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Community and Q&A

XPS vs. polyisocyanurate

jason.hache | Posted in Energy Efficiency and Durability on

I am currently redoing the exterior of my house with a new vinyl siding and changing all windows. My house was built in the mid 80’s and have 2×6 studs with Batt Insulation and mostly without a vapor barrier… but the insulation is not great, especially the air goes through the house.
My question is what is the best insulation for me to put outside the house underneath the vinyl siding. My windows will allow me to go up to 1 inches of exterior insulation.
So XPS with Typar? Polyisocyanurate (IKO Ener Air )? Others?
IKO Ener Air (polyisocyanurate) (
I would like to know what is the best thing I should use.
I live in a Zone 4a-4b.

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  1. Expert Member
    Dana Dorsett | | #1

    As long as the facers on the batts are kraft rather than foil you'll be fine with either. You'll get slightly better thermal performance and much better drying performance out of the IKO Ener-Air (R5.6 @ 1") than XPS. (R5 @ 1") .

    IKO Ener-Air is greener too, since polyiso is blown with pentane rather than HFCs, which does less than 1% of the greenhouse damage of HFC134a, the predominant blowing agent used for XPS.

  2. Richard Beyer | | #2

    Explanations about XPS blowing agents for those concerned:

    Honeywell Solstice Gas Blowing Agent (GBA) has been developed specifically for extruded polystyrene and pressurized one- and two-component PUR foams.

    With a global warming potential (GWP of) <1, it’s a replacement for HFC-134a and other fluorocarbon and non-fluorocarbon foam blowing agents.

    Solstice Gas Blowing Agent offers:

    Nonflammablility by ASTM E-681 and EU A11 test methods
    GWP of <1
    Low Maximum Incremental Reactivity (MIR)
    Low photochemical ozone creation potential (POCP)
    VOC-exempt in U.S.

  3. jason.hache | | #3

    Thanks a lot for the info. I will go with the poly!

  4. jason.hache | | #4

    Just another question regarding these two product, which one ''Breathe'' better? Or wich one let water out?
    I don't want to trap moisture inbetween de walls with a product that seals it in?

  5. GBA Editor
    Martin Holladay | | #5

    Although Dana is a fan of types of rigid foam that have slightly higher permeance than other types (in other words, any type of rigid foam without a foil facing), the advantages of these types of foam compared to low-permeance foams are illusory in your application.

    Let's face it -- you're not going to get much drying to the exterior once you install rigid foam on the exterior side of your wall. And you don't have to. This type of wall is designed to dry to the interior.

    For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  6. Expert Member
    Dana Dorsett | | #6

    Honeywell's Solstice® is a trademarked name for HFO1234yf (probably with a few other "special sauce" ingredients) which had previously been thought to have a GWP of about 4. HFO1234yf is mildly flammable, which has been much studied relative to it's use as an automotive air conditioning refrigerant, but isn't dramatically more flammable than many other alternatives.

    DuPont's Formacel® 1100 blowing agent is similar to the Honeywell product, but primarily HFO-1336mzz, which has a 100 year GWP of about 9. That is still not a big deal compared to HFC234fa and HFC134a at greater than 1000x CO2. It has a slight advantage over HFO1234yf in that it is not flammable.

    In a zone 4 climate using foam with a permeance greater than 2 will still be providing significant drying capacity toward the exterior. That would be 4x the drying capacity of kraft facers on batts (which may exist in this wall), and the same approximate drying capacity you would have with standard interior latex paints (which can be as low as 3 perms.). Most 1.5lb or 2lb polyiso will have a permeance greater than 2 @ 1" unless it has a more vapor retardent facer.

    R5+ would be more than required for dew point control without interior side vapor retarders in that climate, but unless you're gutting the wall to remove any pre-existing vapor retarders it's important to pay attention to the drying capacity toward the exterior as well.

  7. GBA Editor
    Martin Holladay | | #7

    Your response is mostly correct, with a few caveats.

    You wrote, "In a zone 4 climate using foam with a permeance greater than 2 will still be providing significant drying capacity toward the exterior. That would be 4x the drying capacity of kraft facers on batts."

    But the permeance of rigid foam products doesn't change with humidity changes. Rigid foams are "dumb" vapor retarders. Kraft facing, on the other hand, is a smart retarder, with vapor permeance that ranges from 0.4 perm to 4.2 perm.

  8. Expert Member
    Dana Dorsett | | #8

    BTW: The short-sheet spec on the fiberglass faced IKO polyiso ( ) is aways greater than 60 ng/Pa•s•m2 at any of the thicknesses offered. (That's a bit more than 1 US perm, = 57.2 ng/Pa•s•m2 )

    This implies that the facers are VERY vapor permeable, since at 2" (the thickest in the lineup) the polyiso itself could come in as low as 1- 1.5 perms. At 1" you are likely north of 2 perms, but probably not more than 3.

  9. jason.hache | | #9

    Hi all,
    First of all thank you for all the feedback... although I might now be more confused ;)
    But I think, from what I can gather, I will go with the IKO Enerair polyiso... I think!
    My wall are two by six with Batt and kraft paper so with the Enerair outside directly on the wall with taped seams and vinyl siding on top I should be good!

  10. Expert Member
    Dana Dorsett | | #10

    Martin: I could just as easily have cited vapor-barrier latex rather than kraft facers, eh? ;-)

    Exterior house paints are typically tighter than 2 perms (often tighter than 1-perm) which is why some amount of rainscreen gap between the foam & siding would be in order. (Even 1/8" would be a useful ventilation gap behind wood clapboards or fiber cement.)

  11. rwbontrager | | #11

    I'm starting my project to insulate by basement in KS. My plan was to use the following products:
    JM AP Foil™ 3/4"x4'x8' R-4.4 Polyiso Rigid Foam Insulation Model Number: W-N5075
    JM AP Foil™ 1-1/2"x4'x8' R-9.3 Polyiso Rigid Foam Insulation Model Number: W-N5150
    JM AP Foil™ 2"x4'x8' R-13 Polyiso Rigid Foam Insulation Model Number: W-N5200

    2" in the rim joist, 1 1/2" on the walls and 3/4" plus 3/4" OSB subflooring on the floor. Foil side facing in. Glue it directly to the concrete and tape all the seams. Use Great Stuff around the insulation in the rim joist area. Any comments? I haven't found too much on the use on the floor. It's a R 4.4 so I'm expecting it to keep any moisture from condensing on the subfloor, but would it wick any up from the concrete?

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