I flower in a 12 x 8 outdoor shed, and right now it's 31 degrees f. (outdoors, lol) I use a small heater to heat the shed...and since heat rises...I use a fan near the roof that blows the warm/hot air directly down on the ladies. Shed stay's closed during the night with no ventilation, but I still provide plenty of circulation.

Keeping the pots off of the cold floors is a must, also. Setting them on a blanket, plywood or milk crate helps keep the roots from getting too cold.

Try to keep the highest/lowest temps within 20 degrees or so. Don't let the high temps get to the 80's if the cold temps get down to 50's. The wider the temperature spread, the more it stress them out.

Any lower than 50ish and virtually all growth stops. (or at least get's painfully slow)