GNU Collaborative International Dictionary of English

Found one definition

1.       
   Insulator (?), n.
   1. One who, or that which, insulates.
   
   [1913 Webster]
   
   
   2. (Elec. & Thermotics) A substance or object that insulates; a nonconductor; as, “polyurethane foam is a popular thermal insulator”.
   
   [1913 Webster +PJC]