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

Continuous insulation – polyiso foiled both sides

Smipau29 | Posted in Energy Efficiency and Durability on

I have read numerous articles on here but cannot find the exact procedure that I am about to undertake, which is polyiso (Carlisle) 2” thick foiled both sides of sheet over a 2×4 wall with out wood sheathing, Simpson 20g L brackets for BWL. 

Austin, TX Zone-3 

R-10 Polyiso 2” foiled both sides over 2×4 with out wood sheathing. Continuous sheathing so will be taped/ sealed exterior. Fiber cement board attached through the Rigid Foam/polyiso with a 3/4” strapping.  Cavity insulation will be R-13. Gyp board interior with either latex paint and sealed edges or more breathable approach Lime wash type paint. (Need some more research on the last part)

Concern and Question:
I was originally looking for a polyiso foil faced on one side so that if any vapor transmission happened and hit the dew point then moisture would be able to escape… or if there’s any water penetration for what ever reason (things happen over time). 

The problem is I am running on a tight schedule to get the structure up and only place in town I found with the correct thickness only has a 2 sided foil faced Polyiso. About to pull the trigger but trying to hold my self back if I use the double sided foil face, will I lead into issues for moisture not to escape?  

From my research, since the structure is 2×4 wall with R-13 batt insulation in cavities along with the R-10 Polyiso to the exterior then I really shouldn’t have to worry about hitting the dew point for where I’m located.

Am I correct in thinking this or should I just hold out and find a single foil face polyiso?

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

    With a foil facer you have a vapor barrier. It doesn’t matter if you have it one one side or both sides, moisture can’t migrate through the sheet of polyiso. If you’re thinking foil on both sides causes a “double vapor barrier problem” since the sheet of polyiso is essentially a sealed system and doesn’t have that problem.

    I would use the regular polyiso with double foil facers and not worry about it. If you really want some vapor permeable, you can get fiberglass and kraft paper faced polyiso that is more vapor open, or use unfaced EPS (although that will give you less R per inch).


  2. Smipau29 | | #2

    Thanks Bill,

    My concern is not about moisture migrating in its about migrating out. Like i said above, I'm almost postive with an R1o insulation board exterior to the studs with an R13 interior bat I find it hard to believe I would ever hit the dew point at the interior side of the polyiso where I would create moisture. Being said, should I really chance the idea of having no moisture present on the interior of the wall at rigid foam board which then would sit stagnant inside my wall cavity unable to escape thus creating mold and eventual rot! .

    I'm still in the same boat as you, go with double foil facer and forget about it, but would really love to hear and opposing side prior to making the choice this coming Monday! Thank you for your comment.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |