No sheething metal roof on a cellulose insulated catherderal ceiling, – GreenBuildingAdvisor

I just put a house under contract in climate zone 5A. I am planning a deep energy retrofit. I have been reading GBA and following the Unbuilt it crew and others for years now and am looking forward to over prioritizing the budget on the building envelope. I will be doing all the work myself. I’m coming off of building a small 800 sqft addition for my father having done detailed air seeling and 2 inches of continues exterior on that. 

I am OK with having plywood floors and butcher block counters for years after moving in because I went with European triple glaze windows, a zehnder, and took my insulation levels deep into diminishing returns, I can upgrade to fancy interior stuff later.

The house was built in 1991, is 28×38 with a full poured concrete wall walk out basement. I bought a small rectangle very much on purpose. 

On to the roof assembly question.    

I plan to pull the roof decking off the 3/12 roof and then turn the rafters into the bottom cord of a 3ft deep DIY parallel cord truss.

I will then cover the roof in commercial tyvek (or Mento if ya’ll think the commercial tyvek is not robust enough). The WRB will be tied in and taped to the wall WRB also commercial tyvek. 

Perlins will be laid on top of the trusses. Then metal exposed fastener roof panels.

I will fill the trusses with loose fill cellulose that I tamp into place as needed as I fill. I’m thinking the low slope roof,the (gentle) tamping, and the vast quantity will prevent any meaningful settling? IE I should have at least 2.5 feet of coverage everywhere in a 3ft cavity.  

On the ceiling beneath I will use drywall as the air sealing method. There will be no holes. Vent stack goes out the wall and up through the eves, no ceiling lights or fans.  

What are the problems and issues to watch out for with the durability of this assembly? Is it possible I could get a meaningful amount of condensation on the underside of the metal? Could the tyvek degrade over time from radiant heat from the roof? 

Thanks all!

Alex