It's all about airflow past the cards.
Not mine at all, but good summary of a lot of thoughts around cooling open air rigs with "hot" air.
https://www.youtube.com/watch?v=59I1KvEpMcoNotice the comments about the improvements made related to the airflow past the cards.
My own rig is spaced up to have 8cm c-c between the cards and a 12 inch table fan circulating the air through the basement. It's only a 9 card 1KW rig and the cards are running below 60 C. The next experiment would be to baffle airflow past the cards using 4x140mm chassi fans (one sheet at riser level and one at the top of the cards), but it works so well already, getting the cards down to 50C will not gain much since the OC ability does not seem to be that affected by temp when they are this low.
The fan circulating the air dropped the card temps about 10C, so general ventilation is very important, as it also helps getting the heat out to the surroundings (It distributes the hot air in the air volume, improves transfer of heat to walls etc) even if you don't exhaust the air.
Remember that exhausting air menas it must leak in somewhere else, and doors suddenly slamming shut can be annoying to someone important in the family...
