Pretty much exactly what the title says. Do you need fan intakes for a dedicated mining room, or just enough exhaust to cycle the air enough, with an open vent to fresh air?

Also for an open air rig setup, I'm having some heat issues already. I have 4 rigs in a single room, hanging on wire racks. Should the cards have fans blowing directly on them as well? The cards are sitting 70-75 at 75-80% fan speed. This was during a major heat wave here, but that was only 90 Fahrenheit temps, which will go up in the summer.