Is Polyiso my only option? – GreenBuildingAdvisor

I’ve got a 1924 home in Seattle, with 2×4 construction. On the exterior side of the 2×4’s, I have T&G sheathing and layer of felt/tar paper, and then cedar siding.

In the process of totally gutting it from the inside, and wondering about insulation options. From what I can tell, it appears that the only way to achieve code compliance is to cut/cobble 3.1″ polyiso into the stud bays. To clarify, I’m not interested in furring out the 2×4’s, giving up interior square footage, using spray foam, or taking off exterior siding. 

Are there any other options, given constraints listed above? 

much appreciated