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Using polyisocyanurate insulation on the exterior of a home

user-1002337 | Posted in General Questions on

I just read one of the discussions’ on the use of polyisocyanurate insulation on the exterior of a home and how it acts as a vapor barrier and not to use it because it doesn’t breath, thus resulting in potential condensation and mold problems within the walls. My concern is that I am currently getting ready to reside my house and I have already installed this on 50% of my home. I wanted to only add 1.5″ thick foam to work with my 8″ windows jams that I have upgraded to So I wanted the most R value possible from a foam. And polyiso gave this and It was also cheaper then the 2″ XPS. I live in Northern Alberta Canada, Zone 5 I believe. My home wall consists of 1/2 drywall, poly vapor barrier, 2X6 walls with Roxul R22, 7/16″ OSB sheeting then the 1.5″ polyiso with tuck taped seems and then a layer of Typar over top. I have not sealed the bottom or top with caulking or tape between the OSB sheeting and the polyiso so won’t this allow any water vapor or condensation to escape, Also I herd the tuck tape will loose its adhesion ability over time so will this save me.

I’m thinking that for the rest of the house I should ditch the polyiso and just add 1.5″ rigid XPS to allow for the walls to breath. However I don’t want the polyiso I purchased to go to waste so I was also thinking of just using the polyiso with the XPS, Placing one side by side on the walls. Will this allow the wall the breath enough so that I can do this? Or would I be able to just get away with continuing to use the polyiso and just leave the joints un-taped and leave the bottom and top to vent and drain.

PS: No rush but if someone could reply ASAP that would be great. I am planning to finish the rest of the house this week.


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  1. GBA Editor
    Martin Holladay | | #1

    The idea behind installing polyisocyanurate on the exterior of your walls is:
    1. To improve your wall's R-value and lower your energy bills, and
    2. To keep your sheathing warm enough to prevent condensation or moisture accumulation in your wall.

    Here are some articles on this practice:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How Risky Is Cold OSB Wall Sheathing?

    If you are located in northern Alberta, your climate is cold -- at least climate zone 7 using the DOE climate zone system. If you have 2x4 walls, your foam should measure at least R-10; if you have 2x6 walls, your foam should measure at least R-15. Thicker foam is always safer than thinner foam.

    Your worries about the fact that polyiso is a vapor barrier are misplaced. A foam-sheathed wall is not designed to dry to the exterior. The benefits of the higher R-value -- which helps keep the wall cavity warm -- mean that moisture accumulation shouldn't be a problem, as long as your foam is thick enough.

    Your polyiso has an R-value of about R-10. That may be enough foam -- if your walls are framed with 2x4s. If you have 2x6 walls, though, you really should use thicker foam.

  2. NtA8ZinKbM | | #2

    I agree with Martin on having more polyiso and I would also think in terms of your interior moisture. Having more insulation covering the exterior will also help block any thermal breaks that would occur through the OSB and studs. The better you are at blocking the exterior air and the interior air from reacting with each other the better your energy savings.
    Also another thing that I think you should consider is that warm moist air will migrate towards a colder surface- exterior walls, attics etc so it becomes important to control that moisture and make sure it is vented to the outside. This way you control where the moisture exits the building and you don't leave that choice for the moisture to make.

  3. user-788447 | | #3

    I'll make the issue more complicated for you ; )

    The benefits of the higher R-value -- which helps keep the wall cavity warm -- mean that moisture accumulation shouldn't be a problem, as long as your foam is thick enough.

    I think Martin misspoke here. Isn't higher R-value intended to avoid moisture accumalation (i.e. water vapor stays vapor and not 'sorbed into the materials?) I think Martin meant to say that 'moisture accumulation should not BECOME a problem.'

