How to maintain temps when lights are off ?

Growing in a 8X7X7 box in my garage.
Using a 1000 watt inline and a 600 watt reflector. I can maintain temps with lights on and exhaust system now but I am wondering what to do when the winter comes and the ambient temp around the box will be down to ten or fifteen degress at night.
My questions are:
when the lights are off and I have a heat source in the room do I still need to exhaust/ventilate the room at the same rate as when the lights are on ?

I have read about using a dehumidifier to prevent condensation and supply heat to the room when the lights are off. How much heat will a dehumidifier provide and do I still exhaust the room at the same rate when the lights are off ?

any ideas about how to heat the room are much appreciated

my grow space was getting a little cold .. so i went over to my local home depot and got a little space heater... a lil ceramic jobby that ran like $20 or less.. it doesn't have a thermostat on it, just a low, medium and high setting and a little dial like a car heater that shows a thicker red line the hotter that it gets.. i plugged this baby in my grow and it works great.. it only kicks on when the temp gets so low.. it took a little bit to get it tuned in to the right temp since it had no thermostat, but once i figured it out, it works great... if you do get a little heater like the one i mentioned, you could probably dial it in so that you would be able to vent still.. but that is up to you really.. i personally think that the air movement it always good for the plants and prefer to have my air circulating all of the time..
hope this helps some.. best of luck with your grow....

this should help you quite a bit. read dirty steves post https://www.rollitup.org/newbie-central/200093-fans-making-my-head-spin.html

Maybe a heating pad underneath the box so that heat goes through the box and rises, and a blanket on top of the box

The electric heater may be the best bet. Look at ebay for "greenhouse thermostat" and then buy a small heater to plug into it. I don't think the thermostat on a cheap heater is worth much (mine was +/- 7 degrees) and an add on thermostat will keep the temp within 2 degrees. I would hook a small exhaust fan to the same thermostat so the fan only goes on when the heater is on. As long as the outside temp doesn't "out cool" the heater you are in good shape. Plants want air when the lights are on but you should still maintain ventilation in the dark also.