GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Adding Exterior Insulation to 1970s home in Climate Zone 6

wood0619 | Posted in Plans Review on

Long story short, I have a hail and wind insurance claim where I will replace the Aluminum siding on my home built in 1976. 2×4 wall construction with a poly barrier on the interior behind drywall. The sheathing is a 1″-1.125″ thick fiberboard, which I believe has an R-value of ~2.5. I have a contractor open to adding insulation and willing to do up to an additional inch on the exterior. I currently have no WRB and plan to add that as well. I know the building guidelines recommend at least 35% or more of the total R-value on the exterior for Zone 6. The Dow Thermax sheathing cold weather performance and stability look impressive with an R 6.9 @ 1″, but I believe the low permeability of this insulation is a show stopper, so my plan is as follows.

For overall wall assembly, here is what I am thinking:

5/8″ Drywall | Poly – 2×4 with R13 1976 fiberglass | 1″ R2.5 fiberboard | 1″ Insulfoam GPS platinum R5 | Hydrogap-SA WRB | Rainscreen SlickerMax | fiber cement siding

The plan is to build the window frames out to place the windows as “outies” with the WRB sealed and flashed out to new window RO

Is the low permeability of Dow Thermax truly too risky?
Is it a problem attaching sheathing and siding to the fiberboard? Will the fiberboard hold nails, or do all fasteners need to attach through to studs?

I would appreciate any help or suggestions to improve this proposed construction plan.

Thank you,

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. Deleted | | #1


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |