A: It is probably a matter of cost. 1/2” XPS is not going to do much, especially if you have R13 already. However, the contractor may be able to add 1/2” XPS without making any structural changes such as extending the walls. If you have to extend the walls it is almost certainly going to be too expensive to make any economic sense. When I had a similar problem I added a 10mm (approx 1/2”) of Aspen Aerogel’s Spaceloft which is R10 per inch or about R5 for the 10mm product. This is probably the best way to add insulation without making any structural changes.