Insulation, Radiant and Vapor Barriers - Heat transfer vs. R-value graph

I am in the market to have a new home built. A contractor just showed me a graph of Heat transfer by percent against R-value. I maintained that R-38 was minimum for an attic or cathedral roof design. The contractor pulled out the 1977 graph which showed that R-20 stopped about 90 percent of the heat and R38 only gained 1 or 2 percent more. R-values above R-38 did nothing. The conclusion was that R20 is all that is needed. I would like to counter that conclusion with current data if it exists but I can't locate it on the internet. If anyone reading this knows where I can turn up good data please post. Thanks in advance.

"Contractor"
If your paying him tell him to put his chart away and give you R38.
Most new construction codes require this anyhow.

http://www.ornl.gov/sci/roofs+walls/insulation/ins_15.html

This is a Fact Sheet on Insulating. The following is the math the contractor used for the graph;

1/20 - 1/38 = 19/380 - 10/380 = 9/380 = 2%

R-20 means 1/20th of a BTU traverses one square foot of this insulation per hour. R-38 means 1/38th of a BTU...

This is referred to as the "Law of Diminishing Returns". As you increase the R-value, the amount of savings lessens. However, this contractor is misusing this principle. Grant it, the savings between R-20 and R-38 is only 2%. But the cost difference of initially installing R-38 instead of R-20 justifies the installation of the R-38. In other words, you compare the savings to the cost difference between installing R-20 to R-38.