Learned why the greenhouse was so cold when the outside temps reached 26 degrees: the ground fault receptacles went bad and there was no electricity past the fuse box. The little back-up electric heater did not come on. The pump did not circulate the water through the heat-sink barrels and the black hoses that collect heat from daytime sunlight.
When everything works as planned, the temperature will not drop to freezing. There is usually a ten-degree spread when outside temps are below freezing.