GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Exterior rigid insulation in remodel

joemain | Posted in Green Building Techniques on

Hi, I have an existing cape cod home with 2×4 walls with 60yr old rockwool in the cavities and 1×8 shiplap on the exterior.  I am thinking of trying to do 3/4″ EPS (r3) unfaced on top of shiplap, then 2×4 laid flat at stud locations, then 1-1/2″ EPS (r6) unfaced in the gaps, then Tyvek, then furing strips for a rainscreen then Hardie plank siding.  The reason behind EPS is its drying ability and permeability and the fact it doesn’t lose R value over time as much as all the other types.  See sketch attached.  I live in on the border of climate zone 4 and 5, so according to this article 
those thicknesses work.  At the top and bottom I am going to use Coravent to prevent bugs.

Does anyone see any inherent issues with vapor, or reliability of this wall system? Should the 3/4″ layer of foam be XPS instead so it doesn’t compress as much?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    Skip the 2x4s. The labor to cut all that foam in is going to be a problem. I don’t see what the 2x4s accomplish either. Just use two layers of foam with the seams overlapped, then 1x furring over that to give you your rainscreen gap. That’s the usual way to use rigid foam on the exterior.


  2. Expert Member
    Peter Yost | | #2

    EPS can resist compression just as well as XPS if you get the right type; see this resource:


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |