How to: Airtight Negative-Pressure Grow Box

Build a growing enclosure out of wood or purchase a grow box or grow tent from an indoor gardening or hydroponics supply store. If you already have a growing enclosure, skip this step. If you build a growing enclosure and it has air leaks, use duct tape or a caulk gun with 30-year acrylic caulk to seal any gaps. When building a grow box, make sure to keep it large enough to comfortably fit the number of plants you wish to grow and high enough to hang a grow light to avoid burning your plants.

Determine what fans you will need in terms of cubic feet per minute (CFM) by calculating the volume of your grow box. To calculate the volume in cubic feet, you must multiply the length x width x height of your grow box. With this number, you will want to exchange this volume of air once per minute if you are not having issues with heat or humidity, or three times per minute if you are battling heat and humidity issues. For example, if you have a 5-by-5-by-6 feet grow box (5x5x6=150 cu/ft) then you have 150 cubic feet of volume. You will need a 150 CFM fan to exchange the air every minute, or a 450 CFM fan to exchange air three times per minute. Note that the fan determined with this calculation will be the larger of your two in-line fans.

Obtain two in-line fans, one having double the cubic feet per minute (CFM) of airflow as the other. The fan with less CFM will be your intake fan and the other fan will be your exhaust fan. Most in-line fans come in 4-inch, 6-inch, 8-inch or 10-inch diameters. Most grow tents have ports with rope cinches that fit up to 8-inch in-line fans, and you can easily fit your fan inside the port and cinch the rope around the fan to close any air gaps. If you made a grow box, you will now use a power drill and a hole saw to cut holes to the size of the fans on opposite sides of the enclosure: one for the exhaust fan (larger CFM) and one for the intake fan (lower CFM).

Use duct tape if you would like to completely seal the fan to the port of your growing enclosure to prevent any air moving between the small spaces between the fan and the port. Once you turn the fans on, the force created by the exhaust fan will overcome the force of the intake fan and create a negative-pressure grow chamber with great ventilation and air flow.