I advise you to install rigid foam insulation on the interior of your walls. (greenbuildingadvisor.com)
At this point, the only way to meet the insulation requirements is to use spray foam, certain types of rigid foam, or to thicken the walls to 2 × 6. (tinyhousebuild.com)
Exactly the same details can be used for rigid foam board insulation. (buildingscience.com)