How much r-value is added when Styrofoam is applied over exterior sheathing?

Question

We are putting an addition on a stucco home and are considering Dryvit or similar. My client is wondering about the increased insulation value of using 2" styrofoam vs. 1". The walls will be 2" x 6" insulated with 5.5" fiberglass with vapor barrier applied, and the exterior will be 1/2" cement board.

Answer 1

Shoot me if I am wrong, but I believe it is R2 per inch of Styrofoam.

Answer 2

The R-value of Styrofoam is 5 per inch.  The EIFS (Dryvit) system uses expanded foam, not extruded (Styrofoam).  Therefore, the R-value will be slightly less than 4.  Hope this helps.