The American Iron and Steel Institute (AISI) has published AISI S250-21, North American Standard for Thermal Transmittance of Building Envelopes With Cold-Formed Steel Framing, 2021 Edition,” as a single source for calculating the thermal transmittance (U-factors) of walls and ceiling/roof envelope assembles that contain cold-formed steel framing. The standard is intended for adoption and use in the United States, Canada and Mexico and is available for free download at

AISI S250-21 was developed to eventually supersede the many methods currently found in various energy codes and standards, most of which are limited to calculating wall assemblies at 16 or 24 inches on center framing only. The standard’s mathematical options are divided into:

  • Wall assemblies that use standard C-shape framing,
  • Wall assemblies that use other than the standard C-shape framing,
  • Roof assemblies that use conventional C-shape rafter framing methods, and
  • Roof assemblies that use cold-formed steel truss framing members.


AISI S250-21 was designed to represent the next level of analysis for the well-known Modified Zone Method, which was originally created in 1997. The advantages of AISI S250-21 over all previous calculation methods include the ability to analyze wall assemblies that have: 

  • Framing members spaced from 6 inches to 24 inches on center,
  • Framing member steel thicknesses from 33 mils (0.0329 inch) to 68 mils (0.0677 inch), and 
  • Wall assemblies with insulation in the cavity, partially in the cavity or no cavity insulation.


AISI’s Committee on Framing Standards created a spreadsheet that performs the mathematical calculations based on AISI S250-21. The spreadsheet is available at no cost and substantially reduces the time needed to calculate various envelope assemblies based on the standard.