Benefits of Interior Installed High Performance Insulating Glass for Commercial Retrofit Applications: A Case Study (ST-16-C012)

Buildings account for roughly 40 percent of all energy consumption in the United States, according to the U.S. Department of Energy (DOE).  As a result, improving the energy efficiency of buildings should be a top priority for building owners, design professionals, utility companies, allied industries, the federal government, and cities across the country. These diverse groups are working not just to build efficient new construction, but also to improve aging properties through energy retrofits.

Energy retrofits for commercial buildings focus on installing high-efficiency boilers, motors, and lighting.  However, efficiency gains from equipment can be offset by occupant discomfort due to inefficient windows, which can account for 25 percent of a typical building’s heating load in cold climates and 50 percent of the cooling load in warm climates, according to the U.S. Environmental Protection Agency.  Because equipment is sized to service a specific building’s needs, improving a building’s envelope should be addressed first, so that smaller equipment can be specified, saving on the upfront and ongoing costs.

A variety of options exist for improving the energy performance of existing commercial glazing systems including: Application of interior window films – solar heat gain. Complete window rip-out and replacement – solar heat gain and improved U-factor. Interior commercial storm windows - improved U-factor. Interior installed Low-E retrofit insulating glass unit - solar heat gain and improved U-factor

Each alternative has specific performance benefits and associated cost and convenience implications.  Solar heat gain is a primary problem with most commercial buildings having lower performance, single-glazing, regardless of climate zone.  This leads to increased cooling loads, larger sizing of HVAC equipment, higher energy costs and lower occupancy comfort levels.  Any improvement in the glazing system should incorporate technology to reduce the impact of solar heat gain through the use of high performance low-e coatings.  Concurrently for heating dominant climate zones, a substantial reduction in U-factor acts in parallel to reduce HVAC demand for heating, reduces energy costs and improve occupancy comfort levels.

This paper compares the performance benefits, cost implications and occupancy comfort factors for each of these systems with a focus on the advantages of an interior installed low-e retrofit insulating glass unit.  Such a system has been demonstrated to provide the full benefits of a rip-out and replacement at approximately 40% of the installed cost.  It includes independent case study energy analysis, installation and cost comparison, testimonial on occupancy comfort and sustainability attributes.