Brenda is correct. It's true that air conditioning was added to Orchestra Hall during the 1966 renovation, but the project was undertaken for far bigger reasons. The majority were calamities, above all the idea that the sound of the hall could be improved, whereas it was wrecked. But another was the practical one of giving the CSO members a reasonably comfortable backstage space for themselves, which they'd never had. I remember standing on Michigan Avenue during the summer of 1966 and looking through the glass on Orchestra Hall's doors. I could see traffic on Wabash Avenue, a block behind the building. Everything in Orchestra Hall, including its stage and back wall, was gone. All was rebuilt and reconfigured during the renovation.
Don Tait