I recently installed insulation in my garden shed and covered it with a plastic vapor barrier. This was done in the summer when the weather was warm and sometime humid. Shortly after I'd finished this installation I removed the plastic from the walls so I could install some electrical. when I pulled down the insulation I found black mold. My question is why would mold be present and why so fast? I thought I insulated correctly.
Vapour barrier goes on the cold side of the insulation.
The problem here is that for an unheated, Un-airconditioned shed, the outside will always be the cool side.
Insulation won't stop heat transfer, it only slows it down. A closed shed, no matter how well insulated, will always turn into a sauna.
Now you have a hot, steamy inside, and a (relatively) cool outside (especially at night). If you put the vapour barrier on the inside, sandwiching the insulation in, the condensation will occur within your insulation. Hence, the mold issue.
You're better off without a vapour barrier in this situation. This way the batts can dry out. Even better would be some rigid foam insulation, but I really think that insulation in this situation is a waste of money.
It may not be a condensation issue.
Since the structure was built as a shed, it is likely that tar paper or tyvek or ... was not installed on the exterior. Because of this, the sheathing/studs may simply be damp. Because air circulation within the cavity is now severely restricted, such damp sheathing/etc would raise the humidity in the wall cavity sufficient for mildew/mold to grow on the damp surface.
This, plus warm summer temperatures, is consistent with the very rapid appearance of your mildew/mold. To get a final answer, you would need to provide more information about the walls, and about the shed's design, location and history.
For a more general house-oriented discussion:
Sounds like circulating air from the outside caused the mold. Any wind/air that gets in from the exterior will carry moisture with it at some point. Vapour barrier always goes on the warm side, so please ignore previous cold side comment. Good luck with every thing.