How cold can it be in my grow chamber?

Happy Holidays Everyone.
Hope some knowledge can be passed down.
Finally have a seedling (Speed Devil #2) growing in my cabinet in the garage. Problem is that it is now getting cold at night, I have insulted the box, but the 600W HPS are barely keeping it above 55. Should I just cut way back on my air exchange rate, which is currently on for 1/2 hour, then off for 1 hour? Also, how cold can my water in the resevoir be?
 
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