Exterior Insulation with Original Framing / Sheathing
We are gut renovating a 150+ year old house in Northern NJ. We’re keeping the original framing and sheathing, but everything else is gone. Given the original sheathing is overlapping boards, we were going to spray 2″ closed cell foam on the interior to air seal, than add 3.5″ rockwool (studs are true 4″ so we’re packing out the walls with 1.5″ strips of EPS/strapping per one of the articles on this website (I forgot the name of the technique but it reduces thermal bridging).
The siding contractors are proposing to add 1/2″ rigid foam on the exterior side of the sheathing because they need a level surface for Hardie planks. We’re speaking to several of them and for some reason they all advise against using 2″ rigid foam on the exterior. While I haven’t confirmed, I suspect contractors use XPS on the exterior, which is denser than EPS?
We know interior spray foam + exterior rigid foam should be a no-no in order to avoid trapping moisture. However, I did want to see whether 1/2″ is not thick enough to be a problem?
I’m hoping we can proceed with the plan to do 2″ spray foam on the exterior + 1/2″ rigid on the exterior (which should also help a little bit with thermal bridging).
EDIT: I should say that my personal preference is to add 2″ rigid foam on the exterior and use fluffy inside the stud bays, however, I do not want to force the siding contractor to do something they clearly are not comfortable with.
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