[TychoMonolyth]

Freaking ground is still frozen here, but the creeks are filling with water so that water's coming from somewhere.

[Athos]

If the ambient temperatures are too cold there's a couple of thing you can do.

In my case night temperatures get as cold as 32F (0°C) and in the grow space I can keep it at 77F (25°C), just with 60 watts of lights.

What I do is decrease the exhaust fan so the heat inside isn't vented out too fast. Use a smaller fan and / or connect it to a variable control (I use a PC fan and have it connected to a variable voltage adapter).

How much CFM? In the design sub forum in this very site there's a formula to estimate how much (or how little) you need to maintain a certain temperature.

It's something like this: CFM needed = 3.16 * watts / Difference in F

Watts been the electrical power that dissipates inside (in my case only the lights)

Difference in F is the allowed difference in temperature that you want between the inside of the plastic tot and the outside; in this case since the difference is quite high, you'll need very little exhaust power.