Thermally insulating fluid

A thermally insulating fluid composition comprising a glycol solvent for a viscosifier, a viscosifier, and optionally, an aqueous brine. The glycol may be selected from a propylene glycol, or under excessive heat temperatures, a butylene glycol, which can be used with or without a viscosifier. Viscosifiers can be selected from hydroxy propyl methyl cellulose, xanthan and hydroxy propyl guar and combinations thereof. In one method of producing the thermally insulating composition, a viscosifier is added to an aqueous brine, water is then added to that solution followed by the addition of dry salt of the brine to increase density if necessary. A propylene glycol solvent is added to the resulting solution. The thermally insulating fluid composition insulates substances stored or transported in multi-walled vessels, tanks, piping and thermal units. A method of applying the insulating fluid composition of this invention comprising the steps of injecting the thermally insulating fluid into the cavity formed by two adjacent walls of the vessel or piping and then sealing off the opening.