Not sure how much this will help, because I don't know your exact setup, but...... I have an 8 x 15 (120 sq ft) compared to your 100 sq ft and I am using a natural gas heater rated at 20,000 BTu. It has a fan that is linked to the heater's thermostat and in addition I have a 6" fan to help circulate the heat. I live in SE Michigan and have no problem keeping my daytime temps about 68 to 72 even in the dead of winter. You may need to add bubble wrap to the walls of the GH to keep the heat in. I also use a 15,000 BTu (240V) electric space heater with fan as backup just in case the pilot goes out on the gas heater. The gas heater is available at Home Depot or Lowes and the electric heater is available online; just google "electric heater"
http://heating-and-cooling.hardwarestore.com/97-567-electric-space-heaters/contractor''s-buddy-heater-108448.aspx
