Note! This tool has been removed from Naviate HVAC&P version 2016.1 and later.
U-Factors is something Revit uses when analyzing Construction Types.
It affects heat requirements, energy calculations and export to gbXML.
Though Revit has a builtin list of Construction Types you cant edit those using core Revit functions.
U-Factor lets you do this.
If you cant find your specific wall type in the builtin list of types just add it with the U-factor function.
Once started click add to make your very own construction type.
You can verify the result afterwards in Revits Construction Type list.