Clean off the underside of the roof panels with methylated spirit and a rag. Wait until the panels are completely dry.
Coat a section of the underside of the shed roof with the polystyrene-to-metal spray glue, and the back of the polystyrene tile as well. Stick the polystyrene tile on to the glue coated spot on the underside of the shed roof. Make sure it is secure. Ensure the glue is immune to temperature fluctuations.
Continue this until the entire inside of the roof is covered. This will not prevent moisture from getting inside the shed, but it will at least help prevent condensation from building up.
Use tin snips -- or a similar tool -- to cut a hole near the high point of one side of the wall near the roof. The hole does not need to be large. Four or five inches in diameter is plenty for most small sheds.
Repeat the process on the opposite wall, so that you have two holes.
Cover the holes with screen netting, to prevent insects and small animals from getting inside.
Secure the netting in place with metal adhesive glue. The holes will allow some portion of the moisture build up in the air to escape the shed.