ok I'm growing a plant outdoors right now.but I'm setting a grow box in my attic it could get up to 100F in there what can I do?! ...for under $50. Also if I want my finished product to be like 20"-24" when do I start 12/12 light cycle. Does the plant grow in height when it budds
Hmmm. That's a lot of half-info, no offense.
Do you mean that you've got a plant growing outdoors that you want to bring indoors eventually, to your attic? If so, why? It's summer and the days are getting longer in light for another six weeks or so.
Or do you mean that you've also got an attic set up and would like to start that in addition to the outdoor plant.
In either case, 100 degree temps are not going to work. That's just entirely to hot for any strain. You're going to have to find a way to vent that heat elsewhere. It's impossible to tell you how, since you haven't provided any info about your attic. You might want to venture over to grow room set ups and look through a bunch of those threads. I can tell you that venting and exhaust and heat are some of the most challenging things about indoor growing.( I had a horrible time my first time around because I needed really quiet fans.) So start there, in the grow room section, but rather than making your own post, you might want to just read a lot of the ones that are already there. You will find SO much info there.
Regarding flowering and how big you want your plants? That's dependent on so many different factors that it's just impossible to answer. Again, you should read a lot of the threads that already exist on these topics so that no one jumps all over you (and they sometimes do--a drag, yes, but they do). As far as how much your plant will grow during flower, yes, they grow exponentially more during that phase than during the vegetative phase.
Im not sure you can really aim for a certain height. That depends on so many things: what type of strain you've got; when you flower it; how much light it's getting; and so on.
So if I were you, I'd go browse in general mj growing, grow room set ups, and don't leave out the threads that are stickied at the top of some of those sections. There's one written by Widow Maker, I believe, that is called 101 Already Asked Questions that is priceless.
Good luck. Hope it all goes well for you.