we are in western washington state, climate zone 4c.

we bought our house with the intent of fixing up the attic, not as finished space, but at least comfortable storage: plywood floors, rough sheet rock, power and lights. but the more i look at it the more unlikely any of that seems.

the ceiling joists are true 2x6 16" oc spanning almost 16' same with the rafters. so there isn't much live load for using the space, and we can't get r38 in a 2x6. we're pulling out the nasty old doubled up foil backed r5 that's currently in the ceiling bays, and want to reinsulate. i thought that if we sprayed 2" cc foam against the sheeting, then r-13 fiberglassed, that would get the roof up to r-23 or so. then if we put unfaced r-19 batts in the cieling joists, we could at least put the plywood floor in and have storage for light bulky stuff like the christmas/halloween decorations...

is it up to code to add the insulation values of the cieling and rafter insulations to total our attic insulation? by using the spray foam, we should have eliminated any vapor condensation issues, right?

as near as i can tell, it's that, or just pile up fiberglass insulation and write off the attic for storage.

You can add rafter + floor insulation to come up with an approximate total R as long as the attic is then completely sealed, unvented, and gable ends are at a similar R. The 2" of foam against the ceiling would be an adequate vapor retarder, but if you're completely sealing the attic you have to take care that it's truly air-tight between the conditioned space and attic. Take special care to weatherstrip seal & insulated the access doors, and any plumbing chase/stacks that might penetrate.

It's somewhat more foam, but a better approach would be to use 2" of cc foam against the roof deck and to seal the attic from the exterior. Wet-spray cellulose into the rafter bays (to form an ignition barrier it needs to be at least 3" of wet-sprayed goods, not dry-blown), which would give you ~ R12+ ~ R15= R27ish at the rafters, then dry-blow cellulose at the floor level for another R20+. The cellulose would act as a hygric buffer for occaisional mid-winter daily swings into the condensation temps at the outermost layer of cellulose, the foam would keep interior moisture from accumulating in the roof deck the attic space itself would stay above the condensation point of 70F 30% relative humidity air, so mold won't get going on any of the stored goods even with a bit of air-leakage between conditioned space and the attic (but try to seal that stuff up anyway.

Read this: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

(From the tables near the end, it looks like you might be able to get away with just one inch of foam against the roof deck, but two would be better if it's in the budget.)

The performance of batts is generally miserable compared to blown or sprayed goods. Blowing cellulose between the floor joists is a dead-easy DIY job using a box-store rental blower (usually you get free days' rental with the purchase of $100+ worth of goods, or similar), but it would take pro to wet-spray the rafter bays over the foam. If you'd rather do it as a DIY, half-inch gypsum against the rafters would qualify as an ignition barrier, and cutting a 2"-2.5" hole per bay with a hole-saw (near the ridge) would allow you to dry-blow celluose into the rafter bays. (Blow down the bay first, until the blower starts to stall, then direct it upward to pack the upper foot or so.) Using a hole saw you can save the plugs for an easy patch job after insulating.

ok. so the next question is:
my roof is metal, with ridge venting. how is that vent blocked, so i don't have spray foam coming out under the ridge when it expands, and what gets done with that space so that it doesn't fill with yellow jackets or the like?

rigger,
your installer should be able to seal the ridge vent when they install the spray foam. If you DIY the foam, then improvise: plastic sheating, eps plugs, whatever works. (Most of the pros make plugs by spraying a thin layer of foam on a flat surface, then peeling it off and stuffing it into the hole before it sets up completely.)
The ridge vent itself should have a screen plug seal already in place to keep out insects and such. Installing the foam insulation will not affect this.

If the spray foam guy likes your idea and does the work... I would do it. Call his insurance agent, see if his insurance is paid thru install date... (all people should do this for all projects) and then have him put specs in contract with a few years of warranty. Business should be over five years in business doing insulation. Continuous list of last year of work. If he is happy to give you all this... visit a site in progress, work ethic is right for you... then make your decision. That is how to do your "do diligence" completely.

Asking about ideas is great at this site... but.. as a homeowner and a contractor... get the above next and you will get the value you are looking for.

As I read Table 4, 2" of cc spray foam might be OK and 3" to be sure. 4" in colder climates. In general, I believe they are correct in modeling based on an assumption of some air flow, not just diffusion. If you take the same assumptions and simulations and apply them to walls, you get an interesting opinion about using spray or rigid CC foam.