Many people (most of which forget that cannabis grows between the tropics mainly) will say heat over 75 is bad, believe me my loft gets up to 90 and as long as you keep an eye on things it is fine. My loft is only 6 ft at the apex so I only grow plants to about 2ft max but fim and lst. If you want to grow sativas then i suggets setting up a horizontal grow. Once the plant gets to about 6'' top or fim them and train the resulting tops (I usually get 5) into a star shape.This way you can slot many plants into a small area as the branches can be trained to fill the gaps of the others. Hard to explain without a picture! Alternatively a scrog is another way of keeping height down without growing loads of small plants. 12/12 from seed is another way that I have been doing for a number of years but really only gets good results (2-4 oz per plant) with Indica's and lots of training. Naysayers on here that havent grown 12/12 will advise against this, mainly because they dont know whats what and believe that bigger is better.