Vent/Heat Question?

Im planing my first grow and having some trouble. The 2 area I have are both less than ideal, need advice on what to do. I have a 400w MH/HPS switchable ballest, with an air-coolable hood, and 6" exhaust fan. So here is the delima: I have a room in the basement with no vent to the outside. If I was to grow in a tent and just vent and intake to/from the room its self would that be suffeciant? Or, I have a space in the loft which is ventable to outside, but not heated or insulated. Would the light create enough heat to keep a 6"x 6" space warm enough when temp drops to 20F in the room?

i think the problem you will have is keepin heat in the room when lights off in the attic,
i think your going to need a heater for the room as temps get colder just get a small heater put it in front of the fan and it will blow warm heat onto your plants