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Going in that direction, the reflective barrier needs an air space between the foil and the metal roof. Add the foam panels, shinny side up on top of your OSB, then add 1x3 perlins and screw your metal to that. If the foil touches the metal, you will render your radiant heat barrier ineffective.

Have you considered board and batten siding? Rough sawn directly from a local sawmill would be cheap and once stained, would last a really long time. No need for pressure treated lumber.