We are in the process of remodeling our primary house and are looking to add 1200 sqft of additional space. This space will have a very low pitch roof which we intend to use as walkable deck. All permits were secured and we will be finishing framing in few days. i did not understand all the in’s and out of flat roofing when we started the process but now as it is time to make final decisions i am little freaked out and concerned.
Roof structure:
–  14″ TJI at 16″ OC, supported with large LVL in middle and acts as a high point to drain water in two different directions
– 3/4″ CDX plywood as primary sheathing
– Westcoat ALX pro custom finish as primary roof/deck finish – We are in Seattle area

Scenario:
– How best to insulate and avoid condensation problems. One of the inspectors on site recommended 2″ of Closed cell spray foam under CDX and then use fiber insulation to achieve R49.   He was not sure if we should leave an air gap between two layers (CC foam and fiber bats). From this forum i learnt that we should completely close it (non ventilated) and there should be no air gap left.  we are not building a cupola on top so i do have concerns on how well this roof will ventilate also especially considering the fact that a giant LVL beam is running right in the middle of the roof and there is no thru circulation.

Questions:
– Should I use all 8″ of CC foam?
– Should I use 2″ of CC foam and then use fiber bats but leave no air gaps and seal it all?
– Should I use 2″ of CC foam and then leave few inches of space between for air to flow?

– Anybody here used Westcoat ALX pro? it came as recommendation from my architect but I cant find much review of info on it. https://www.westcoat.com/systems/alx-pro/ After reading more here I would have gone with putting insulation on top and make it a warm roof but seems like that may not be an option at this point as ALX pro can be put only on plywood. I have reached out to few insulation companies who will be providing bids in coming days, I want to educate myself before i dive deeper into discussions with them and finalize things.