Common wasp

These are actually my favourites of my garden. I am usually placing bowls of fresh water for them and for bees as well as the birds in the yard. And many times I have saved them from our little swimming pool. Now I noticed them building a nest in the fig tree. First there was one only and now there are bout 4 of them. As I noticed the queen must have placed some eggs in the cells - I have those latest photos in my camera. I would like to get some information - shouldn't they form a ball around the cells because the weather is rather hot here and so the wasps are usually just sitting over the cells during the hottest time of the day. Can somebody explain?