Summer house ventalation

i have just had brick one built at the end of my garden it is about 20 square meters with glass and bi fold doors across the front. It has insulated all round including the pitched roof. I locked it for over a week and have just come back to see ceiling and the walls all covered in mould. There seems no damp so I think it's a ventilation problem. The building has no windows what are my options to add ventilation if I don't have or add a window?

An airbrick maybe, or if you have an electricity supply down there a demumidifier, or a an extractor fan controlled by a humidistat, that will only operate the fan when the humidity gets above a certain level. Seems a bit odd that it has become mouldy so quickly though. If there's not a leak, you're not running a tumble dryer, or other moisture producing device in there are you?

No there was only some old paint brushes in a bag and some paint trays that may have had a small amount of paint in them. It might have been a bit more that a week two at the most. If there was a leak I assume the walls would be damp or ceiling but both were dry the mould was particularly bad across all of the ceiling and two of the other walls the long back wall and one side. The other wall was hardly effected not sure if this makes a difference?

Plastic airbricks on the outside . Replace one brick at each end on the back at high level. Then on the inside you can cover them with a plastic grille - not one with a flyscreen cuz it'll block with dust. Easy retro fit and cheap as chips. Just put them where they won't be seen Plastic ones have at least 10x the flow of teracotta ones.