URD 1/0 wire for 100amp subpanel 60' away from main box
Here is the UL standard summary for that type of cable:

https://standardscatalog.ul.com/standards/en/standard_854_12

It says:


  1.2 These cables have insulation of solid, extruded dielectric material(s) that are for use in wet locations at 75°C (167°F) and
  lower temperatures. Cables that are marked with a conductor type that
  includes the letters "HH" have insulation that is for use in dry
  locations at temperatures as high as 90°C (194°F) as well as in wet
  locations at 75°C (167°F) and lower temperatures. Cables that are
  marked with a conductor type that includes "-2" have insulation that
  is for use in wet or dry locations at temperatures as high as 90°C
  (194°F).


So unless you attic is getting above 90C (which is REALLY hot!) you should be good.

Are you sure you need 1/0 here?  That's a huge gauge for 100A.  But yes, that will work.