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") Enterprise NX-01's outer hull was lined with duranium. (ENT: "The Xindi")