The room (storage unit with a roll-up door) I ferment in stays cool (65f.) during the day which is perfect for ales but at night it gets pretty cold, I would say somewhere around 50ish this time of year. This cold environment can cause the yeast to become inactive and essentially prevent them from fulfilling their responsibilities, they will go dormant and settle to the bottom of the fermenting vessel, resulting in an under attenuated beer. Not good.
Line voltage thermostatic control
My idea was to have a thermostat inside an insulated box big enough to contain two fermenters. The device would control a 60 watt light bulb inside the box to keep the temperature at 65f. turning the light on and off as needed. In my situation, (a small enclosed space) it doesn't take more than a light bulb to keep it warm. Well, I wanted to avoid using a standard thermostat along with a transformer and had heard of a 'direct line voltage thermostat' but didn't think I would find one. To my surprise Orchard had them and I purchase one for about $45 with tax. The ease of use is what is so valuable here. The line voltage thermostat plugs directly into the 110v outlet and then the devise that you want to control plugs into the thermostat. Set the temperature that you want and when the temp. drops below that setting it powers on the devise of your choosing, in this case a light bulb, but could also power on a heater or cooling unit just as well. The unit I bought is the LUX win100 which is also programmable.
If you don't have a lot of electrical skills but want to create this kind of controlled environment, I would recommend getting one of these thermostats. This would be perfect for a small room or shed that already has an electrical outlet in place and you wanted to turn on/off a space heater or cooler.