    You are creating a wall type that has little drying potential to either the exterior or interior. The wall won't dry to the interior because of the vapor barrier. The wall has little drying potential to the exterior even with 2" of XPS. Martin's answer concern the hygrothermal dynamics of vapor diffusion but water can get into your assembly in 2 other ways: bulk water intrusion from the outside (rain getting past your cladding) and air exfiltration during the heating season.

    If your flashing details are good and your house has been tested with a blower door/ infrared camera for locations of air exfiltration you may be OK with the outboard foam insulation.

    Here is a table of material properties:

    XPS has a perm rating per inch of 1 (similar to the OSB). EPS has a perm rating of 3.5 per inch. Would even ~2" of EPS allow enough drying potential if you walls did get wet by some mechanism? I don't know.

    I understand Martin's reasoning that thicker foam is safer however when retrofitting thicker foam can be more difficult to install at all transition points, flash well, and seal well at protrusions. Adding insulation should happen in tandem with creating a proper continuous air barrier.

  4. user-1002337 | | #4

    Thank you guys for the quick responses. I believe I understand a little more now. What I got from this was that I'm OK with the polyiso however I just didn't add enough. I wish I would have consulted early about this on here but I'm to late for that. Anyways I have already added 1.5" to most of the house. So wouldn't this be better anyways than not having added any foam at all. Sure I'm not at R15 but for climate zone 7 at least I have added some foam which should help with preventing thermal bridging. Or am I just making matters worse. From what I noticed around here is that most people when upgrading there siding have only add roughly 2" foam.

    Since I have still some of the house left to do. Should I still stick with the polyiso or should I go xps? If I go xps though then I will loose some R value since I have to stick with 1.5" for my window jams?

    I wish I could go thicker now but I'm pretty much stuck with 1.5".


  5. GBA Editor
    Martin Holladay | | #5

    Go with the 1 1/2" polyiso. Install rainscreen strapping and pay very careful attention to flashing. Get a humidity meter and monitor your indoor relative humidity levels during the winter to be sure you interior isn't too humid, and you should be OK.

    J Chesnut,
    I fail to see the distinction between the two sentences, "moisture accumulation shouldn't be a problem" and "moisture accumulation shouldn't become a problem." If Shane installs an adequate thickness of polyiso, I think that moisture accumulation shouldn't be a problem -- neither immediately after he installs in, nor in some future time (when a situation might arise that could result in something "becoming" a problem).

    Be= present tense (as soon as the polyiso is installed); become = future situation (???).

  6. jklingel | | #6

    Question: Isn't the poly vapor barrier going to prevent drying at all? Wouldn't he be better off to remove the poly vb if he uses polyiso foam? thanks.

  7. GBA Editor
    Martin Holladay | | #7

    Yes -- but removing existing poly behind drywall is expensive and disruptive.

    John Straube makes the point that although many experts are worried about retrofit wall foam on houses with existing poly, no one is seeing failures. Thousands of Canadian houses with existing poly have been retrofitted this way, and the predicted disasters aren't happening.

  8. user-788447 | | #8

    OK the language distinction is too obscure but the distinction I meant to communicate is that while sufficient insulation outboard the sheathing protects the sheathing from moisture accumulation via diffusion there are other mechanisms of moisture accumulation that can occur. In these other cases the outboard sheathing restricts drying potential. There therefore are scenarios that moisture accumulation can be a problem.

    I think it is most prudent to design with drying potential in mind. I'm concerned that the outboard insulation rule of thumb will turn out sometimes problematic in the field as earlier vapor barrier rules of thumb. That said I don't think Shane has to lose sleep moving forward. A wall assembly with both a vapor barrier and outboard foam insulation may work fine but it is neither ideal nor recommended.

  9. user-1002337 | | #9

    Hello, Martin.

    Just a question about your reponse earlier. Im confused on your response about min foam requirements on wall thickness. You said that. If you have 2x4 walls, your foam should measure at least R-10; if you have 2x6 walls, your foam should measure at least R-15. How come a thicker wall needs more and a thinner wall needs less? I was thinking it was the opposite. Wouldn't a thicker wall (2X6-R22 with Roxul) keep warmer and prevent the warmer air from coming in contact with the cooler air which in turn will cause less moisture issues? Could you explain the reasons on this.


