I have 2 rooms in one big shed and they are not totally light proof but pretty good. I'm not sure if it is a big as deal as made out to be, but really have no clue, I have had grows where there was total darkness and grows where there was a bit of light leakage (not unlike an urban outdoor grow) and there was no disconcernable difference. I have even opened the door .... :O... I do have a green light for working in dark periods though. That is not to say that total darkness, when achievable, is probably better. But I don't concern myself with a bit (glow of control panels, etc) of light in my grow. What is the effect of a small amount of light leakage (zipper incl.)? Is it smaller growth, longer finish time, loss of potency?