Integrate windows to achieve energy performance

Here are the top 5 myths about the building façade and its impact on operations.

By Robert Bolin, PE; Kristopher Baker, PE, Syska Hennessy Group June 21, 2013

Learning objectives

  1. Understand the use of windows in nonresidential buildings.
  2. Learn how to integrate the façade with mechanical and electrical systems.
  3. Learn the 5 myths about façade and energy performance. 

True or False? Architectural glass is a necessary evil.

False. Glass is a fundamental element, often the most recognizable feature to an architectural design, and is the primary means with which the building creates a human connection to the outdoor environment. With the proper application and the right supporting building systems that leverage façade opportunities, glass façades can be a net energy benefit to just about any project.

There are great misconceptions among building design and construction professionals in regard to the use of fenestration. Some are due to the extrapolation of one climate zone upon another or one building type upon another, but many stem from the understanding of the residential environment. For example, although it is generally true that one of the best things a homeowner can do is add insulation to his attic, increasing insulation levels on the roof of a commercial building isn’t nearly as impactful. In contrast, no one would build a single-family home without operable windows, and yet rarely do designers even consider them for a commercial space.

Often, this misapplication of knowledge impedes the designer’s ability to think beyond the traditional to what is truly energy efficient in each climate and for each building’s individual set of circumstances. No longer are building façades merely energy losers that the building mechanical and electrical systems must counteract.

Today’s commercial building façades provide the first opportunity for the building to capitalize on the natural environment and minimize thermal and lighting loads. Though glass will always impact the HVAC performance, a façade that is well integrated into the architecture and building systems can have a net positive impact on the total energy performance and is the first step to achieving what is considered a high-performance building.

5 myths about façade and energy performance

1. The façade has the single biggest impact on the energy expenditure of a building.

From a thermal load perspective, buildings can be identified by one of three categories: external-load, internal-load, or ventilation-load dominated. The external-load dominated building is typified by a residential home or multifamily housing unit. In a building of this type, the thermal energy flowing through the building envelope drives the total annual energy consumption; as much as three-quarters of the total can be directly attributed to counteracting the heat gains and losses from the outside. Energy codes and standards such as ASHRAE 90.1, IECC, and ASHRAE 189.1, not to mention many other state energy codes, have acknowledged the increased importance of the envelope by requiring increased levels of energy performance for the façades of hotels, hospital patient rooms, and residential projects.

In contrast, most of the load in the internal-load dominated commercial office building comes from within, that is, the computers, lighting, equipment, and occupants. The rough energy use characterization for a typical commercial facility includes one-third lighting, one-third plug load, and one-third HVAC systems (made up of the effect of the envelope, lighting, plug load, occupants, and ventilation). The envelope is typically responsible for no more than a third of the HVAC energy expenditure and thus has a small impact on the building’s annual energy use of just 5% to 15%. It should be noted that even though the envelope plays a considerably lesser role in the annual energy consumption, it often remains a significant portion of the peak cooling and heating design load. Interestingly, under temperate outdoor conditions of 55 to 70 F, internal investigations by the authors have shown a typical office building often consumes less energy with reduced levels of insulation.

Ventilation-load dominated buildings are ones where air change rates and indoor air quality (IAQ) are paramount, for example, labs and hospitals. Airflow rates are dictated by codes and standards like ASHRAE 170 for hospitals and a medley of guidelines from OSHA, NFPA, and NIH for labs. Each requires minimum total flow rates as well as minimum outside airflow rates. Typically, heat gains and losses through the façade are relatively small compared with the energy needed to heat and cool a building’s high airflow rates; thus the façade plays an even less significant role.

While a focus on the building envelope design is important, the idea that the greatest energy use is due to the façade is typically false. Designers must understand both the climate surrounding the building and the dominant load of the building type, noting that many buildings comprise all three categories, for example: hospital patient rooms (external), offices (internal), and operating rooms (high ventilation rates). So, it is important to target energy consumed by other building systems while still seeking opportunities to limit the negative impact of façade performance.

2. Low U-factor and SHGC values and high VLT are always better.

It’s often said that a higher performing glass simply translates to a lower solar heat gain coefficient (SHGC) and U-factor. Realistically, the optimum glazing selection involves tuning the U-factor, SHGC, and visible light transmittance (VLT) to the purpose of the window, the building, or space type and its particular orientation.

Windows have two primary functions in most commercial buildings: they provide a view that creates a connection between the occupant and the outdoor environment, and they provide daylight that displaces electric lighting and reinforces the connection to the environment. Closely tied to these functions are solar gains and heat losses that must be addressed by a building’s HVAC system. Lower SHGCs reduce solar gains throughout the year, effectively increasing the need for heating and decreasing the need for cooling. Lower U-values similarly reduce heat transfer throughout the year, reducing the heating and cooling at the extreme conditions, but often increasing cooling during temperate conditions. Every project has its own unique optimum case.

Importantly, how the glass is being used must also be evaluated. While the SHGC and U-value are usually the performance metrics to be optimized for glass providing views, they are not typically the driving metric for a daylighting aperture. Rather, a high VLT and light-to-solar gain (LSG) ratio are typically more important for these applications.

One interesting element to investigate in optimizing glass performance is the sometimes divergent relationship between annual energy consumption and peak design loads; annual energy reductions can sometimes be associated with peak load increases. This occurs because the peak load is associated with sizing the equipment that maintains thermal comfort inside the space during the worst condition, i.e. how large is the chiller or boiler and how much airflow is required. In contrast, the annual energy performance accounts for the ongoing operational efficiencies of all building systems over the entire year. Therefore, careful attention to envelope performance trade-offs is crucial to balancing peak load sizing with typical operating load performance for an overall optimal solution.

For example, a patient room has a minimum air change rate required to maintain acceptable indoor air quality (IAQ) levels that generally provides more airflow than is needed in some circumstances to meet internal and external loads. As a result, the air is usually reheated to avoid overcooling the space. In this instance, a higher SHGC, with its associated additional solar heat gain, can often help limit the reheat required by the HVAC system, subsequently reducing the annual energy usage for the space. In essence, whenever the space is in full sun and the external load requires airflows greater than the minimum required by the code, there is an energy penalty, but whenever the space has a load that allows airflows below that of the code, it is an energy benefit—it is essentially having some passive solar heat in the patient room. There is little doubt that the higher SHGC will, however, require more peak cooling capacity.

Finally, the VLT should always be considered for occupant comfort. It should never be assumed that the highest VLT is the best for occupants. A tuned fenestration system with glazing appropriately selected based on the optimum SHGC, U-factor, and VLT will rarely be composed of the minimalist option.

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.

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.

It 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.