Shed ventilation

[Simpo Two]

So I bought this cute little 6x4 shed last year, and went over all the joints with sealant to keep the nasty rain and bugs out and make it as weatherproof as possible...

Except that after a couple of miserable damp and freezing months, I went back in to find a thin film of fine white mould on the rubber handles of my loppers, and also on the wooden shaft of a pruning saw.

So I guess I need to allow some fresh air in after all - but preferably not spiders etc. What sort of ventilator would be best, and whereabouts would it go - above window, below window, somewhere else, or do I need two to get a draught through?

[(MrB)]

Try these, I fitted 4, two at either end (apex) of a 10x12 shed.

http://www.screwfix.com/prods/77573/Heating-Coolin...