I am a homeowner near Seattle (Zone 4C, marine), trying to figure out the right way to do a flat roof warm conversion. 

Bought the house about a year ago – mid-80s expansion built off a smaller home. Come to find out the roof was replaced five years ago — but during that reroof, the exhaust vents in the middle of the roof were removed and the perimeter intake left in place, so the assembly ended up neither properly vented nor unvented. The result is condensation on the underside of the deck, which is what I'm now dealing with. There is no space between the joists and sheathing to even take a chance on a vented assembly. The proposed fix is a warm-roof conversion: sheathing → Class I vapor retarder → 3.3” polyiso (leave current R-30 batt)→ cover board → PVC membrane. Block the old intake vents with wood blocking and spray foam. From online research, that seems correct. 

At what point does a job like this justify a dedicated envelope consultant rather than just a roof contractor designing the assembly scope? I am nervous about spending $100k+ (have gotten 5 quotes) only to have it turn out to be a disaster. 

I haven’t found any envelope consultants who work at a residential scale - does anyone have any Seattle area recommendations? I have spoken to one consultant, but they are not located locally, and were in the $15k+ range. There are also signs of moisture damage around windows, but I haven’t yet experienced any moisture, so there may be more to the project.

When do you bring in a building-envelope consultant vs. trusting the roofer's design?