DSpanoudakis -
"Makeup air" is the air required to provide adequate ventilation based on the CFM of the fan.
Imagine that you have a 16" box fan in a window (fan completely sealed and no screen on the window) in a small 8'x8' room and the door is closed. You would notice that not much air is being pulled through the room and exhausted out the window. That is because there is very little "makeup air" coming in from what little space there is around the door that allows air through. Now, open the door and you would see that there would be plenty of airflow through the room.
NuTone (a fan manufacturer) recomends 1 square foot of open ventilation for every 300 CFM.
You need to see if there is any soffit (sp?) vents, gable vents, ridge vents, etc. and then determine what the sum of their square footage is.
The best "makeup air" venting would be soffit vents.
If you do have a gable vent and are gong to install a powered fan in it, make sure that you close the space around the fan or you will just be drawing air from it and not the soffit vents.
Hope this helps.
Steve
