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The roof system observed was a structural lumber deck, self adhered underlayment, polyiso insulation between lumber battens and a snap lock standing seam metal panel. The question arose if was proper for the underlayment to traverse atop the polyiso insulation or if it was proper atop the lumber structural deck? I can see pros and cons to both options.
Self-adhering "ice and watershield" underlayment is required by its manufacturers to be installed on top of clean decking only.
I would recommend checking with the manufacturer of the self-adhered underlayment for guidance on substrates their products will adhere to. There are manufacturers who allow adhering SA underlayments directly to the polyiso (in some cases requiring backnailing the underlayment). I would think that installing the underlayment on top of the polyiso would be beneficial to waterproof the insulation as well as everything below it since polyiso is not meant to be exposed to water.
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