Temperature Control

Im starting a grow box (4x4x4) up in my attic, and this is the one problem I can't figure out. My attic is prolly around 40 degrees around this time of year, and I need about 74 degress in my box. I'm ging to use a 400W MH/HPS light, and i was thinking of putting a 80cfm fan in for ventilation. my dilemma is, one, should i get two fans, one for intake and another for exhaust, and if i do, would the cold air coming in offset the heat given off by my light? If I do not get an intake fan, will there be enough fresh air? I was thinking of a small space heater with a thermostat on it for heat, but if i get an intake fan its going to be on constantly. Any suggestions????

I can tell you I use a closet (grow area is about 4x4x4) in my basement about 60 degrees f this time of year with one fan and it does not get warm in there. The airflow in is through the slatted door on the closet. I don't think two fans will hurt. Why not use the heater as the intake when the light is off? and keep the exhaust fan going? Just a thought good luck.

4 x 4 x 4 = 64 cubic feet. ideally your exhaust fan would be able to exhaust your grow area three times a minute so your ideal exhaust fan would have a CFM rating around 190 (64 x 3 = 192).

as far as heat/cold - my box is similar (2 x 4 x 7) - i'm using a 1000w light and 440 CFM exhaust fan and a 170 CFM fan to cool the lights. it is in an environment that stays around 60 degrees and during the winter i have to turn off the fan to cool the lights for the box to stay warm enough. In the summer however - it's all i can do to keep it cool.

with the right exhaust fan - no intake fan is necessary (passive intake holes still are though!)

if you cant keep the temps tropical you will be in trouble fast! cold air will slow growth substancially