Shonick said: Can you elaborate more on this? Are you saying, adding an insulated wall below the foundation, meaning to extend the greenhouse wall into the ground?
Take a look at the first picture in this article.
https://ceresgs.com/solar-gree... Of course, this is an extreme example, but you will get an idea of sinking and extended, insulated wall. This type of wall will keep the cold from the ground from getting under your greenhouse. If the sunken wall goes down to the frost depth in your area, you actually can take advantage of the constant warmer ground temperature under the greenhouse with this type of wall, or the easier Swedish Skirt mentioned further down in the article.
