Calculate your grow room\'s volume. Multiply the room\'s length by the width by the height. For example, if your grow room is 8 feet in length, 10 feet in width and 9 feet in height (8 x 10 x 9), the room\'s volume is 720 cubic feet.

Determine the minimum cubic feet per minute to exchange the air in the grow room. The exhaust fan should exchange the air in the room at least once every three minutes. Divide your number of cubic feet by three minutes to calculate the minimum CFM (720 / 3 = 240 CFM).

Add 5 percent to your minimum CFM if you are using carbon dioxide enrichment. For a room with 240 CFM, add 12. The real minimum CFM required to ventilate your grow room is 252.

Select the proper size exhaust fan based on your calculations.

In the wall of the grow room, create a hole that is the same size as the exhaust fan. Make sure the hole is near the ceiling, at the top part of the wall, so that the exhaust fan pushes out the warmer air that rises. Install the exhaust fan in the hole.

Make an air-intake vent. Create a hole that is 1 1/2 times the size of your exhaust fan for your intake vent. Make the intake hole on the lower part of the wall that is opposite the exhaust fan.

Insert a length of ducting into the intake hole. Cover the intake hole with a piece of screen or porous cloth to prevent insects from entering the grow room.