Constructing a Greenhouse or High Tunnel [video]

Greenhouse vs High Tunnel

reference this link to "Very Small Greenhouses for Back Yards" 

A commercial greenhouse is a structure intended to be heated with fuel to allow production of plants during a time of year when the plants could not develop on time or survive outdoors due to low temperatures. A greenhouse performs  the competing roles of keeping heat inside at night, while still allowing in sunlight necessary for plant growth during the day.  Without a heat source, a greenhouse would cool off very quickly when the sun goes down and get almost as cold as the outside air as the night progressed.  Also, because greenhouses do a good job of allowing sunlight in, they can quickly heat up during the day to temperatures damaging to plants, even on very cold days, so they must have ventilation systems to allow excess heat to be released. 

A high tunnel, by contrast, is a heated or unheated greenhouse structure over soil. These are used primarily to allow planting in ground earlier in the season or to keep crops growing later in the year than they would be able to if unprotected. The soil in a high tunnel in New Hampshire doesn’t freeze over winter, so these can be used to grow cold-tolerant crops all winter long with the use of specialized “frost blankets”.