beenthere, i tested the exhaust with the door opened and closed and it didnt seem to make much of a difference. there is a narrow 1/3 inch gap on the bottom of the door that i can feel air being sucked through.
scottr, the crawlspace is not terribly cold. it was a bit cooler than the rest of the house but not freezing so i dont think insulation in there is an issue. thanks for that suggestion anyway.
hvaclover, the flex tube is not insulated and is not like the pic you attached. it is very thin aluminum. i am not sure what this flex duct is rated as. i think i threw away the packaging already but i can go back to home depot to find out what it is. should i be looking for "rated for bath fans" or something like that? finally, the flex duct was not pulled taut. i believe its supposed to stretch out to 12 feet but i only used about 10 of it (including the bends). is it really bad to have the last 2 feet un-stretched? i would prefer leaving this flex duct as is because modifying it would require ripping apart the soffit i just built above the shower. at this point, it would be easier to just replace the fan with a 150 CFM unit.