My project is a timber frame that has 1x8 T&G Doug Fir over the roof timbers.  Since the project needed to go through the winter before being enclosed with polyurethane SIPs, I attached a layer of non-vapor permeable, waterproof synthetic underlayment on top of the T&G.  In addition to waterproofing the exterior of the T&G, I wanted to protect the interior skin of the SIPs from interior (vapor) moisture once the structure is enclosed.

I am about to have the roof PU SIPs attached on top of the existing underlayment.  Aside from roofing felt, which has a vapor permeability rating of 5+, is it apporpriate to use a synthetic underlayment that has no vapor permeability?  If I did so, what is the possibility I would trap vapor between the lower and upper sheets of non-permeable underlayment.

Advice and comments are eagerly welcomed.