Thanks all, well I made some progress. I found a few remaining air leaks coming from the bottom of one of the walls. Nothing some spray foam couldn't handle. After sealing those leaks I could then feel a gentle air coming through the intake vent. The fan is rated for a room 20x20ft and with my new wall in the darkroom in now 17 1/2 x 11ft, however the ceiling is on a slope up to 9ft so that ads extra volume. I'm not sure if I'll need a vent hood or not? Is there a trick to determining this? Would like to hear from people with a darkroom my size to see what they do. Also the fan's out nozzle had a stamped metal cover with holes in it (I assume to keep critters out), I got a nibbler tool and cut that out and I think it's probably increased airflow atleast 20%.