[DD.]

I've noted a few posts recently where members are keeping fingers crossed and hoping a unheated  greenhouse will keep their seedlings safe overnight.

This is not going to happen.

During the day time, light, (it does not have to be direct sunlight), is converted to heat as it strikes solid objects. Even a dull day in winter is sufficient to raise the daytime temperature of your greenhouse by quite a few degrees. This heat is trapped in your greenhouse, in the same way that certain gases in our atmosphere trap it in.

At night time, of course this does not happen. The retained heat in your greenhouse overnight will eventually leech out. This can be slowed down, but not entirely stopped by insulation. The inevitable will happen and the greenhouse temperature will approach that of the great outdoors.

Before consigning seeds to their fate, it is very important to understand the physics of a greenhouse and take due regard of the current and forecast weather conditions. An unheated greenhouse will not be your saviour.

[aelf]

very true. I only have a few dwarf sunflower seeds sown and in my greenhouse (did it last week) They are also covered with a plastic cloche to double-insulate them and I have a parrafin heater in there as well. despite these precautions, I found that the surface of the pots were frozen this morning 

Best to wait a bit longer befor planting more seeds, I think.