Area, placement, light redirection
The amount of fenestration and its placement is also critical to daylighting performance. There needs to be sufficient fenestration to admit enough daylight, but positioning is critical. The higher the glass is located on the façade, the greater the depth of daylight penetration. The rule of thumb is that the depth of daylight penetration is 1.5 to two times the window head height. That is why clerestory fenestration is so effective at daylighting. Glass below desk height is least effective for daylighting. If furniture is moved away from the outer façade in order to create a circulation space and light-colored flooring allows for re-direction and mixing of the light in the room, then some daylight from this area can be harvested.
Optimal daylight designs often have a ‘split-façade’ where there is vision glazing (for views to the outside) where glare must be controlled more often for the occupants and clerestory glazing above which provides more consistent daylighting. Figure 6 illustrates how a light shelf is used to redirect the light coming through the clerestory windows further back into the space, increasing the depth of daylight penetration. In this case, EC glass in the view section is controlled independently from the EC glass in the clerestory to maximize glare control through the vision window and daylight admission through the clerestory and light shelf combination.
There is a trade-off between putting more fenestration in a wall and the perimeter zone’s energy performance. This trade-off becomes more significant for buildings designed for more effective daylighting (i.e. those with larger perimeter zones). EC glass can help relieve that trade-off by reducing peak solar loads during cooling periods, offsetting heating loads during winter, yet letting in enough daylight to turn off electric lights (which are also a heat source) throughout the year. Many studies have shown significant energy savings of EC glass over static glazing solutions, exceeding 20 percent. (There are many studies showing the energy savings of electrochromic glass. These include a study by Paladino and Company, titled “Performance Assessment of … Electrochromic Coatings and Control Strategies,” a study by E. Lee et al called, “The Energy-Savings Potential of Electrochromic Windows in the U.S. Commercial Buildings Sector,” a Public Interest Energy Research (PIER) Report design guide for early-market electrochromic windows complied for the California Energy Commission (CEC), and another Lee study, “Advancement of Electrochromic Windows. California Energy Commission.”) In fact, because of the dynamic solar control performance of EC glass, more glazed area can be used to provide the needed daylight penetration, without taking an energy penalty.
An example is shown in Figure 7, which illustrates the results of energy modeling of the standard three-story medium office building used by PNNL for assessing the performance of ASHRAE 90.1. The performance of the building in Phoenix with EC glass was compared to the performance using the ASHRAE 90.1-2010 baseline glazing and the baseline glazing with manual blinds. A building with a 50 percent window to wall ratio of EC glazing has the same energy performance as 20 to 30 percent window area with conventional glazing. This is significant, given this large difference was shown in a building with a relatively small perimeter zone compared to the core area, in which impacts on building energy consumption from changing the envelope conditions are normally relatively small because of the dominance of the core.
Skylights are an excellent way of providing daylight to the core of buildings, but of course are only effective for daylighting the top floor or where there is an inner courtyard. That being said, skylights come with significantly more challenges around solar heat gain control, as is demonstrated by a skylight installed in 2009 at Ball State University in Muncie, Indiana (Figure 8).
Converted from an open central courtyard into an enclosed space, the university wanted to preserve the open feel of the space and create a general purpose area serving as a lounge, entryway, and a venue for large group audio visual presentations. When first installed, the specified glazing was an insulating glass unit (IGU) with a standard solar control low-emissivity (low-e) coating on the exterior lite with a 50 percent frit pattern on the inboard lite. Within only a few months, the school found there was too much light and heat coming into the space, making it very uncomfortable for occupants. They considered many options, including mechanical systems on both the inside and outside of the skylight, but found EC glass to be the most low-maintenance and aesthetically pleasing solution. (It was also first-cost-competitive.) With EC glass installed a year later, the space immediately became thermally and visually comfortable. The ability to tint the entire roof to one percent means the university is also able to project audio visual (AV) presentations under the roof for entertainment events.
Another benefit of EC glass is it lowers peak loads, which can reduce the size of the HVAC system—in some cases, enough to allow the transition to more innovative and sustainable heating and cooling systems such as chilled beams and radiative cooling and heating. An example is shown in Figure 9, where EC skylights were used to control solar gains as part of a day-lit and naturally ventilated space at Chemeketa Community College in Salem, Oregon.
While glazing choice does not impact interior design per se, interior design can make or break a daylight design. It is critical interior design choices support the effective distribution of daylight in a space. The color (preferably light and neutral) and reflectivity of walls and ceilings are critical choices. The Illuminating Engineering Society (IES) recommends 50 percent reflectivity for walls and 80 percent for ceilings.
Additionally, the height of workstations should be minimized so daylight can penetrate deeper into the space. If perimeter offices are needed, glass walls can be used to share the daylight with the interior.