Growing cannabis indoor means recreating the climate of the plant's native environment.
Close your eyes and imagine all those wild Indicas growing in the wild, on the hilltops of central Asia or South America.
That is the climate you want to achieve if you have your eyes on high yields of top quality marijuana.
Room temperature od approx 20C, not to mention some abandoned factories, basements or attics with their 10-15C, is not high enough for cannabis.
Too much of the good thing is no good either - HPS light bulbs emit lots of heat, which in closed boxes could lead to temperatures reaching 40C.
What you should be aiming for is 26-28C, evenly distributed in the whole grow area.
The same goes for humidity: too low, and your plant will grow slower then it might; too much, and mould will develop.
At the first stages 60-80% humidity will be perfect, dropping to 40-50% once the buds get dense with resin - that is when the risk of mould is greatest and the cost highest.
Last but not least is carbon dioxide (CO2) - the main feed of your cannabis plants.
If you keep your plants in closed spaces, there is a risk of a low level of CO2.
That will severely affect the rate of growth.
CO2 deficiencies have to be avoided at any cost - and actually by bringing the level above the natural one of some 340 ppm (parts per million) you can positively affect your plant, leading to even higher crops of marijuana.