My (5) Outdoor White Widows are chaining up nicely 1 week prior to Lunar/solar 12/12 season. The temps hit 37 degrees F. overnight.
Sped up installation of Veg Chamber and converted it to a Bloom Chamber. It is located in an attached shed wired up and the chamber sectioned off. 2 walls concrete, 1 wall framed sheet metal and last side a door with built in fan and port for portable AC/Heater. Cieling is rafters for plenty of ventilation, (2) concrete walls, (1) framed sheet metal wall and last wall is a door.
. I was thinking of covering the exposed rafters with sheeting and Mylar leaving the rafters still open at the ends. Is this beneficial as far as Lumens per plant?
Outdoor plants now inside. More light blocking to be added. (Shed will be dark).
Lighting: (1) 300 watt Feliz CFL “Red”’ (1) 300 watt Feliz CFL “Dual” and a bank of (4) 54 watt, 3000K x 4’ T5’s. All can be changed over to Veg.
Is Temp set 65 degrees F for energy savings be sufficient for the Plants?. It can be adjusted from 61-78 degrees F.
Will installing Mylar on the ceiling increase Lumens to the plants to make the work pay off.
The temp can be set for 61-78 degrees F. I have it set for 65 to conserve electricity 
Sounds like a plan. Be sure to keep a journal in order to have reference to all the changes you go through. Cannot wait to view the results. 
After 8 nights I noticed the chamber lights on at 4:00 am when it is programmed to turn on at 8:00 am. I saw them shut off at 8:00 pm several times.
I checked the program in the digital/bayberry back up timer and found an error in one of the channels.
I fixed it and will resume with verified 12/12 cycle.
I did not notice anything wrong as the plants are budding like crazy.
What coarse of action can I take to minimize impact? How May hermies affect my crop? 
Harvest Day 8 weeks, 4 days in Bloom Roberts White Widow. Rescue shed finally hit the low 40s degree F. As outside the temperatures get down to the low 20s. Total Success.
I want to winterize the shed so I require dark period heating. Hoe low temperatures can I let the pants get down to on a routine basis and not do them harm?
It is great so see you succeed under such hard environmental circumstances. 
Glad you are happy!
Rescue Chamber now All Season:
With a successful harvest completed the temporary chamber located in a concrete walled shed it is time to go full season.
Light system upgrade: from (2) 300 watt CFL and T5 4’x(4) tube fixture to 600 watt HPS/Mh and (2) 300watt LED.
Heating System: Added oil filled radiator inside removable door pass thru enclosure with dispersion fan. This enclosure can be removed and replaced with a stand alone air conditioner for hot weather.