venting to the outdoors

i have just about finishted my growroom in my attic i built a front wall to section off the room. It is an a frame because of the sloping roof it is all insulated and i am going to lay down plastic to make a clean environment. My problem is with ventilation. Im using a 400w mh and about 400 w of cfl's. I have a fan for the mh light but i think i need an exhaust for the room in general and i will also need an intake vent. I have two options as far as access to fresh air. A window on the far side of the attic about 25 ft from the grow room, or a vent that is on my back wall of the grow room. My plan is to crack the window on the far side to let fresh air in and put an intake fan on the front wall of my room and use the vent on the back wall as an exhaust. This i can handle my concern is how to stop cold air and light from coming into the vent and through the exhaust and into my room. If any one can tell me what they would do it would be appreciated. Odor is not really an issue where i am i just dont want my babies getting cold. I am willing to scrap this exhaust plan if someone has something better

 

i had thought about reversing the lights to run at night but come november and december its going to be cold outside like new england cold and im afraid to have that draft coming into the room lights on or off i would just like to control as much as possible

 

i have a nice space heater that works really well i just wasnt sure if that would be detrimental in any way but i guess dry heat is better than cold air

 

thanks for the advice i knew there had to be some type of gasket or valve and the thermostat is ingenious thanks again