NFRC Ratings of Windows in Energy Codes
As owners, architects and energy modelers strive to create sustainable, energy-efficient buildings, it is crucial to consider the impact of windows on the overall performance of those buildings. Windows are a primary source of heat gain and loss, affecting a building's energy consumption and indoor comfort.
What Is NFRC?
The National Fenestration Rating Council (NFRC) offers an independent rating system that evaluates the energy performance of windows, doors, and skylights, helping to ensure consistent performance of fenestration systems. This article will discuss the importance of NFRC ratings for architects and other building design stakeholders, highlighting their significance in meeting the requirements of common energy codes like the International Energy Conservation Code (IECC) and ASHRAE Standard 90.1.
NFRC ratings are designed to help architects, builders, and consumers understand and compare the energy performance of various window products. The system rates windows based on a variety of factors, including:
U-Factor: Measures the rate of heat transfer through a window, indicating how well it insulates. Lower values indicate better insulation.
Solar Heat Gain Coefficient (SHGC): Represents the fraction of solar radiation that passes through a window, contributing to heat gain. Lower values indicate less solar heat transfer.
Visible Transmittance (VT): Measures the amount of visible light transmitted through a window. Higher values indicate more natural light.
Air Leakage (AL): Represents the rate of air infiltration through a window. Lower values indicate tighter seals and reduced air leakage.
When Are NFRC Ratings Required?
In addition to facilitating informed decision-making, NFRC ratings are crucial for architects to comply with common energy codes. Applicable codes and standards must be reviewed, but most states and jurisdictions reference codes that require NFRC ratings when any new windows are being installed in a commercial or residential project. The IECC and ASHRAE 90.1 are two energy codes / standards adopted by building departments and certification programs that reference NFRC ratings. These will require that all window systems (glass + framing together) are rated in accordance with NFRC procedures for the performance values listed in the previous section.
Beyond the ratings themselves, energy codes also mandate specific energy performance levels for windows. The minimum required values for a given project can be looked up, usually based on the location of the project (climate zone), and prescribe maximum U-Factor and SHGC. Since a lower U-factor means better insulating property, this usually results in greater energy efficiency in warm and cold climates. SHGC, on the other hand, is more complex and the optimum value will depend on the climate, building type, internal loads and many other factors. To optimize the SHGC for a project, we recommend the use of an energy model - especially in moderate or cold climates.
Recommended Steps for Architects
Architects document compliance with energy code requirements for NFRC ratings on their drawings by clearly indicating the necessary information for each type of window used in the project. This information is typically presented in a window schedule or as notes on the drawings themselves. Here are some steps architects can follow to document compliance:
Window Schedule: Create a window schedule that lists all the different types of windows being used in the project. The schedule should include information such as window size, window type (e.g., fixed, casement, double-hung), and the manufacturer's name.
Rated Performance Values: For each window type (glass + frame assembly), include the NFRC ratings: U-Factor, Solar Heat Gain Coefficient (SHGC), Visible Transmittance (VT), and Air Leakage (AL) values on the drawings. These values can be obtained from the manufacturer's product data, the NFRC Certified Products Directory, or reports provided by certified entities who can simulate custom products and provide NFRC label certificates.
Require NFRC Labels: Specify requirements for contractors to include NFRC rating information in product submittals, NFRC labels adhered to manufactured windows, and NFRC certificates for site-built or field-assembled systems. This will help ensure the right products are installed and the rating information is available if requested by a building inspector.
By following these steps, architects can effectively document compliance with energy code requirements for NFRC ratings on their drawings, ensuring that the selected windows contribute to the overall energy efficiency and performance of the building. Proper documentation also helps streamline the plan review and approval process, reducing the likelihood of delays or additional revisions.
Example NFRC Label Certificate for a commercial window.
Common Mistakes
Center-of-glass ≠ Assembly U-Factor: A common, and very dangerous, misconception is reporting a center-of-glass measured U-factor from a glass manufacturer’s cutsheet as a fenestration assembly U-factor. A common insulated glass unit has a CoG U-factor of 0.29 whereas the same glass in a standard aluminum framing system could easily jump to U-0.60.
More Questions?
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