As the primary barriers between indoor and out, the openings of the building envelope, high performance windows, doors, and skylights (fenestration) are essential to an energy efficient building. Choosing such products involves several considerations, including appearance, energy performance, human factor issues, technical performance, and cost. Other factors like building orientation, natural and artificial shading, and climate will influence the properties selected for windows, doors, and skylights installed throughout the building.
The first building energy codes adopted by many states were the 1992, 1993 and 1995 versions of the Model Energy Code (MEC), the predecessor of the International Energy Conservation Code (IECC). Today, many energy codes contain prescriptive requirements include the fenestration product minimum performance criteria (U-factor, Solar Heat Gain) based on climate zones.
Dramatic improvements in fenestration technologies in recent years have revolutionized performance. A good example of this change is the adoption of Low-E window technology in the 1990s, a period when almost all windows installed were clear glass. By 1995, the first national standard for measuring solar heat gain co-efficient (SHGC) for windows was developed. When minimum SHGC ratings were first incorporated into energy codes in the year 2000, most manufacturers were no longer making clear glass windows. Technology had transformed the market faster than national codes could incorporate the new standards.
Today, high performance windows, doors, and skylights provide myriad benefits, including:
- Lower energy bills
- Better views and ambiance
- Increased comfort
- Less condensation
- Less fading
- Lower maintenance expenses
- Positive energy tradeoffs
Terms to Know
U-value: A measure (Btu/h-ft2-°F) of how well a material or series of materials conducts heat. U-factors for window and door assemblies are the reciprocal of the assembly R-value. The smaller the number, the less the heat flow. Window U-factors typically range from 0.2 to 1.2, while the U-factors of other building assemblies (e.g. walls, floors) typically range from 0.02 to 0.10 when insulated. Using efficient windows decreases the energy consumption of the structure and helps the project comply with the code.
SHGC: Solar heat gain coefficient (SHGC) is the glazing’s effectiveness in rejecting solar heat gain. SHGC is part of a system for rating window performance used by the National Fenestration Rating Council (NFRC). SHGC is gradually replacing the older index, shading coefficient (SC), in product literature and design standards. If you are using glass whose performance is listed in terms of SC, you may convert to SHGC by multiplying the SC value by 0.87. The solar heat gain coefficient (SHGC) for your window may be found on the National Fenestration Rating Council (NFRC) label affixed by the window manufacturer prior to sale.
Commercial windows and COMcheck
In collaboration with the National Fenestration Rating Council (NFRC), NFRC certified products are now linked from a Component Modeling Approach (CMA) database. This new feature allows users to provide valid and certified fenestration thermal properties in order to improve compliance. Read below to learn about how to verify energy code compliance for commercial site-built windows.
Commercial fenestration has been identified as one of the most non-compliant provisions in the energy code. One reason is that many commercial window assemblies are site-built. Widely informing the design community about this requirement through regular trainings and at the beginning of each permitting process can help improve the compliance rate.
For site-built windows, under the 2009 and 2012 IECC, a NFRC Label Certificate must be provided for all windows on a given project. NFRC’s commercial window energy rating process enables manufacturers to combine frame, spacer, and glazing components into project-specific, final window products—listing U-factor and SHGC as the NFRC and the IECC require. Visit NFRC’s website to access the label certificates or ask the manufacturer (often the frame manufacturer) to provide the label certificate. The U-factor and SHGC values on the label certificate should be compared to the specifications on the building plans. Projects lacking these certificates must use the IECC default tables for U-factor and SHGC values (table 303.1.3-1, 2 & 3).