Insulation question - fiberglass only or fg plus xps
I'm looking for some feedback on the insulation we are considering in a 2-story addition for a 1938 colonial in the northern Virginia area (outside DC). The walls of the current home are brick and block with plaster, so maybe an R4 at the most. And we still have the original windows. The exterior of the existing house is 26X30 and we are going to add about 850 square feet off the back (425 on each floor) with the addition running the full length of the house (30 feet).
Exterior walls will be 2x6 and we are looking at either r-19 fiberglass or using a combination of 2 inch XPS foam plus r-13 fiberglass. The roof on the addition will be r-38 fiberglass or using 2 inch xps plus fiberglass. Although the foam increase the r-value and helps tighten up the envelope, it does add a good bit of cost and I'm not sure it is totally worth it. Now if we were just building an entirely new home, that might be different, but super insulation an addition on a basically un-insulated home seems questionable. But it is what one contractor is suggesting - saying it will make it much more air tight and is cheaper than going with the spray in foam option (icynene for instance).
As part of the project we do plan to replace the windows in the existing home and will be insulating/finishing the basement so everything should be getting a little more energy efficient.
What are your thoughts? Are there any health concerns as well? What about issues in case of a fire?
Thanks in advance for any suggestions/advice.