49 C.F.R. § 179.220-4 Insulation.
Title 49 - Transportation
The annular space between the inner container and the outer shell must contain an approved insulation material. [Amdt. 179–9, 36 FR 21340, Nov. 6, 1971]
Title 49: Transportation
PART 179—SPECIFICATIONS FOR TANK CARS
Subpart D—Specifications for Non-Pressure Tank Car Tanks (Classes DOT-111AW and 115AW)
§ 179.220-4 Insulation.