I've actually had this room going for about 3 months now, starting from 0 experience, and trying to rush myself through all knowledge from every forum, and everyone's personal experience. I do have mylar in the flowering room, 4-5 foot high.. It's also in the veg area. The flower room is blocked off by black/white poly plastic stuff, and a velcro door. The two rooms are somewhat air joined so air from both rooms is vented. I managed this, keeping it air tight by making a maze-like structure from 2x4, and putting the blackwhite poly over both sides. One hole in the flowering, one in the veg.