Pioneer;

I wouldn't do an unvented roof in your climate, use soffit baffles to keep air folw and go deep on your FG, pay based on bags used

what about these methods with a vaulted interior ceiling? how to you stop your blow in insulation from settling to the heals?

Whats the pitch on the vault?

5/12

Yes, it is true that every additional R provided diminishing return, but that return is still sometimes very very good, even if it is less than the initial return. Just to illustrate my point, lets just guesstimate the first inch or two will probably return 500 to 1000% savings per year, based on the cost of insulation and your energy savings compared to a home with no insulation.(My dad is in the heating oil business, one 1800 sf home he used to deliver to that didn't have any insulation used 700 gallons of heating fuel every month!) The next 10 or so inches to get to a foot thick will probably return about 50 to 100%savings per year, which is diminished, but still great. The next 6 inches will probably return 20 to 30% savings/year, but this is still much greater than the interest on your mortgage. I know my numbers probably aren't perfect, but my point is this: Diminishing returns don't mean they are bad returns.

To use some real numbers,
At R-30, or U-.03333, if it is freezing outside (32) and you like it warm inside (72) there is a 40 degree temperature differential. U-0.0333333 BTU/ ft²·°F·h * 3000ft^2 * 40 °F *24hrs/day= 96000BTUs/day. To put this in terms better understood, since there isi about 140,000 BTU's in a gallon of heating oil, your attic alone will use .685 gallons/day to keep the house warm.
At R-49, or U-.0204, same scenario, 59000BTUs/day, .419 gallons/day, a savings of 38.9%. In my area, the attic is usually about half of our heating bill, this would be like a 20% lower total bill.

You're missing the part where the efficiency factor of the heating system comes in.   Assuming an 85% mid-efficiency oil burner,  that 96000BTUs in the first instance is likely more like 96K/0.85= 113K in souce-fuel BTUs, which is 0.81 gallons of oil, not 0.675.

But the fractional relationship remains- the 59K cost you 69.4K in source BTUs, or 0.496 gallons, not 0.419.

And your main point remains fundamental: "Diminishing returns don't mean they are bad returns."

Whether you get any return at all can depend on the price of the R-value though- eventually at some level the present-value of the insulation exceeds the return for the forseeable future.  (This happens far more quickly with expensive foams than for cheap stuff.)  It seems most homeowners won't budge on retrofits without a 2 year ROI, which is a RIDICULOUS rate of return to expect on any legal investment. 

But if you calculated a NPV for all upgrades that go positive after 10 years almost all existing structures would have a very long "to do" list indeed...  And 10 years to present-value positive is pretty good compared to where lots of peops have invested their money, eh? ;-)

But that's a long termer's view.  Short termers will get a bigger & quicker return on a kitchen or bath makeover than an efficiency upgrade.  (I'm not advocating, just acknowledging...)