The NFRC Window U-Value Rating Procedures

During the 1980s, the availability of energy-efficient window components and products grew at a rate faster than the window and building industrys ability to analyze their performance accurately and efficiently. As a result, a coalition of industry and public sector groups formed the National Fenestration Rating Council (NFRC) in an effort to provide standard methodologies to rate the thermal performance of windows. The NFRCs first task was to develop a methodology for evaluating the thermal transmittance (U-values) of fenestration products. This procedure, published in 1991 as NFRC 100-91 (NFRC 1991) and already referenced by state codes in Alaska, California, Idaho, Minnesota, Oregon, and Washington, allows the manufacturer to use a unique combination of advanced computer simulation tools coupled with improved laboratory test methods. Since most manufacturers offer dozens, and often hundreds or thousands, of individual products, each with significantly different U-values, these simulation tools are an essential component of the rating systems cost effectiveness. This paper discusses this procedure and its intended use in more detail, and outlines the NFRCs future plans for developing rating procedures for solar heat gain coefficients, optical properties, infiltration, condensation resistance, and annual energy impacts.