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Polyiso vs. EPS Foam Insulation Over Stucco

Nate G | Posted in Energy Efficiency and Durability on

I have a stucco house in high desert New Mexico (zone 5B). The stucco is in bad shape after having been terribly abused by the previous owners, so I’m taking the opportunity to have the whole house wrapped in continuous rigid insulation as part of the re-stucco project. The boards would be applied over the existing deteriorating stucco, fastened through it and the sheathing and to the studs, and then a new 1/2″ two-coat cementitious stucco cladding will be attached over it.

They say they can only use a maximum of 1.5″ of foam because they’ll need to use masonry nails to attach it to the studs because the nails have to penetrate the stucco, and the thickness of the stucco and the Celotex sheathing plus 1.5 inches of foam (2.75″ total) is the most they can do to allow the nails to penetrate the required once inch into the studs given that the longest masonry nails they have are 4″.

I have two options for the foam: Polyiso or EPS. The bid is about $11k ($9.50/square foot) for Polyiso and $10k ($8.60/square foot) for EPS. Here are my estimates of the foams’ long-term R-values in my climate (taking into account Polyiso’s R-value reduction over time and the performance characteristics of Polyiso and EPS in different temperatures as documented in the “Polyiso Penalty” article:

Polyiso R-7.5 in the heat; R-6.7 in the cold
EPS R-5.7 in the heat; R-6.8 in the cold

Either choice would exceed the code-minimum of R-5 for continuous insulation applied over 2×4 studs in climate zone 5.

I’m considering going with the EPS option for the following reasons:
1. Lower upfront price.
2. My manual J modeling shows that it’s basically a wash: compared to EPS, Polyiso yields a lower summer peak cooling load by 200 BTUs, but the peak heating load is increased by 80. Big whoop.
3. My peak heating is much more expensive than my peak cooling (horrible oversized 80% efficient gas furnace @ $1.25/hour for 2 hours a day vs a wonderful swamp cooler @ $0.06/hour for 12 hours a day); the better cooling performance is likely to produce few financial gains.

What do folks think? Is there anything I’ve overlooked? Any reason why I should go with polyiso instead of EPS? Is only 1.5″ inches of EPS likely to make much of a difference?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Another reason you might want to go with EPS is the wall is built up with poly or foil under the interior gypsum, or if you have foil or vinyl wallpaper on the interior. Foil faced polyiso is an extreme vapor barrier, less than 0.05 perms, whereas unfaced 1.5" of EPS is still in the ~1.5-2 perm range, offering a real drying path toward the exterior.

    Even if you went with fiber-faced polyiso, most versions are still guaranteed to be less than 1 perm (but might test higher than 0.5 perms, depending on facers and thickness.)

  2. Nate G | | #2

    There's no interior poly or foil or any wallpaper. I've opened the wall in multiple places from both the inside and out in the process of doing DIY projects so I know what's in it. Here's the current wall sandwich:

    - Many layers of latex paint (40 year old house)
    - Drywall
    - 2x4 studs with 2" R-8 kraft-faced mineral wool in the cavities, terribly installed
    - Celotex sheathing
    - Single layer of tar paper
    - Chicken wire stucco lath
    - 3/4" cementitious three-coat stucco
    - Latex paint (ugh, this makes me so sad)
    - Cementitious stucco color and texture coat (peeling off in a million places, naturally, as it didn't stick to the paint)

    Here's what will be added:
    - 1.5" EPS or Polyiso
    - More chicken wire stucco lath
    - 1/2" cementitious two-coat stucco

    FWIW, this area gets less than 9 inches of rain per year. Water management detailing around here is poor, I suspect because builders realized they could safely get away with it. There is no sign of moisture damage anywhere and relative humidity hovers below 30% year-round unless I'm running the swamp cooler.

  3. Expert Member
    Dana Dorsett | | #3

    It's worth packing in cellulose from the exterior before adding foam, which will reduce any convection through the cavity from the interior side. If you do that you'd be winter-safe even with foil faced polyiso (if that's what you choose.)

    If you don't completely fill the cavities it's likely the R8 econobatts won't block much convection from the interior. Even if installed perfectly the studwall + original stucco with econobatts is giving you at best R5-ish performance, and filling the cavities with fiber (any type or density) will bump to about R9-10. If the batts are still hanging OK rather than balled/bunched up in sections it should be possible to dense-pack it, but even if you have to poke 3 -5 holes in to fill the cavities in a more haphazard way to get round deteriorated batts it would be good enough even with 2-2.5lb cellulose. With fiberglass it needs to be 1.8lb density to get the air-retardency, which is a dense-packed density for fiberglass (but not for cellulose.)

  4. Charlie Sullivan | | #4

    Nathaniel, I think your decision logic makes sense. It seems a little crazy that there are no longer masonry nails available, but you'll have pretty good results with 1.5", so it might not be worth inventing a different solution.

  5. Ryan H | | #5

    Nathaniel,

    I'd like to know if you finalized your decision for your home. I'm in ABQ and am debating an identical/very similar project. Wouldn't mind knowing how the project has progressed. We're debating extending the parapets, turning the stucco system over the top, terminating into a PTO roof over Polyiso/EPS (a chance to avoid the New Mexico roof diaper look when using a membrane roof, and removal of roof cavity venting; turning the system into a "hot roof").

  6. Nate G | | #6

    Funny you should mention it. The foam is being applied at this very moment! So far I'm pretty happy. I'm only gaining about R-6, but it's probably doubling the effective R-value for the wall on this old tract house. The only thing I would have preferred is thicker foam, but it was limited by the length of the masonry nails they could source. After having busted up a bunch of the stucco to redo the windows, I honestly think removing the stucco entirely would have been an option. I could have just pulled it off in sheets! And that would have allowed any foam thickness I wanted because they could have used like 8" screws or something. Oh well,

  7. Ryan H | | #7

    Would you be willing to share some of the construction details? (eg Termination at base of wall, parapet, etc). I've attached a PDF of my basic ideas for the system... will be drawing it up over the next few months.

  8. Nate G | | #8

    So far it looks to me like they're doing things right. They terminated the base with a weep screed on the bottom of the new wall assembly (previously there was none), they're taping the seams, and they're lathing over it in a manner that looks to my untrained eye to be indicative of quality work. My house has a pitched roof so there are no parapets.

  9. Nate G | | #9

    And for what it's worth, it's made a huge difference already. The house feels noticeably warmer with fewer cold spots, and the furnace doesn't come on as often.

  10. jjmancini | | #10

    Hey Nate! I am in ABQ and looking to do a very similar project. Are you willing to share who you used and whether you are happy with the work? The only diff is my house is an older Adobe.

  11. SierraWayfarer | | #11

    Hi Nate,
    I am in Southern NM. 1000s of older houses down here could benefit from what you are doing. I hope you will add to this post from time to time. Whatever you can give from your experience might benefit many. Is adding rigid foam to the outside of traditional stucco getting to be common in ABQ or are you breaking ground? Thanks!

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