By John Bendt
Facility managers rightly have high expectations for improved energy performance when upgrading their existing buildings. Helping meet these goals, window system manufacturers have made significant progress in developing glass and aluminum framing products for renovation and replacement applications. Modern, high-performance window systems also can have a very noticeable impact on existing buildings’ operational energy costs and their associated HVAC and lighting capacity.
While savings from lower operational energy costs are always welcome, even a small increase in employee productivity can have a large, positive financial impact as salary costs are generally 10 times higher than energy costs in U.S. office buildings. Energy-efficient, high-performance buildings provide occupants with access to daylight, comfortable temperatures, and better air quality. These environmental characteristics are correlated with lower absenteeism and higher productivity, and can save up to $2,000 annually per employee.
As the square footage of space allotted per employee continues to decline, building owners and tenants can maximize the efficiency of their floor plans by using the interior perimeter space right up to the windows. With improved window technology, occupants no longer complain about unwanted heat and cold associated with older glass and aluminum technology.
Climates, Codes and Coefficients
Climate has a major impact on the energy use of commercial buildings, and energy codes and standards rely on a clear definition of climate zones to convey requirements. To determine which code is adopted for a particular building’s location, an interactive map of adopted codes is available at the International Code Council website. Many states have adopted the International Energy Conservation Code (IECC).
The IECC requires consideration of thermal conductance of window systems and the amount of solar heat gain that is admitted into a building through the glass. Conductance values are quantified in the form of a U-Factor and the amount solar heat gain is stated in the form of Solar Heat Gain Coefficient (SHGC).
- The U-Factor is a measure of thermal transmittance in an assembly, such as a window expressed in units of BTUs/hr.sqft.°F. When comparing window units of the same size, a lower number indicates better insulating properties. The 2015 IECC lists prescriptive compliance option U-Factor requirements for window systems that are fixed in place and do not open, for window systems that do open, and for entrance doors. For fixed windows, U-Factors range from 0.28 to 0.36 in Northern climates to from 0.44 to 0.48 in Southern climates.
- SHGC quantifies the total solar energy admitted through a window, both the energy directly transmitted plus energy absorbed by the glass and flowing inward. SHGC is expressed as a number between 0 and 1. A lower number indicates better performance – less solar heat transmitted. IECC Table C402.4 details SHGC requirements by building elevation and climate zone. IECC’s prescriptive compliance option SHGC requirements for south, east and west-facing glass range from 0.31 in Southern climates to 0.43 in Northern climates.
Through the 1970s energy crisis, commercial buildings largely relied on tinted or reflective glass to control unwanted solar heat gain. The first glass to use microscopically thin, low emissivity (low-e) coatings to reflect solar, infrared heat energy was introduced into the market in the 1980s. Today, glass manufacturers can “stack” multi-layer coatings on the glass that total only one-ten-thousandth the thickness of a human hair. Inert gas infills rather than air, improved insulating glass spacers and room-side low-e coatings can further improve the energy performance of glass.
In the U.S., roughly half of all existing commercial buildings still do not even have insulating glass. A building with non-insulating, quarter-inch-thick, clear glass upgraded to today’s top-performing, insulating glass products could experience a 341% increase in insulating value and a 241% decrease in unwanted solar heat energy. This upgrade could significantly reduce occupant discomfort and disruption.
In addition to glass improvements, technology advancements in aluminum framing systems also have improved overall energy performance. Aluminum is recognized as a highly conductive metal. Used in window systems, it requires provisions to reduce the heat flow through the metal. Recent reports indicate 38% of all aluminum window and door products in North America have frames with a non-conductive thermal break to disrupt the unwanted transfer of heat or cold.
There is an opportunity for more buildings to benefit from the improved energy efficiency provided by aluminum window and door products with thermally broken frames. The state-of-the-art in thermal barrier technology, utilizes polyamide nylon strips Compared with other methods, the polyamide strips can provide more separation between aluminum extrusions and more flexibility in design, offering a more economical option for different finishes and/or colors on the window system’s exterior and interior surfaces.
Choose Window Systems Carefully
New window system technology can have a dramatic effect on HVAC and lighting capacity when these systems are being upgraded along with windows in an existing building.
It is important that facility personnel work with a reputable window manufacturer that can provide the necessary engineering support and experience to compare replacement window alternatives. It should be expected that the manufacturer can estimate the anticipated energy savings, the reduction of HVAC capacity and the reduction of lighting capacity.
This comparative energy modeling, combined with finite element (FE) heat flow analysis, is used to select the optimal window system for the building. For existing buildings, operating parameters – such as internal heat gains, HVAC set points and equipment efficiency – are often difficult to quantify. For these reasons, a comparative approach is recommended, looking at the relative impact of one window system versus another. Comparisons should not be interpreted as expectations for overall perimeter zone energy consumption.
In choosing a partner, select a qualified window manufacturer that provides:
- The U-Factor for the actual window system sizes– Since glass is generally a better insulator than the framing, it is very important to compare this parameter on equally sized, complete window systems with framing included.
- The performance analysis for the glass in the window system – This should include SHGC, as well as:
— Visible light transmittance, the percentage of visible light that passes through a window
— UV transmittance, the measure of ultraviolet (UV) A and B radiation through glasses of different types
— Interior and exterior reflectivity, the measure of light reflecting off the surface of glass
- The most reliable energy modeling data – There are many energy modeling tools available. Ensure the chosen window manufacturer has a proven method and solid record. For example, THERM is a state-of-the-art computer FE thermal analysis program and COMFEN is a perimeter zone, energy modeling tool. Both were developed at Lawrence Berkeley National Laboratory and are trusted by window industry leaders.
FE thermal analysis of the system is extremely important. Because it provides estimated interior surface temperatures of the glass and metal under wintertime design conditions, FE analysis will identify potential condensation issues prior to a new system being installed. Insist that this model be completed with inclusion of the material around the window opening, such as concrete, steel or wood, to achieve the most accurate results.
Carefully selecting a window manufacturer will ensure that facility executives arrive at the proper window system to provide existing buildings with the intended, improved energy performance and increased tenant comfort in a competitive marketplace.
As Apogee Enterprises, Inc.’s vice president of building retrofit strategy, John Bendt assists building owners and facility managers in evaluating the benefits of energy-efficient building envelope renovations and upgrades. With more than 25 years in the commercial building industry, Bendt has helped many improve productivity and increase value.
- American Institute of Architects and Rocky Mountain Institute, “Deep Energy Retrofits,” 2013
- American Architectural Manufacturers Association (AAMA) and the Window and Door Manufacturers Association (WDMA) 2012/2013 “U.S. National Statistical Review and Forecast”
- International Code Council
- IECC Table C402.4