...it's not just a residential issue anymore!
IECC Table 802.2(2)
This proposal eliminates the distinction between "factory assembled" and "site built" fenestration products, adjusts the U-factor for commercial glazed systems to a reasonable level that manufacturers of commercial glazed systems can meet while still providing systems that have the strength and durability needed in commercial buildings and revises the SHGC requirement for residential type windows to be consistent with the requirements for residential windows in Chapter 4 of the IECC and Chapter 11 of the IRC.
IECC Section 402.5.1
402.5.1 Maximum fenestration U-factor and SHGC. The area weighted average maximum fenestration U-factor permitted using trade offs from Section 402.1.3 or Section 404 in zones 4 through 8 shall be 0.48 for vertical fenestration and 0.75 for skylights. The area weighted average maximum fenestration SHGC permitted using trade-offs from section 404 in zones 1 through 3 shall be 0.50.