  10. jklingel | | #10

    The thicker wall is cooler, being farther from the heat source, and thus needs more r, if I am not mistaken.

  11. GBA Editor
    Martin Holladay | | #11

    As John Klingel explained, a 2x6 wall has colder sheathing than a 2x4 wall, because a 2x6 wall has more insulation than a 2x6 wall. Cold sheathing is risky, because cold sheathing is subject to moisture accumulation; warm sheathing is not.

    To keep the sheathing on a 2x6 wall warm enough -- above the dew point -- you need more rigid foam than you would on a 2x4 wall.

  12. GBA Editor
    Martin Holladay | | #12

    Todd Vendituoli,
    Good to see you visiting the GBA site! I hope everything's going well over in West Burke. I haven't heard from you since you left Miller's Run School.
    Martin Holladay, former school board member, Miller's Run School

  13. GBA Editor
    Martin Holladay | | #13

    J Chesnut,
    There's no end to the debate about exterior rigid foam on walls. During the 1980s, an entire generation of builders was taught the importance of allowing walls to dry outward. So a lot of builders are suspicious of exterior foam.

    However, all of the data from monitoring studies show that properly specified exterior wall foam lowers the risk and keeps the wall dryer. In the 1980s, we all built homes with cold plywood or OSB sheathing. We thought that was the right way to build. But a lot of those homes have wet walls.

    Cold wall sheathing really isn't that safe; it's risky. The benefits you get from exterior foam -- namely warm, dry sheathing -- far outweigh the risks.

    Of course, you do need to detail your wall properly to address rain. A good rainscreen goes a long way to solving the problem of wind-driven rain.

  14. user-1002337 | | #14

    Hello Martin.

    Thanks for all the Help. I have been doing some more research and I am feeling the need to add more foam. Like is said I have added already 1.5 " polyiso on half of my house and installed windows with 8" jams to except this. After more research and from your reply's I'm feeling I should bite the bullet and add another 1" of foam on top of the already 1.5" to get above that R15 like you had mentioned. My concern this time is that I have installed typar house wrap over the polyiso that I already installed. Would I be able to install this 1" polyiso foam over the house wrap and then apply another layer of house wrap over the 1" foam? Will this cause any problem? I'm thinking no since the polyiso doesn’t' breath anyways. Or should I just stick with my 1.5" foam. I'm trying to not add to much more foam because I am already going to install 1 x 4 furring strips for the siding and I didn’t incorporate this into my existing plans so my windows are already lost the flushness to the siding and I don’t want to added 2 layers of window trim to make up for it.


  15. GBA Editor
    Martin Holladay | | #15

    Q.."Would I be able to install this 1-inch polyiso foam over the house wrap and then apply another layer of house wrap over the 1-inch foam?"

    A. Yes.

    Q. "Will this cause any problem?"

    A. No.

  16. user-1002337 | | #16

    Hello martin,

    Heres another question for you. If I was to put 1.5" xps on a few of my remaining walls and then the 1.5" polyiso over that will this then allow any mositure to alteast breath through the xps and not just stop at the osb where mold/rot etc could occur. Of course it would then stop at the polyiso but then it could atleast drain between the xps and polyiso down to the flashing and out instead of trying to drain/breath between just the osb and the polyiso. Would this thought even happen with this scenerio or am I just out to lunch on this one. Or is the 3" polyiso still even better then adding the 1.5" xps with the 1.5" polyiso.


  17. GBA Editor
    Martin Holladay | | #17

    Q. "If I was to put 1.5 inch XPS on a few of my remaining walls and then the 1.5 inch polyiso over that, will this then allow any moisture to at least breathe through the XPS and not just stop at the OSB where mold/rot etc could occur?"

