operation is simulated. The simulation of occupied conditions in this building for a full year predicts that 3,285 kWh of cooling energy and 71 therms of space-heating energy are required per year. Heat gain through the windows is the largest component of envelope load, and it constitutes more than 30% of the total cooling energy load (see Figures 2 and 3).
Figure 4 presents the daily load profiles of air conditioning electricity use on a typical cooling day for four combinations of glazing and shading. In this case, the shading includes both the architectural overhangs and the site shading from adjacent buildings. Standard glazing without shading represents the worst case, and spectrally selective glazing with shading (the existing building) represents the best case. The combination of high-performance glazing and shading achieves a 0.4 kW (14%) reduction in afternoon peak electricity demand and a 12.4 kWh (30%) reduction in daily total electricity used for air conditioning. Architectural and site shading reduces daily cooling use more than upgrading the windows does. The shading combination reduces daily cooling energy use by 9.4 kWh (22%), as compared to 4.4 kWh (11%) for just upgrading the windows.
Architectural shading is clearly very important in reducing cooling loads. It reduces the annual cooling requirement by approximately 23%, whether one starts with standard double-pane glazing or with spectrally selective glazing. In both cases, the heating load increases as the solar gain is reduced, but thanks to the combination of the Tucson climate and the wellinsulated tight building shell, this has little impact. Even in the worst
case scenario, less than 80 therms per year of space heating is required.
In this housing development, site shading plays an important role in reducing morning and evening direct solar gain. The test house is shaded to the east and west by adjacent, two-story houses. This site shading not only reduces the solar gain through the windows, but effectively shades much of the exterior wall area, reducing overall conductive gains as well.
Annual Energy Costs
The cooling and heating loads are combined into a single value by converting the energy requirements to costs. The study assumed that electricity costs 10.5¢/kWh, and natural gas costs 79¢/therm for the first 20 therms a month and 75¢/therm above that. Figure 5 shows annual cooling and heating costs as a function of glazing type, two types of shading, and the orientation of the front of the house. Using the data from this figure and referencing a base case building with standard windows with no overhangs but with adjacent building shading, Table 2 presents the reduction in cooling and heating costs for a subset of combinations.
The existing building has a south orientation, and the combined features lead to a 26% reduction in cooling and heating costs. The total cost of cooling and heating is reduced by more than 10% by adding the presence of the adjacent houses. As expected, the maximum effect from architectural shading occurs if the front of the house faces west, which orients most of the window area to the south. The maximum effect of site shading occurs if the front of the house faces north, which orients most of the window area to the west.With the front facing east, the majority of windows are on the north side, and neither architectural nor site shading has much effect on cooling and heating costs.
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