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Your plan should work but could be quite costly.

Depending where you live, you could need R-30 or more of insulation on the roof to make any difference.

This equates to 6-10 inches of foam placed over the roof deck depending on your location.

Might actually be cheaper to remove the cedar ceiling, insulate the rafters with conventional fiberglass, then reinstall the cedar.

'Sweating' should not be an issue regarding the metal roof and foam, however, provided the space between the metal roof and foam is ventilated.