I just want to mention everyone is right here. You can grow in an attic, I do. But i also have a 24,000 btu mini split. And a friend who studied thermodynamics to become an engineer. The real trick is fans, use fans to equalize the temp to the temp outside, then crank the ac. I run a 6k room, with one 24,000 btu ac and dehumidifiers. temps go 80- 86. run at night. ( Dont push your ac, it will break.. )
So with all that being said, ive put my soul into this. If you want to do it in an attic, and its all you got..Do it, but the summer will be tuff until you get equipment. Winter is your friend though, so when that comes around by some real light! anyways have an awesome day!