I've been feeling down about what seems like an insurmountable issue regarding humidity, in my garage workshop (a detached concrete structure with MDF panelling inside and a concrete tile roof over steel joists)

I came home last week with a digital meter, which recorded 45% RH all day (nice day), but since it has been reading consistently over 60%. 

Can I ever hope to control this mechanically, and at what cost?

My strategy will probably have to be to keep glued pieces in the house between jobs.

Bloody Scotland!