Have a greenhouse approx. 14' long and 8 feet wide. Plastic dome type you install yourself. Put it up about 4 years ago.. Works fine. But...here in the mountains of north central PA I can't start my seeds in it due to the weather and the house is filling up fast with seedling trays. I have thought of putting a heater in it...but did one time and the wick went bad and spent Mothers' day cleaning black shoot off the whole thing. A real mess. So...it isn't really air tight as the barn cats "winter' in it and are able to come and go as they please under the bottom spots on the ground. Temp this morning at 7 AM was 40..we have had some unusal warm weather. yesterday aroung 65 but it won't stay this warm this time of year. I could run an electric cord and put heat lamps hanging all over the greenhouse and then sit and watch the electric meter spin...but would be willing to do that for those few short weeks. At this point I use the greenhouse to put the seedlings in about 3 weeks before going into the garden. Anyone have any ideas ???