Daylight design process
EC glass can help designers balance the need for sufficient daylight admission while managing solar gains and glare.
Daylight design starts as early as building massing and shape determination. Building shape has a significant impact on the size of the floor area that can be day-lit—especially by vertical windows.
For the best daylight penetration from side lighting (i.e. vertical windows), the floor area near the windows should be maximized—meaning those buildings that are long and skinny, have inner courtyards, or are shaped like the letter ‘I’ or ‘E’ (akin to some European hospital designs). Figure 3 shows different building geometries of the same area, but with increasing perimeter area. As the perimeter to floor area increases, the importance of envelope performance becomes increasingly more critical—both from an energy perspective and for occupant comfort. EC glass can help reduce the impact of increased solar loads by attenuating the sun’s heat during cooling periods, and offsetting heating loads during winter. Thus, the impact of increased fenestration to the floor area is less significant. In colder climates, the combination of EC coatings in triple-pane glazing with high-performance, thermally broken framing systems provides additional thermal loss reduction as well as solar control.
Building orientation also has a critical influence on daylight design effectiveness. Ideally, buildings should be oriented with their long sides facing north and south to minimize solar loads (Figure 4). However, designers do not always have the luxury of an unconstrained site to ideally orient the building. High solar gains through windows on east- and west-facing façades are much harder to manage since fixed-exterior horizontal shading systems are not effective at controlling low-angle sun. In many cases, reduced window areas are needed with these orientations due to the high solar gains, or a higher air conditioning system capacity. EC glass can help reduce the negative impact of sub-optimal building orientation.
Figure 5 shows the impact on building energy performance of a three-story prototypical medium office building, used by Pacific Northwest National Laboratories (PNNL), to assess the performance of American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, of the change from an ideal north-south orientation to 45 degrees off optimal orientation in Phoenix for three different fenestration options including:
- EC glass;
- code-compliant glazing; and
- code-compliant glazing with manual shades in use.
Even though this building has a small perimeter zone relative to the core, the increase in energy usage with EC glass is much lower (i.e. less than two percent) than with the static glass options (i.e approximately five percent). It is important to note the use of manual blinds (i.e. where they are modelled as pulled and left down for a few hours until re-opening) has a negative impact on energy performance, and is not taken into account in most building modelling during the design process.