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Looking for an effective R-value for polyiso as outboard insulation

PaulRFalcone | Posted in Energy Efficiency and Durability on

I live in zone 6 (and not happy about it). I was looking for an effective R-value for polyiso as outboard insulation over 7/16 OSB, 16” O.C. 2×6 stud wall with high density cellulose. R-value needed.
Paul (new to site and love the reads)

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  1. GBA Editor
    Martin Holladay | | #1

    Your question is a little unclear. If you want to know the R-value of polyisocyanurate, that's simple: it's in the range of R-6 to R-6.5 per inch.

    If you want more information on how to install rigid foam on the exterior of walls, here's an article to help you: How to Install Rigid Foam Sheathing.

  2. PaulRFalcone | | #2

    Sorry I was not clear, I am looking for an r value adequate enough to keep my OSB warm enough so it misses the dew point. zone 6 Western NY. Thanks in advance for more info. Regards .Paul

  3. GBA Editor
    Martin Holladay | | #3

    R-11.25, or 2 inches of polyiso.

    More info here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  4. Expert Member
    ARMANDO COBO | | #4

    Well... I don't know why the JPEG attachment is not coming up. Perhaps Martin can fix the issue.

  5. GBA Editor
    Martin Holladay | | #5

    As far as I can tell, the file isn't coming up because you have named the file with a forbidden character (an @ symbol). Rename the file something simple, like "Image to post," and it should work.

  6. Expert Member
    ARMANDO COBO | | #6

    Thanks Martin... I learned a new thing today! ;-)

  7. Expert Member
    ARMANDO COBO | | #7

    I see it slightly different. For condensation control and cold climate dewpoint test, which is an approximation and simplification of the dewpoint method of ASHRAE Fundamentals, and it assumes the vapor pressure on the assembly to be miniscule, no air leaks and it is very conservative, if sheathing condensation depends on temperature and relative humidity, performing a dewpoint analysis using 70°F & 40%RH inside of the house and 26°F as the average 3-month cold temperature in Buffalo, you will need 3” of polyiso. Now your job is to make sure your indoor RH stays at or below 40%. You can see in the attachment how much insulation you would need at different RHs.

  8. Expert Member
    Dana Dorsett | | #8

    The thickness answer is still the same (2"), but performance-wise it's safer to de-rate the iso to R5.6/inch when the average temp through the material is 25F or below. If the average January temp is 10F or higher you're golden, since the average performance will drop no lower than R5.6/inch, and the winter average will still be high enough. But in a colder place like Fargo or International Falls simply using iso's ASTM C518 rating at the minimum code-prescribed R would be a mistake, since it will under perform it's rated value for much of the winter.

    Conversely you used 2.25" of XPS to meet the R11.25 minimum code prescription it's performance would be north of R12 in the dead of winter when it counts, and even 2" would be enough, despite not meeting code on it's ASTM C518 rated R.

    Similarly, 2.5" (usually labeled-R10 per ASTM C518) of Type-II EPS will perform better than R11.5 when it's cold enough that the warm side is 40F. Most Type-II EPS is tested and rated at R4.5/inch @ 40F average temp, and it's legit to use it's 40F R-value in this application (but not others.)

    FWIW: It would take quite a lot of exterior R to miss the dewpoint at the 99% outside design temp (which is what required by code in Canada if skipping the interior poly), but with even the modest interior vapor retardency of latex paint and a reasonably air-tight build, it's the average temp of the sheathing is what will determine how much moisture it takes on over a winter, not the 99th percentile peak. The IRC requirements are based on averages, not peaks.

  9. PaulRFalcone | | #9

    So what is the best ridgid insulating board for above grade exterior ? polyiso, EPS, eurathane, etc.
    (no poly on inteior studs)r

  10. GBA Editor
    Martin Holladay | | #10

    There are a lot of opinions out there. Most green builders have concluded that polyisocyanurate is the most environmentally friendly foam, but there are advantages to all three types of foam.

    More information in these two articles (one of which I linked to in my very first response, so I'm beginning to guess that you aren't reading the articles I'm linking to):

    How to Install Rigid Foam Sheathing

    Rigid Foam Insulation

  11. Expert Member
    Dana Dorsett | | #11

    Iso outperforms EPS at equivalent thickness in most US and Canadian climates even though the derating slope is in opposite directions (EPS performance increases with declining temp, iso performance decreases), but both use blowing agents less than 1% as damaging as those used for XPS or most closed cell polyurethane. Water- blown polyurethane is even less environmentally damaging, but not nearly as common as the HFC blown goods, and SFAIK not sold as rigid-board anywhere.

    In Europe most XPS is blown with CO2 rather than HFC134a, with less than 0.1% the greenhouse potential of the goods sold in the US, but also closer to EPS in R value at any given thickness than the HFC blown stuff.

    Iso works, but it's important to factor in the low water vapor permeance of the facers in the stackup. That permeance extremely low if foil faced, but typically between 0.5-1 perms if fiber faced (read the specs!).

  12. PaulRFalcone | | #12

    Martin, I have reviewed your first post and yes you did supply the link. I have now read it, thanks that answers the question and gives me more insight to other products (rock wool) etc. thanks again Paul

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