Converting from traditional aluminum siding to smooth skins?

Ditto on not using fiberglass. Not only low R, but with vibration and moisture it tends to drop down-leaving voids.
The best would be polyurethane and sprayed. Lot of high-end houses are build this way with excelent results, but that is expensive.
Still buying polyurethane pads and finishing with "Great stuff" will do good job.