Firefighters responding to a report of a two-story structure burning out Croy Canyon west of Hailey on Thursday night arrived to find the upper floor of the building fully engulfed in flames.
"The whole second story of the building was fully involved," said Wood River Fire & Rescue Assistant Chief Jeff Nevins, the incident commander on the fire.
The two-story barn with apartment on top is at 98 Pioneer Dr., west of Hailey in Blaine County.
Nevins said firefighters were dispatched to the fire just before 9 p.m.
He said the fire caused "extensive damage" to the upper 800-square-foot apartment, which he described as "a total loss."
The fire caused no injuries.
Nevins said the lower portion of the building received minimal damage due to the quick reaction of the 37 firefighters who arrived from Wood River Fire & Rescue and from the fire departments of Hailey, Bellevue, Ketchum and Carey.
"It was really good work on our firefighters' part," he said.
Nine fire engines from the various departments were called to the scene under auto-aid agreements.
Wood River Fire & Rescue Chief Bart Lassman said the fire started after towels or some other fabric full of massage oil residue spontaneously combusted after being placed on top of an electric warming blanket. Lassman said the towels had been laundered, but still contained enough massage-oil residue to begin burning.
He estimated damage to the building at $120,000 to $150,000.
According to information provided by Wood River Fire & Rescue, the last fire engine left the scene of the fire around 12:30 a.m. on Friday, Feb. 1.