Ok,
Ventilation is important. But its not enough to add another 700CFM fan.
Think of it this way: Your grow room is in a room say just for kicks 6' x 8' and it is surrounded by a balloon. If you into the balloon without getting the old or heated air out, you are not ventilating. You can only blow so much into a balloon, and if there is no escape of the warmer air, you will only increase air pressure. 2 1000W lights produce right around 8,000 BTU. I don't want to get technical, but for your information, I have a gas fireplace that puts out a max of about 12,000 BTUs. That gas insert will heat my entire house quite comfortably in the winter time without any help from my furnace. I am sure your grow room is a lot smaller than my house. So now imagine that fireplace turned on high in your grow room in August, when the coldest air you can find might be 75F... and there is no air conditioning to help!
RULES:
Exhaust 10% more CFM than incoming fresh air (That way you will encounter less odors coming into your living quarters), so your grow room will be at negative air pressure.
Get your fresh air (esp when lights are on) when they are the coolest air temperatures.
Doing it this way, if your outside air temperature is say 70F, you should be able to maintain 80F. In other words a 10 degree difference.

Good Luck!
SN