Integrate windows to achieve energy performance
Here are the top 5 myths about the building façade and its impact on operations.
- Understand the use of windows in nonresidential buildings.
- Learn how to integrate the façade with mechanical and electrical systems.
- 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.
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