The National Fenestration Rating Council (NFRC) is a not-for-profit trade association dedicated to identifying true performance of fenestration systems and products. Since 1989 they have championed the process of fairly and comprehensively rating windows, doors, skylights, and similar products. They have developed a uniform testing and rating process that quantifies the key elements of fenestration performance including:

• a procedure for determining the total product thermal transmittance ("U-factor"), not just the U-factor of the glazing
• solar heat gain coefficient ("solar heat gain" or "SHGC")
• visible transmittance ("VT")
• air leakage ("AL") in residential window units and
• condensation resistance ("CR")

Together, these individual rating procedures are simply known as the NFRC Rating System which employs both computer simulation and physical testing by NFRC-accredited laboratories. The NFRC Rating System is supplemented by two separate product certification programs, one for residential products and one for commercial (nonresidential) products, where fenestration manufacturers or responsible parties may certify and label fenestration products to indicate the performance ratings achieved. Both of these product certification programs have been updated as of May of 2012.

Residential window performance: NFRC ratings for residential windows look at the total performance of a manufactured window unit and typically carry an NFRC label similar to the one above. Source: NFRC

For residential fenestration products, NFRC's Product Certification Program (NFRC 700, PCP) sets forth the specific requirements for rating, certification, and labeling of a residential manufactured product. For windows, it essentially requires a pre-glazed, manufactured window unit to be tested as a total product. Any given manufacturer will have multiple manufactured window types, sizes, and shapes, and each significant variation needs to be tested and rated. When complete, a standard label can be applied to the finished product that identifies the key elements of performance, much the way mileage rating stickers are applied to cars or nutrition labels are applied to food.

For commercial (non-residential) projects, the NFRC's new Component Modeling Approach (CMA) Product Certification Program enables whole product energy performance ratings for fenestration systems of all types. This program recognizes that commercial buildings often employ custom fenestration systems where architects can select from a myriad of choices related to glazing, frame material, configuration, size, and shape. Therefore, the concept behind component modeling is based on performance data from the three primary components that make up a fenestration system of any type:

• Glazing: Glazing manufacturers do their own testing and identify optical spectral and thermal data for the glazing that they produce. This data is submitted and recorded in the International Glazing Database (IGDB). The NFRC CMA program uses this data as it applies to a specific glazing and incorporates it into the overall rating determination of a specific commercial fenestration system. 
• Frame: Manufacturers of window frames provide the thermal performance data of frame cross-sections based on testing and computer simulation as appropriate.
• Spacer: The efficiency of different glass spacer components is identified based on  geometry and materials.

Using the basic data above, the NFRC has also made available the Component Modeling Approach Software Tool (CMAST) which establishes a set of performance libraries of approved components (frames, glass, and spacer). These libraries can be accessed for configuring fenestration products for a project, and obtaining a U-factor, solar heat gain coefficient (SHGC), and visible transmittance (VT) rating for those products. All of this can then be reflected in a CMA Label Certificate for indications of performance and code compliance.

Commercial fenestration performance: NFRC ratings for commercial fenestration systems are generated through a combination of physical testing of components and computer software to demonstrate code compliance. Source: NFRC