Growing Indoors, Outdoors (if you catch my drift).

Ive got a 1x1x1.6m budbox with a 400w HID and a 100mm extractor fan. the tent is setup in an outdoor concrete shed that can get pretty cold at this time of year. my babies are on 18 hours of light at the moment (7pm till 1pm next day) but im not sure how long to leave my extractor on. I want them to get O2 but dont want to chill them by dragging in too much cold air. has anyone got any advice for me on this?

if its cold now... what about in a few months?

well it'll be colder in a few months which is why Im asking the question now. It rarely freezes over in ireland but it can get pretty damn cold. does anyone know the minimum temperature marijuana can survive???