My colleagues had organised an event at the Lähteen Sauna, in the area of the Lapinlahti mental institute. Ex. institute... and Lähteen sauna is as far as I know, the oldest still operating sauna in Helsinki that is still open to public.
Very good heat in the sauna! Apparently the easy access to water is kind of far away, however, even if the sea is right next to the sauna. Still, much recommended. 7€ for entrance fee, to be paid at the cafeteria inside the main building.
Photos and text (c) 2019. All sauna photos have been taken with permission, when the facility is closed, and/or when there are no other guests present.