Because our beautiful greenhouse is glass, it leaks energy. We did seal every nook and cranny with silicon and experimented with many of the suggestions in the Greenhouse Gardener's Companion--from building thermal walls of water to wrapping the entire interior with bubble wrap. Two seasons of experimentation has convinced us that due to cost and effort, the climate in our greenhouse is only somewhat under our control and we do not heat and cool the space year round.
We have been pretty surprised to learn that our biggest issue with the greenhouse is not actually heating it, but cooling it. So lets start with the simpler of the two: heat. I read a lot about thermal mass and natural ways to insulate my greenhouse from temperature changes and we have experimented with creating water walls (just large, stacked containers full of water positioned along the walls), wrapping the interior walls with bubble wrap and other insulating materials, heat producing lighting, and heaters. Some people even paint the greenhouse walls in an effort to better trap and maintain heat from the sun.
Our experience has been that given the small amount of floor space we have and our desire to keep the space attractive both inside and out, our best option is simply sporadically heating the space as necessary. To do so we have used both a standard space heater and a ceramic heater. We keep both on a smart plug so that we can control them from inside our home. This way, if we decide we want to take guests out for a New Year's champagne toast, we can turn the heater and electric blankets on an hour before, insuring it will be toasty by the time we head out. When the space becomes functional for growing in March we use the ceramic heater set to a minimum temperature. This way we can insure that our baby plants stay above 55 or 60 or whatever base temperature necessary. I have also used electric heat mats from my seed starting set up to give a few extra degrees of under heat to young plants.
Cooling the greenhouse is FAR more complicated and necessary than heating it. Where you position your greenhouse will determine a lot of your cooling needs. Our greenhouse is located in the NE corner of our yard with fencing along the N and E sides of the structure. The south facing wall is where the door is positioned and the W wall is where we placed the growing bed (8x2x2 feet). The W wall already had a redbud tree growing about 10 feet away which is ideal as it provides dappled shade in the summer but none in the winter. Our greenhouse gets sun from late morning to sundown. As such, even on a cold winter day with temperatures in the 40s F, if the sun is shining (which it usually is in Colorado) the inside of the greenhouse can get to 120F, yet at night, the greenhouse stays only about 10-20F higher than the outside, so it can freeze inside. These are pretty harsh and varied conditions to ask a plant to weather so we don't grow in the greenhouse from November to March.
One of the best features for cooling our greenhouse actually came built-in in the form of temperature controlled vents. These ingenious contraptions use a piston containing oil that automatically expands above 60 F causing the vents to open. We have two of these vents so year round when the inside of the greenhouse reaches above 60F the vents slowly pop open allowing for air flow. We also use shade cloth in the summer. The Exaco shade system ran upwards of $500 so we fashioned our own out of bulk shade cloth we purchased. Airflow and shade do a lot to control mounting temperatures within the greenhouse but even with both at play I have seen my greenhouse get to over 140F in the summer. To combat the greenhouse effect working overtime and killing my plants I use a portable evaporative cooler in the heat of the summer. Every morning when I go open the door of the greenhouse I refill this cooler and replace the ice packs in the reservoir. I will then run the evaporative cooler and an additional fan during the hottest parts of the day, I will also occasionally spray down the floor with a hose when things get really toasty, this will generally lower the temperature by about 10F for an hour or so while the moisture evaporates.
Heating and cooling our greenhouse has turned into an ever evolving science experiment that continues to challenge and grow me as a gardener. While it has been at times stressful (like Christmas morning when we woke to all of our citrus trees in the greenhouse frozen because we'd tripped the fuse by putting too many Christmas lights on the greenhouse--face palm), it is always rewarding. In that vein, I leave you with a photo of two foxglove that I started at the same time from the same seed packet, the first plant was transplanted outside and never bloomed, the second still lives in my greenhouse and produces spike after spike of spectacular flowers. Oh the power of the greenhouse!