Integrate windows to achieve energy performance


3. Unwanted solar gains are most troublesome on the south side.

Most owners and architects still think of a building’s south orientation as being the worst side for a glazed façade in terms of solar gain, but in actuality the east and west sides are typically far worse due to a wider solar angle range and the challenges in controlling solar gain. Though greenhouses and solariums should in fact be located on the south side, once the particulars of the solar gains are accounted for, it becomes clear how minimally impactful the solar gains on the south side actually are. A simple calculation would indicate that on an annual basis the south façade typically receives slightly more solar energy than the east or west façades. As your latitude increases, more solar energy will land on the south face, while as the latitude decreases, more solar energy will land on the east and west. This is only half the story.

The true comparison needs to begin by not only looking at the amount of incident solar energy, but when and at what angle it arrives. A similar comparison of solar energy during only the summer months shows that the east and west orientations of a commercial building could have roughly double the energy landing on those surfaces than on a southern exposure. The latest versions of ASHRAE Standards 90.1-2010 and 189.1-2011 acknowledge this and now require that the total area of glass on the east and west sides each be less than that on the south—simple for a rectangular building with its long face oriented south, but significantly more difficult for the same building rotated 90 deg. This is intended to specifically limit the amount of glass on these east/west façades in order to target solar gains that are the most difficult to control.

Figures 5 and 6: On an annual basis, the south façade receives the greatest solar energy. During the summer, the south receives less solar energy than both the east and the west. Courtesy: Syska Hennessy GroupFigures 5 and 6: On an annual basis, the south façade receives the greatest solar energy. During the summer, the south receives less solar energy than both the east and the west. Courtesy: Syska Hennessy Group

Additionally, when focusing on just the summer season, the sun is high in the sky throughout the day, which makes exterior solar shading elements extremely effective on the south side. For example, an 18-in. overhang placed on a 5-ft-tall window would block nearly 70% of the solar energy over the course of the summer at a mid-North American latitude of 40 deg. Due to the effective lower solar angles located on the east and west orientations, the same overhang would block less than 30% of the solar energy making it far less effective for the identical projection.

Understanding the solar radiation on a rectilinear project with no neighbors is relatively simple, though we rarely are allowed this luxury. Most projects will require the use of a solar radiation or building simulation design tool to help identify the areas of problematic solar radiation and areas of potential solar harvesting. It is this tool that will identify any given project’s most troublesome spots and identify the most effective solar gain control strategies.

4. Optimum daylighting requires maximum amounts of glass.

It is a common strategy to justify more glass in a building with the intent to maximize daylight. The more the glass, the better the daylighting—right? Wrong. Instead, quality daylighting is about creating the proper balance of illuminance and luminance within the lit environment.

Figure 7: The inner chart illustrates where the energy goes within the building, while the outer chart illustrates the building component that is driving the HVAC energy use. Courtesy: Syska Hennessy GroupIt is true that displacing electric lighting with the incorporation of a quality daylighting design provides a unique opportunity and can be a façade’s single greatest impact on a building’s annual energy use. Remember that electric lighting is typically around one-third of the total energy consumed in a building, and a quality daylighting system can reduce that by up to 75% for areas where daylight-responsive lighting controls are implemented. This is roughly equivalent to a 20% to 25% reduction in the total building energy due to the glass and the required integration of building systems (lighting and HVAC).

While this is the simple opportunity, realizing this energy savings is a much more challenging prospect. A proper daylighting design must first realize that direct solar penetration and visual glare is the Achilles heel to a comfortable visual environment. Some form of glare control must be provided to occupants under certain circumstances. As such, the most typical side-lighting daylight strategy divides the window into a continuous daylight portion above the 7-ft level and vision portion below. The occupants then have full control over the vision area, but the daylight aperture above remains open and unobstructed. As a result, lots of glass does not guarantee an improvement in effective daylighting. In fact, added glass to the vision portion does little to increase the reliable daylight, while adding glass to the daylight aperture without also increasing the associated daylighting design elements (light shelves, daylight reflecting devices, etc.) will often provide too much daylight.

5. Operable windows negatively impact energy and should never be used.

While natural ventilation may not always be appropriate, it should always be considered. Natural ventilation can be a passive solution supporting an active HVAC system that still provides heating/cooling in a mixed mode capacity where windows operate according to controls, either seasonally or daily.

In addition to the benefits that include an expanded comfort range and improved satisfaction for occupants, natural ventilation can also promote passive survivability—the ability for a building to continue to function in some capacity without power. A building with operable windows and a good daylighting design can still maintain its occupancy and business operations during an outage, regardless of the building’s climate and geography.

When using operable windows, a variety of practical control strategies can be implemented. For one, the BAS controls can be set to use natural ventilation by signaling the HVAC system to react to whether the windows are open or closed. Practical strategies may include window interlocks or a red light/green light mechanism that alerts occupants to open or close their windows.

At the National Resources Defense Council (NRDC)’s Santa Monica headquarters, Syska Hennessy Group designed a mixed-mode system relying on occupant-controlled natural ventilation as the primary means of conditioning for the facility. This design included manually operable windows with automatic interlocks to the HVAC system, as well as CO2 sensors to signal users when the windows should be opened (or opened wider). When choosing potential control strategies, a frank discussion among designers, builders, operators, and occupants of the pros and cons of each is crucial to the system’s success.

While a number of considerations need to be a part of the mechanical design when specifying operable windows, as long as they are part of the HVAC solution from day one, optimal operational efficiencies can still be met. Issues of maintenance, security, and weather infiltration can easily be overcome when designing operable windows into a commercial building. These challenges, as well as climate, should be considered when specifying the window type.

A holistic view

For many years, design professionals have considered the building façade as the first level of defense against the outdoor environment. To achieve the aggressive energy consumption goals increasingly mandated within the architecture, engineering, and construction industry, this mind-set must be changed. Instead, the façade in general, and glass in particular, need to be viewed as the first opportunity to harvest energy from the outdoor environment and provide passive lighting and conditioning through daylighting, passive solar heating, and natural ventilation. The associated heat gains and losses then need to be minimized through optimum glazing selection, shading elements, and orientation. Each façade is unique to its given project and should be considered as its own building system that must be integrated with the HVAC, lighting, and other building systems.

Understanding that current energy codes and standards dictate blanket performance criteria as the minimum allowable thresholds, today’s design professionals are challenged to steer clear of using ASHRAE standards as a design goal. Rather, they need to identify codes and standards as a starting point for the high-performance building to be optimized. Tomorrow’s designs cannot simply incorporate better components than a prescriptive building does; they will have to be designed with completely integrated systems. The code must be a first step to engaging the other building team members in a discussion about managing glare, specifying the right windows, and achieving the best performing façade with the right performance criteria, at the right orientation, with realistic solar expectations.

Robert Bolin is a senior vice president and national director of high-performance solutions for Syska Hennessy Group, based in the Chicago office. Kristopher Baker is an associate partner and building performance modeling and design consultant for Syska Hennessy Group, based in Denver.

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