I’m building a timber frame house and will have exposed beams and rafters with 2” tongue and groove ceiling decking on top of the rafters that will be visible from below.    I’m trying to decide what is the best way to dry in the decking.    I’m considering sharkskin ultra which is basically an ice and water shield and not vapor permeable or mento 5000 which is vapor permeable.   There will be 8 inches of foam board on top of the membrane and then firing strips to provide ventilation with 5/8 zip on top of the furring strips followed by a metal roof.   Any suggestions or thoughts would greatly be appreciated.