ASHRAE Journal - June 2010 - S24

Note that the requirement for assemblies is 10 times more stringent than for whole buildings. This is because of the added air leakage that occurs due to penetrations and gaps that occur when whole buildings are tested compared to individual assemblies. Similarly, the requirement for materials is 10 times more stringent than that for assemblies because of the air leakage that occurs when materials are taped or pieced together to form assemblies.
Fenestration Shading

Photo 1 Overhang on west face of building in Climate Zone 5 (photo courtesy of CTLGroup).

The standard requires that there be permanent shading projections on the west, south, and east walls of buildings in Climate Zones 1 through 5 unless the vertical fenestration receives direct solar radiation for fewer than 250 hours per year because of shading by nearby structures or topography. This requirement is intended to reduce the solar heat gain to the buildings in these climate zones. Since the requirement is for an area-weighted projection factor of 0.5, not every window is required to have a permanent shading projection or overhang. Requirements also can be met by recessed windows, the use of balconies, building shapes that shade themselves or other architectural features. A projection on the west face of a building in Climate Zone 5 is shown in Photo 1.
Building and Fenestration Orientation

Figure 2 U.S. photovoltaic solar resource map: Flat plate tilted at latitude. (NREL: www.nrel.gov/gis/solar.html.)

The prescriptive fenestration orientation requirement for Climate Zones 1 through 6 encourages design that orients the building with the longest façades on the north and south. This discourages large amounts of fenestration on the east and west facades, which are subject to the most solar gains in the summer months due to the lower angle of the sun on these faces. Lower SHGCs on the east and west façades can be used to meet these requirements in Climate Zones 1 through 4. A similar requirement is in place for Climate Zones 5 and 6, but only restricts fenestration on the west face. This recognizes the benefits of solar gains on the east façade for morning warm-up in colder climates.
On-site Renewable Energy

The project committee recognized that today, renewable energy systems are not always cost effective. However, the committee also realized that once a building is constructed, the opportunity for future installation of such systems is often lost forever. Meanwhile, the capital cost for such systems is expected to decline as their use increases. To enable the future installation of such systems as they become more cost effective, the standard has a mandatory requirement that the building design provide for the future installation of a photovoltaic, solar thermal, geothermal energy (but not including ground-source heat pumps) or wind system with a minimum rating of 13 Btu/h. ft2 (3.7 W/ft2) multiplied by the total roof area. Provision for future installation means to show allocated space
S24

for solar collectors, pathways for conduit, piping and associated equipment on the construction documents. This minimum requirement was based on the assumption that a typical photovoltaic array generates 8 to 10 watts of power per square foot of photovoltaic panel when operating at peak capacity and that roofs on most building can allocate 50% of their roof area for these systems. The only exception for the installation of such systems is if the building is in a location with poor incident solar radiation, defined as 4 kW/ m2·day on a collector oriented due south and tilted at an angle equal to the site’s latitude and as illustrated in Figure 2. This exempts the upper Midwest and New England states, as well as portions of the Pacific Northwest. Sites with adequate permanent shade also will often meet the exemption. Under the prescriptive requirements, buildings that do not qualify for the same incident solar radiation exception must install on-site renewable energy systems at the time of
June 2010

ASHRAE Journal’s Guide to Standard 189.1 ashrae.org

ASHRAE Journal - June 2010

Table of Contents for the Digital Edition of ASHRAE Journal - June 2010