Site-built Glazing Systems FAQs

How do I show compliance for site-constructed glazing systems?
How do I show compliance for a curtain wall system when I have assembly performance data from the manufacturer?

Q: How do I show compliance for site-constructed glazing systems?

Compliance for site-built glazing systems is comparable to other assemblies within the building envelope but can be challenging because documentation on glazing performance values-U-factor, Solar Heat Gain Coefficient (SHGC), etc.-is not usually available. Without documentation, default values from the code may be used, but these values assume the worst in terms of thermal performance and often present code compliance challenges. Default values can be taken from tables in Chapter 1 of the IECC, Appendix A of ASHRAE 90.1-1999 or 2001, or by right-clicking on the appropriate cell in the COMcheck™ software.

The National Fenestration Rating Council (NFRC) also has a rating program for site-built windows. See http://www.nfrc.org/sb_programoutline.aspx for more information. If the site-built glazing used in your project has been rated through this program, the performance ratings can be used for energy code compliance.

Figure 1: Determining Overhang Projection Factor (PF)

Code compliance challenges can be alleviated somewhat if glazing overhangs that provide significant shading are used. Overhangs must be a permanent part of the building, however, before the credits can be applied. The projection factor (PF) will determine how much credit will be given. See Figure 1.

The PF is calculated by measuring the distance from the edge of the overhang to the window (A) and dividing that distance by the distance from the bottom of the window to the lowest point of the overhang (B).

Q: How do I show compliance for a curtain wall system when I have assembly performance data from the manufacturer?

If the wall assembly is essentially all glazing, the easiest way to show compliance for these systems is to use the COMcheck software, Version 2.4 Release 2c or later,^* following the steps below.

Within the "Envelope" tab, enter an exterior wall and window for each curtain wall:

Create Exterior Wall
- Select "Ext. Wall" and choose "Other"^** as the assembly type from the drop-down menu.
- Enter the total square footage for the entire assembly (including the framing-the framing area should be less than 10% of the gross area of the wall).
- Enter the U-factor from the manufacturer for the entire assembly.
Enter Window
- Select "Window" and choose "Other" as the assembly type from the drop-down menu.
- Enter the estimated square footage of the glazing. This number should be greater than 90% of the total square footage of the entire assembly.
- Enter the U-factor and SHGC from the manufacturer for the entire assembly. Enter PF, if applicable. The software accepts PF values from 0.0 to 1.0. However, for all 90.1-1989 based codes, the PF ratio credits are limited to 0.5.

^*To download the most recent version of COMcheck go to www.energycodes.gov/comcheck/ez_download.stm.
^**The Georgia Energy Code does not allow the use of the "other" category for envelope assemblies. Please see your local code official for other compliance options.