Insulation was a lining used to maintain the temperature or environment of an area.
Trellium-D was primarily used as insulation for interstellar vessels. (ENT: "The Xindi") When traveling through the Delphic Expanse, ships that were not insulated like this could suffer damage from spatial distortions. (ENT: "Anomaly (ENT)") Enterprise NX-01's outer hull was lined with duranium. (ENT: "The Xindi")
As part of an answer to a 2151 "poop question" posed by Molly McCook, Trip Tucker mentioned that waste was turned into useful items for their ship, including insulation. (ENT: "Breaking the Ice")