CO2 burners.. Heaters and de hums. just like an indoor room.. Most greenhouses aren't completely seal and plenty of fresh air gets in. Plus in the ideal growing season the sun will allow you to open vents and have fresh air flow and exhaust at the top... I don't know much about the expensive greenhouses but i would imagine most of the questions are gonna depend on if you want to add lights and do winter runs.. Also how cold your area gets..
Most people that I know that aren't doing light dep get away with plugging in a couple cheap heaters in the early months of feb and march.. Then in april no heaters are needed at all... Totally depends on your location though
