tomthehippie is right, and depending if you're using 1000+ watts of HPS or MH, I doubt your fan is going to be sufficient. My room exchanges air every minute, and it has heat build-up problems. Without air conditioning it's tough to keep the girls cool enough when the outsid temperature hits over 80F, and each 1000W light produces 3000 btu of heat.
By the way, I was in the heating/airconditioning biz for 10 yrs.