Reducing Building Energy Loads through Improved Fenestration Design

Charlotte Matthews

This study analyzes the means by which building energy loads may be reduced through improved window insulation, application of low-e coatings and optimized fenestration design. Buildings consume 36 percent of United States energy for the purpose of keeping occupants "comfortable". Maintaining the ideal temperature within the building through mechanical heating and cooling takes almost half of that energy. A United States Department of Energy statistic, attributing 25 percent of heating and cooling loads to poorly insulated windows, suggests the replacement of these units as a first step to reduce unnecessary energy use.

Of course, the replacement window must, in fact, have a greater resistance to heat transfer. Air infiltration through windows is not a significant source of building heat loss, so blindly replacing an old window does not guarantee energy savings. The window specification for Carr House at Brown University was used as a case study to determine the thermal performance of the typical new window installed today. The stipulated windows were found to be "poorly-insulated" as they lacked most of the technologies known to enhance thermal performance. Though the nation's leading glass manufacturer, IPG, incorporates warm edge technology, Brown still specifies aluminum spacers, which cause heat loss and condensation. Argon, a low conductivity gas, improves window insulation by 33 percent over air, with only a 10 percent higher price tag. Nonetheless the Brown window specification calls for air—insulated glass. Additionally, the University sole—sources wood and conventional aluminum windows. Pure—wood frames require significant upkeep that can be avoided with claddings or hybridization of the material with plastic. Mass-market aluminum frames conduct unnecessary quantities of heat due to insufficient thermal breaks. High performance aluminum windows, manufactured by Kalwall and Visionwall, have not been considered for a University project. Nor has the invitation to bid on projects been extended to manufactures of high performance windows that are comprised of alternative materials, like fiberglass.

Another consideration for fenestration design is the regulation of heat gain. Heat gain, primarily in the form of solar energy, increases the cooling load of a building but also reduces the heating load. The visible light component of solar energy reduces the lighting load of the building, as well as the cooling load incurred from electric lighting. Optimizing the transmittance of solar energy to minimize the overall building energy load is critical for the recovery of any portion of the energy loss innate to fenestration. Factors involved in establishing the optimal balance include the relative costs of heating, cooling, lighting, the internal heat gain of the building, and the exposure to sunlight that the building can expect in winter. The urban location of Carr House and its proposed commercial use (thus daytime occupancy with significant internal heat gains from lighting and office equipment) dictate that the focus for passive solar design is on cooling load avoidance rather than heating load reduction. Furthermore, due to the University’s particular heating and cooling systems, the cost of an increased cooling load resulting from the greater transmittance of solar gain for passive winter heating would negate savings to the heating bill.

Solar gain and visible light transmittance are regulated with fenestration design (area, orientation, and construction) and the application of low-e coatings on the glass. The optimization of these qualities requires the consideration of so many variables that computer-modeling software specific to building energy has been developed to facilitate the process. After experimentation with one such energy design tool, PowerDOE, I concluded that comprehensive energy analysis is too cumbersome for an architect to incorporate at building conception for ideal fenestration design, but a great asset for specific component changes later in the design process. In addition, the breadth of analysis empowered by PowerDOE provides engineers with an equally dependable, but far more accurate, means of sizing mechanical systems than that offered by current methods. The incorporation of energy design tools in the design process raises issues of energy efficiency that might otherwise be missed and stimulates greater collaboration between the architect and engineer. Generally, too little attention is paid to the significance of building design on operating costs and energy expenditures.

The mandate for an architect to use an energy design tool throughout the design process may force simplistic designs ideal for the tool’s method of energy modeling rather than an architect’s creative abilities. However, educating architects on the general rules of thumb of passive solar design would vastly reduce the energy use of a building inherent to its design.