what insulation system are people generally going with for TJI framed ceilings?  My design uses 11 7/8" TJI 24" oc.  Do people usually put foam on top of the roof deck to create the thermal break and then roof ontop of it? (most likely use a metal roof).  It is also my understanding that metal roofs may not need vented.
One thought I had was to install the sheathing and then shoot 1" spray foam to seal it up, followed by dense packed cellulose.  I am thinking this would be in the r-45ish range.  The other thought would be to dense pack the entire thing and then put 1" of XPS on the exterior of the sheathing, then metal on top of that.  This would give me around an R-50 for about the same cost in materails.  Is not venting either one of these systems an issue?

What's your zip code?

NE Iowa- 50669

To protect the roof deck with exterior insulation the goal is to keep it above the average mid-winter dew point of the interior conditioned space air. Even an inch of XPS reduces the drying capacity in that direction, so you need to go thick to make it work. In Reinbeck IA you'd need something on the order of 40% of the total R on the exterior of the roof deck if going with a rigid-foam approach, otherwise there's a risk of moisture accumulation in the roof sheathing despite the hygric buffering of all that cellulose.

By contrast, spraying the interior with SPF maintains the drying-toward exterior at the wood deck, while slowing down the rate of moisture flow from the cellulose to the wood, keeping it drier. But it would take about TWO inches of 2lb closed cell polyurethane foam (and NOT 2lb Icycnene) under the roof deck to get the right balance of vapor-retardency, fast enough to not risk too much moisture in the cellulose, slow enough that the roof deck can dry. At only one inch the roof deck still susceptible in that climate & roofing type. Vented metal roofing on purlins or furring has substantially more drying capacity than metal roofing laid directly on roofing felt, which probably gives you sufficient margin to go with 1 inch of SPF under the roof decking.

Two inches of SPF would yield ~R12, and the remaining 10" would be another R37 if dense-packed to 3.2lbs per cubic foot, maybe R39 if you make that 3.8lbs/ft^3, for a total of R49-ish.

Alternatively, 4" of 2lb Icynene under the roof deck, with the remaining 8" dense-packed cellulose to 3.2lbs would run R50, and would work: The cold edge of the cellulose would stay warm enough to condense very little, and the vapor retardency of the foam would keep moisture flow to the roof deck slow enough. The roof deck would still dry in both directions, even through 4" of closed cell foam, whereas with R20 of SPF it would only dry to the exterior. (Closed cell Icynene usually quotes out favorably compared with SPF per unit R, even though it's only ~ R5/inch and needs more inches. It's rated 1.3perms at 3" as opposed to SPF at under 1perm @ only 2".)

For an exterior R approach you might price out 8" EPS rigid board (or an 8" SIP) on top, and lower-density wet-sprayed cellulose rather than dense pack in the cavities, for a super-insulated ~R75.

thanks for the reply.

So with drying as a concern, what about friction fit inserting 2" of XPS just below the TJI flanges (between the TJI), and then filling the remainer with cellulose.  Then possibly use the zip style roof deck product, followed by metal roofing.  This would give me not as high, only about an R-40, but it would allow for a 1.5" gap between the deck and the insulation to vent.

What about placing additional XPS on the interior side of the TJI, before the gyp?

2" of spray foam would run around $5k.  8" EPS SIPs on the roof run around $6 sqft.  The roof is 2765 sqft. 

I would just ditch the TJI design, however I am working on a passive solar concept, and the roof slopes are critical (can not use a truss) to get my lighting where i need it at.

I have been doing some more math on this.  Here is what I am thinking.  I can use 2 layers of type IX EPS at 2.5" each (R-4.8/inch), giving me R-24.  I would friction fit these between the flanges of the TJI, but run a bead of caulk between the foam and the top TJI flange prior to pressing it against the bottom side. Then install the 2nd layer of foam. After that I simply net off the bottom of them and dense pack the rest with cellulose, so 7.5" at r-3.7 would give me r27-30.   This gives me a total R-51+, plus still leaves about a 1.5" gap between the bottom of the sheathing and the insulation to vent it.  Then place metal on top of that.
I have an EPS supplier that could even supply me with pre cut to 1'-11" (or whatever my webspacing would be) blocks 3" thick by whatever length (say 10'). 
what are the downfalls to this system?

Sounds like a winner- you'll have sufficient R in the foam that the condensation risk at the interior side of the foam is nil, and the cellulose will never saturate. The diffusion rate through 5" of high-density EPS is very low (probably less than 0.5 perms), so the key will be air-sealing it well on the interior, to keep air-transported moisture off the roof decking. The cellulose dries toward interior-only, the roof deck toward the exterior.

Any reason why you can't put the EPS on the exterior though? With purlins/strapping to hold it down, through-screwed to the deck and providing something on which to mount the metal roofing would be less work than 2 layers between the TJIs, sealed separately. It makes for some long screws though- maybe only half the foam on the exterior would be easier, and the perm rating in either direction would then be such that the roof decking could dry in both directions. You'd then have sufficient room to use lower cost low-density wet-spray at ~ R3.5/ inch and still end up with a 40/60 ratio for foam/fiber, and you'd have and ~R10 of thermal break over the TJIs out over the roof deck, keeping THEM warmer & drier too.

I thought about that, and they actually have foam desking, which is basically a SIP panel with osb on one side.  I considered getting the 4" panel, placing that on the TJI, then suing sips screws to the TJI.  Then simply putting down the roof membrane and metal.  Filling the entire void with cellulose, giving me an R-69ish and a 4" thermal break.  However the problem there is, I may as well use an 8" urethane sips panel and cut my depth in half.  Cost wise, I would be able there anyway. 
The big problem with that is in the design of the clearstory.  This would make my roof structure around 18-19" thick with all components installed.  I am designing it to have the TJI set on the wall bearing, and then build 2x8 fascia extentions to create my overhangs.  20" roof lines look VERY heavy and out of proportion.  Then I run my EPS exterior up the wall and over the front of the TJI to completely insulate the end of it as well. 

However this roof is too thick to work in the clearstory space I have available if I want to get some narrow horrizontal windows in there.  Above the window I need around 10" header space due to my roof loading, plus then if I have another 6-8" of TJI within that space, plus a 16-24" tall window...

Hard to explain, but I am working out the design.  I will post it and the details after I get everything modeled.

I do appreciate the feedback and help.  I design buildings, but this whole superinsulated deal is new to me, and I have been researching the crap out of it to add these influences into my daily projects.
I have already convinced the owner to do a change order to add 2" of XPS to the exterior of their new community center for their town.

I get it- not all buildings are shaped like a shed, and things have to be able to fit mechanically without a major redesign of the structural members...