It was quite cold by our standards overnight, and a light coating of frost was deposited on the propagator covers.
In the warmer months, these covers are fitted with a very light (20%) shadecloth to protect the seedlings from the sun when the weather gets hot and from flying insects and birds. In winter, I replace the shadecloth with a twin wall polycarbonate sheet to protect the seedlings from frost and high winds. It traps heat from the sun during the day and insulates the enclosure at night. The sheet's structure resembles a series of small square tubes glued together longitudinally from one end of the frame to the other. The ends are open, and to maintain its insulating capability, the sheet's ends are blocked with plumbers silicone adhesive.
The bottom of the propagator enclosure is permanently covered with a layer of heavy duty shadecloth. This layer keeps insects and other pests out of the enclosure but allows drainage of surplus water, and facilitates air circulation in summer.
In winter a layer of closed cell plastic insulation is fitted above the shadecloth to insulate the bottom of the enclosure and block ingress of cold air. Its a simple measure but in sync with the polycarbonate cover, protects the seedlings from frost and speeds up germination of seeds in cold weather.
The slatted floor in the propagator is easily removed to insert or remove the foam.