Do you need a vapor retarder or a vapor barrier with SPF insulation?

It depends of the use of the building, the climate and the materials of construction. In normal occupancies and moderate climates, SPF insulation typically does require a vapor retarder. Extremes of climate and building use may require vapor retarders/barriers. Check with your design professional for specific recommendations and refer to technical document, Moisture Vapor Transmission for further information.