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

Polyiso over XPS

Michael Menkens | Posted in Energy Efficiency and Durability on

Thanks in advance for all your great input. I have a split-level house and have recently gutted and started to rebuild the lower level which is made up of half cinder block and half wood frame. In order to keep square footage to the maximum I opted to apply 1″ XPS directly to the surface of the block and seal it very well. I then framed a wall in front of the XPS. I want to add additional insulation but it would need to be in the stud bays. I don’t want to use fiberglass because 1″XPS is probably not enough of a thermal break. My question is should I use 2″ XPS directly on top of the 1″ XPS or would it be better to use 2″ polyiso directly on top. My concern is vapor barrier related. Either one should yield enough r-value. Also note that almost all of the wall is above grade with the exception of some is about 18″ below grade. Also I’m in New Jersey which is climate zone 5. Thanks for your input

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
    Dana Dorsett | | #1

    Most of NJ is in zone 4A- only the NW counties are zone 5A.

    XPS is a terrible choice from an environmental cost/benefit analysis- it's by far the WORST insulation material in common use today on a CO2e basis, with 3x the damage per R of second-worst HFC blown closed cell spray polyurethane, and almost 10x that of EPS:

    Worse yet, it loses performance over time as it's HFC-soup(mostly HFC134a, with a CO2e about 1400x CO2 @ 100 years) blowing agents diffuse out over time, and is only warranteed to 90% of initial value @ 20 years. At full depletion it performs the same as EPS of similar density.

    By contrast EPS and polyiso are blown with low-impact hydrocarbons, usually variants of pentane at about 7x CO2 @ 100 years.

    It's fine to install other rigid insulation over the existing 1" XPS.

    It's not clear what is meant by "I then framed a wall in front of the XPS." Are the stud edges pretty much snugged up to the XPS, or is there a gap? If gap, how much gap? Is it a full 2" (which seems to be implied by your plan, but isn't clear.) With 2" of continuous polyiso (R12-R13) and a fully depleted R4.2 on the XPS it would still beat the IRC's R15 continuous insulation requirements for basement walls, and would come close to meeting the IRC's above-grade wall performance on a U-factor basis even without cavity fill.

    If the studs are tight to the 1" XPS it's fine to install up to R15 rock wool or fiberglass batts tight to the foam. Most of the block wall is below grade, and thus at a warmer average winter temp than the above grade portion. A 2x4/R13 + R5 continuous insulation meets IRC 2018 code for above grade walls, and has sufficient dew point control at the foam/fiber boundary to allow use of a Class-III vapor retarder (= standard interior latex on wallboard). Even though long term that foam will be R4.2, the fact that the entrained air in the below grade portion won't have condensation events the moisture from short term minor condensation events above will redistribute and dry toward the interior rather than accumulate.

    If using polyiso, be sure to leave a gap between the cut bottom edge of the foam and the slab. Any bulk moisture or ground water in the slab might otherwise be wicked up into the polyiso undercutting thermal performance. A half inch of air is plenty, unless there is a flooding history (in which the high tide mark would be your guide for where the polyiso stops, installing EPS below that mark.)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |