2x4 Exterior Wall Insulation
I planning an addition to my house and am starting to think about insulation. Based on my reading of the local code, the exterior walls need to be insulated at r-18 value. As this is an addition and we have relatively limited space, I had been planning on building it out of 2x4, but using batt insulation, I believe I would be limited to r-15, no? So, any thoughts on what I can do?
So far as I can tell, my options are:
1) Frame using 2x6 - This is not a preferred option because space is so limited. Even 2" will make a difference.
2) Install r-15 batt insulation in the stud cavity and a layer of r-5 foam board insulation on the exterior, for an r-value of 20
3) Install spray foam insulation in the stud cavity, 3" thick for an r-value of 21.
4) I have read of some people doing a 1" spray of foam insulation in the stud cavity and then installing batt insulation on top of that. Any experience with this method? Would you have to compress the batt insulation?
Any other thoughts? What's your preference?