I have a 6x8 twinwall greenhouse that I would like to use as a coldframe and season extender with two 2x8 raised beds for cold season crops and wire shelves above for starting plants. No central heat but thinking of soil heating cables for the cold crops along with a glass bed cover for a cold frame within the greenhouse. All the soil cables I see state they keep soil at 74 degrees F. Do I need that much heat? If not I can go with a shorter less expensive cable. Should I insulate the bottom before installing the base, cable, hardware cloth and soil? What is an upper limit to soil thickness a cable will heat? Thanks for any input or suggestions.