heating and ventilation problem

hello, im going to set up my first growing box with 1.20m x 1.20m x 2.00m into an old shed. Problem is that the temperature in the shed are almost same as outside, winter is coming and temperature might get even -20c at some point. So i have a little problem with heating, i also have 400W hps lamp, does the lamp and ballast produce some heat, if i'd put ballast also into the box or what would be the best option to heat this little box. Another problem is also with ventilation, if i want some fresh air coming from outside the box i think it would be too cold then, maybe some little cracks in the box also help air to circulate, if i just put ventilation into the box?

i thought insulating inside with this white thing which consists with those small soft balls, dont know how they're called in english , but yes i also need extra heating device

That's a real small area for a 400w...

I actually have the opposite problem with my grow room is going to be to hot.

I think you would need a heater at all times, definately when the lights are off. The ballast creates heat, but not enough to compensate for below freezing weather.

I would insulate the room, and try getting a portable heater.

Get a small honeywell with a built in themostat and set it up for your desired temp and it will shut off once you reach the tempand come ononce it drops below but always have circulation or it will lead to a mold problem