I will be building a home in southern nevada and have an insulation question.  The house will be a single story with 2210 sq ft.  Walls are 2x6 with spray foam.  They are using  A-trusses to frame the roof.  My question is this, I would like to have as much insulation as possible R40+ for the roof.  I intend to use spray foam on the underside of the roof but since the trusses are made with 2x4's I won't get much of an R value there.  The roof has a 5/12 pitch so would I gain any r-value by blowing in insulation on top of the ceiling there by creating a dead space between the roof and ceiling.  I hope I explained this correctly.  Any advice would be ppreciated.

The thickness of the spray foam on the underside of the roof is not controlled by the 2x4's in your truss. The foam can be sprayed to any thickness, totally encapsulating the 2x's, to achieve any R value you want.

Tom,
From your description, I think you  are using a scissor truss.  I would ask the builder to specify a truss with an "Energy Heel" of 4-7 "  Basicaly this is a spacer between the top and bottom cord that raises the roof ~4-7 inches and allows more insulation  at the ends where the truss depth is smallest.  

Good Luck
Eric

Stacking 6-7" of panelized fiber-faced polyisocyanurate ABOVE the roof deck will get you the R40 you're looking for. (4' x 8' x 3" fiber faced iso is a standard R20 panel size for commercial roofing insulation, often used in place of EPS.) Taping the seams and staggering the layers 18" guarantees low air-movement/high performance. It'll need a surface nailer-deck (held in place with long screws) if you intend to use standard shingles with it. Fastener spacing for the nailer-deck depends on local code for wind-worthiness, etc.

Whether that's cheaper than blasting a foot of half-pound foam on the underside of the deck (as wes rightly recommends) kinda depends... In my neighborhood you'd be looking at spray foam @ ~$4.50-5.00/ft^2 installed-price, VS. 3" iso panels stacked to R40 @ $3.00-3.50/ft^2, plus nailer-decking, plus labor. YMMV.

And whatever else you do, going with a "cool roof" with high solar reflectance and moderate infra-red emissivity will improve cooling performance measurably. (In nearby CA climate zones cool roofs are now required by code, well vetted on cost-effectiveness.)

As for the second part of your question, yes, insulation on the micro-attic floor does indeed add R-value, and it's legit to split the R value (as long as the dead space is truly dead, which it would have to be for roof-decking insulation to work fully.) If you have any AC supply ducts running up there, burying them in heaps of cellulose to at least R15 or so at the thinnest is the right thing to do, even WITH R40 above. (Return ducts would be less critical- it's the temperature DIFFERENCE that counts.) Be sure that the duct joints are all mastic-sealed before insulating over them though. (Some prefer to seal & insulate in one step with half-pound foam.)

Thanks for all the great input. I will be talking to the builder about all these things. It tough living in Phila and having the house built in Nevada and explaining things over the phone.

Alton, in regards to your response, the ceiling are 10' so that should not be problem with any of the windows.

Dana1, thanks for the info on the panels and I'll ask the builder about the cost and if it would interfere with the concrete spanish tiles.

Would it be fair to say that when the underside of the roof is sprayed, ie: 6" of foam, and the attic floor filled with blown insulation that I could achieve a r-40+ rating with this combination

Six under the deck + six on the attic floor MIGHT get you the performance you want, with some attention to detail:

In both the roof decking and blown insulation, getting at least a couple inches of covering over the framing timber greatly reduces the thermal bridging. 6" of foam would give you ~2.5" of thermal break over the truss, which is good. With an energy-heel framed in you can pile in the blown stuff deeper than 6" for the bulk of it. If the ceiling joists are 2x6 you should opt for a minimum of 8" to reliably cover the joist tops.

If you leave the framing edges exposed, the whole-surface R-value plummets by ~20-25% due to the thermal bridging effects. Wood has an insulative value on the order of ~R1/inch, compared to ~R3.5-3.8/inch for half pound foam or blown fiber. By covering the 2x4 with 2" of foam you've more than tripled the R-value of the bridging element (what was a thermal-bridge of R3.5 is now bridge of about R12). Doing the same on the floor/ceiling end will also boost performance. By applying 2-3" of thermal break on the framing at both the roof deck and ceiling/flloor joist end you can almost pretend those breaks don't exist in the whole-surface R-value estimate.

Panelized R40 iso above the roof deck has very minimal thermal bridging (the long screws) and the full R-40 is pretty much there no matter what.

Also, this is an instances where blown fiberglass vs cellulose will have subtle but real differences. Fiberglass is somewhat translucent to infra-red, absorbing some of the radiated heat coming through the roof deck will penetrate within deeper layers of the FG insulation. The peak temperature in the fiberglass in a hot attic isn't at the top surface, but rather an inch or so into it(!). This fully negates the R-value of the top couple of inches when that occurs. Cellulose is fairly opaque to IR by comparison, and the peak temp will be at the top surface, with a fairly linear temperature gradient from top to bottom- you get pretty much the full performance out of it under all conditions. If you go with fiberglass, allow at least 3" of thermal break above the framing timber to get the best cooling-season performance out of it.

Dana1 thanks for the input again. I spoke with my builder and he is all for the energy heel. He mentioned he told me about it last year but I guess it went over my head. He also said he will blow in cellulose above the celing to get me an r-40+ rating at that point plus whatever I get from the spray foam at the roof joists. Thanks again, Tom