Energy-conservation codes and good design practice demand careful consideration of the various energy performance characteristics of fenestration. The 2003 International Energy Conservation Code (IECC) and ASHRAE/IESNA 90.1, Energy Standard for Buildings except Low-Rise Residential Buildings, require that fenestration be labeled and certified by the manufacturer for energy performance, based on ratings established by the National Fenestration Rating Council (NFRC). It is the Architect’s responsibility to determine the required energy performance of the fenestration and confirm with manufacturers under consideration that they can meet it.
According to the 2003 IECC, the term fenestration products includes windows, doors, and skylights. According to the ASHRAE/IESNA 90.1, the term fenestration includes all areas (including the frames) in the building envelope that let in light, including windows, plastic panels, clerestories, skylights, glass doors that are more than one-half glass, and glass block walls.
Primary energy performance requirements usually include thermal transmittance (U-factor), solar heat gain coefficient (SHGC) and air infiltration.
Thermal transmittance (U-factor) is the primary measure of heat loss and is essential to characterizing the building-envelope heating loads. The lower the U-factor, the more energy efficient the fenestration. The 2003 IECC and ASHRAE/IESNA 90.1 require that fenestration be certified and labeled according to NFRC 100, Procedure for Determining Fenestration Product U-Factors. The 2003 IECC designates default U-factor values for use when the manufacturer has not determined values for non-labeled fenestration products.
SHGC measures how well a product blocks heat caused by sunlight. SHGC is the primary determinant of building cooling loads. The lower the SHGC value, the more energy efficient the fenestration. Low SHGC values can be achieved, but mostly at the expense of visible light transmittance. The 2003 IECC and ASHRAE/IESNA 90.1 require that fenestration be certified and labeled according to NFRC 200, Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence.
Air infiltration of the fenestration can have a significant effect on overall building performance, especially for heat loss and heat gain. The 2003 IECC requires air infiltration testing according to ASTM E 283, Test Method for Determining the Rate of Air Leakage through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences across the Specimen. ASHRAE/IESNA 90.1 states that fenestration and doors must be certified and labeled according to NFRC 400, Procedure for Determining Fenestration Product Air Leakage, except where site assembled. NFRC 400 is intended to establish certification of a manufacturer’s standard window product line and is intended for use on windows, doors, and skylights; it is not applicable to an overall building-enclosure system.
Related and additional energy performance considerations include condensation resistance (CR) and visible light transmittance (VT).
CR is a measure of a product’s ability to resist the formation of condensation on interior surfaces at a specific set of environmental conditions. Condensation forms when the surface temperature is at or below the dew point temperature of the surrounding air. The surface temperatures of each component of the assembly, coupled with overall building and HVAC operations characteristics, contribute to the likelihood or potential for condensation to occur. The CR is determined from the lowest rating for the frame, edge of glazing, and center of glazing. ASHRAE/IESNA 90.1 states that fenestration and doors must be certified and labeled according to NFRC 500, Procedure for Determining Fenestration Product Condensation Resistance Values. AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, uses a condensation-resistance factor (CRF) that does not correlate with NFRC’s rating system.
Second Quarter 2006 SpecPress 5 The VT of the fenestration can have a significant effect on overall building performance, especially with regard to daylighting. VT measures how much light comes through a product. When daylighting and high visibility are desired, select assemblies with high VT values. ASHRAE/IESNA 90.1 states that fenestration and doors must be certified and labeled according to the NFRC 200.
The NFRC is currently developing a new program for component-based fenestration product certification. This new program is intended to replace the existing non-residential energy performance rating system for site-assembled products.