The 800-square foot room is heated to 185 degrees Fahrenheit and can fit almost 100 nearly naked Gothamites. 5 Bathhouse Flatiron features one of the largest saunas in the country. It’s a sexy space.”Indeed Bathhouse Flatiron — an 18-and-over spa that debuted earlier this month and is open until midnight every day — was designed to make people look good. “It’s something for the hardcore sauna-heads.”The Bathhouse space wasn’t always so sexy. Some say the sauna’s size, while sexy, is almost intimidating.