What can be in an unheated greenhouse?

So how worried should I be about my hardier seedlings overheating during the day?

Do you have an automatic window opener on the GH? Its worth getting one if you haven't. If its forecast to be hot, leave the door/window open.

An automatic opener is on my long long list of things to get to.

Knowing me, the chances that I will go up the garden to open the greenhouse door before I dash to the bus-stop to go to work are pretty low! Not a morning person   So an automatic opener is definitely a must. At some point.