Wall dew point calculation

[t1.shep]

Is there a way to calculate the dew point in a wall assembly?
If a wall is made up of individual materials with a given thickness, and I create a R-Value parameter that I can assign to materials, walls, roofs, etc, is there a way to calculate the total R-value of the wall to determine the dew point location?
I know I need to add the formula to determine where the dew point occurs in the wall assembly. What I can't figure out how to do take each material that makes up a wall type and its corresponding R-value and calculate each portion and add together as a complete assembly.

[sthedens]

I saw a demonstration put on by Dupont (makers of Tyvek) that did exactly what you're talking about. It stepped through a bunch of different wall material layer configurations and let you pick the best one for your situation, then changed the wall type in Revit to that type.