Condensation on ceiling spray foam insulation?

[RobertPW]

Hi all

I’ve built a lean to shed on the side of my garage, consisting of concrete slab with DPM and half brick dwarf wall with timber frame, plastic clad walls atop with a grp roof.
After completing the ceiling became damp and mouldy so I added soffit vents and air bricks.
The ceiling is still showing signs of damp and mould.
The roof is a cold deck with no insulation.
Any suggestions on how to reduce/remove this condensation? Could it be the bottom 2 courses of the original garage wall which is slightly damp below the dpc? I could add PIR to the ceiling or spray foam, but fear I’ll still have the same problem.

Any advice appreciated.

 

[freddiemercurystwin]

Is there cross ventilation at high level ie on opposite sides of the roof?

 

[RobertPW]

No, I thought there would be sufficient ventilation with the 6 air bricks and soffit vents. The underside of the deck is open with no overboard if or insulation at present.
The easiest option would be to vent through into the abutted garage but not sure if this would make it worse?
Alternatively I could add mushroom vents.

 

[freddiemercurystwin]

In principle this is what is required (you can ignore the insulation in the diagram).

 

[RobertPW]

Thank you, I’m going to have to have a good think about the best way to achieve this. It would be easier if my joists ran the other way.
the other option I am considering is spray foam insulation if anyone has tried this?