The folks at the extension did some real heavy labor this summer and insulated the perimeter of the foundation. This insulation prevents heat loss inside the structure via the soil. It will also keep the ground warmer longer, further extending our growing season.
From this view you can see some of the insulation . The vertical sheets go down to a depth of 16 inches. Under that are some horizontal sheets 24 inches wide, extending out from the high tunnel.
The l-shape with the insulation is something that Michigan State tried. They went out four feet but we didn’t have room for that on a couple sides.
I've read about growers installing a solid wall about three feet around the perimeter to prevent heat loss and add to the thermal mass that can absorb solar heat. But as Bill explained to me, we're not set up for that as we don't have the rocks or concrete to store thermal heat.
I also read about someone else who insulated down four feet and loved the results. Bill agrees that deeper would have been better, but the Extension doesn't have access to a backhoe to make that reasonable.
Still he says, "When it comes to insulation I guess you could always do more."