I'm going to build a grow box, but before i begin i'd like to hear what u guys think of it. This will be my first time growing and building something like this so feel free to leave any comments and suggestions you may have.

First off, I'll be constructing two of these things, each 36"x36"x36" using plywood. The inside of the box will be covered entirely with mylar. I'll use one box for vegetating and the other for flowering.

For lights I will be using 400w MH (400 Watt Metal Halide Agrotech Reflector Grow Light System) and HPS (400 Watt High Pressure Sodium Agrotech Reflector Grow Light System) lights.

For ventilation im considering using this fan i found on homedepot website (NuTone ValueTest® Wall/Ceiling Fan - 696N at The Home Depot). its called NuTone ValueTest wall/ceiling fan and has 50 CFM. I plan on having two of the fans on each box. one to remove the air inside and the other to bring in fresh supply of air.

You might want to have both fans as an exhaust and have a passive intake.

Can you make the boxes taller. You need to factor in the space the light hood and the pots you may only end up with 12-18 inches for growth.

I'll be growing some white widow by hydroponics and at most i only want it to be around 2.5ft tall after vegetative and flowering. how much taller should i make the box to accommodate that? the lighting system with reflectors and all will be 6", so should i just make the box six inches taller so there is room between the top of the plant and light. and by passive intake do you mean i should just have some sort of opening in the box to let air flow in naturally?

you need a stronger fan, get an inline duct fan from home depot at least the 6 inch 250cfm. You will need a taller box as well, I made the same mistake. My box is 3x3 and 4 feet tall and I still feel it isn't tall enough. Your pots will take up 12 inches and your light will take up the top 10 or twelve inches. and then your left with only 12 inches of usable space.

Oh yea, you are going to need a stronger fan, like gangababy said an inline fan will work great. Heat will build up fast in a cab without proper ventilation.

you will want to get an air cooled hood too it will significantly lower the temps. I have a 400 watt hps in a sun systems II air cooled hood and the temps stay around 75-80

I agree i think 4 feet minimum. Personally i wouldnt go below 5..

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Current Grow - AK47 Day 33 Flower Grow Cabinet Build First Grow in new Cabinet C99 X A11

You are going to need a good exhaust fan for a 400 watt HID lamp. My cabinet is 4ft tall with 250 watts of lights and a 120CFM fan just barely keeps temps in check. If you are constrained with height then you can always grow scrog or go lowryder (like I do), but the other posters are correct - if you have the space then go as tall as you can and you won't be limited by the height of your grow. Ventilation is the key to a cabinet grow - get a big exhaust fan. You can always dial it down if you need. Make sure you exhaust out the top and draw in air from the bottom. HOT AIR RISES. If sound is an issue then get a S&P Mixvent fan - they are quiet and move alot of air. I would avoid the Home Depot inline fans. I had the 6 inch which did well, but it was loud/whiney. Do it right the first time and you will save alot of cash.