    A. No, you are misunderstanding the purpose of the foam. The purpose of the exterior foam is to keep the OSB above the dew point. If you do that, the OSB can't accumulate moisture because it will never be cold enough. It stays warm and dry all winter.

    Q. "Of course it [the moisture] would then stop at the polyiso but then it could at least drain between the XPS and polyiso down to the flashing and out instead of trying to drain/breathe between just the OSB and the polyiso."

    A. First of all, there will never be liquid water between the XPS and the polyiso -- unless, of course, you have terrible flashing details and introduce rainwater into your wall. Secondly, there isn't enough room between the XPS and polyiso to allow drainage.

    Q. "Would this though even happen with this scenario or am I just out to lunch on this one?"

    A. Out to lunch, I'm afraid.

    Q. "Is the 3 inch polyiso still even better then adding the 1.5 inch XPS with the 1.5 inch polyiso?"

    A. Yes, the R-value of 3 inches of polyiso will be a little higher than 1.5 inch of XPS plus 1.5 inch of polyiso.

  18. user-1002337 | | #18

    Hello Martin,

    I have another question concerning my project. I have begun finishing my project and took your advice about more foam. Im adding 2" more instead of just 1.5" polyiso. However I have found a article about polyiso and how it losses its R value over time.

    Here is the link:

    So if this is the case. Whats the point of using it other then the green point of view. Even though XPS is being made green now. I am at a R value of 21 with 3.5" with the poly but according to this I could be 20% less in a few years. So if this is true, should I have even bothered with the poly and just went with xps? The highe R value with less thickness made me go with the polyiso in the first place. But it looks like I might be just hurting myself in the end. If this is right I could be below a R value of 16.5 (20%) less in years to come. If i went with 3.5" xps i would be at 17.5. Im using IKO Enerfoil polyiso - It is Foil faced on both sides. I am taping up the ends before I put it up however im poking many holes in it with plastic cap nails to secure it + furring strips.

  19. GBA Editor
    Martin Holladay | | #19

    The document you linked to was not prepared by a disinterested researcher; it was prepared by the Extruded Polystyrene Foam Association, a trade organization that has an ulterior motive: undermining polyisocyanurate insulation in hopes of increasing sales of XPS.

    The bottom line: even after 15 years of "thermal drift," polyiso will still have a higher R-value per inch than XPS.

    Here's what I wrote on this issue in an article in Energy Design Update (April 2003): "The thermal performance of closed-cell foam insulations — including polyurethane, polyisocyanurate, and to some extent, extruded polystyrene — drops as the insulation ages. The cause of this “thermal drift” is the gradual dissipation of gaseous blowing agents, which are replaced by air as they exit the foam. (Expanded polystyrene, an open-cell foam, is not affected by thermal drift.) ...

    "A new test method for determining the R-value ... dubbed “long-term thermal resistance,” or LTTR, was implemented in the US by industry consensus on January 1, 2003 for polyiso insulation ... “The LTTR value is a 15-year time-weighted average,” says John Geary, marketing services manager at Firestone Building Products, a polyiso manufacturer. “If you were to take the R-value every year for 15 years, and average those 15 numbers out, the result should equal the LTTR.” ...

    "The LTTR method results in polyiso R-values ranging from 6.0 to 6.25 per inch...

    "The R-values provided today by polyisocyanurate manufacturers are more accurate than ever, and can be reasonably used to make comparisons between competing insulation materials. ... It can be expected that these [R-]values will hold, on average, for about 15 years. Beyond 15 years, it is probably prudent for conservative designers to continue to figure on a long-term value for polyiso of R-5.6 per inch, as the NRCA advises."

  20. user-1002337 | | #20

    Good to hear. I was a little worried.

    Thanks Martin.

  21. albertrooks | | #21

    Martin and all,

    I really wish we would add mineral wool to our "collective vocabulary" for exterior insulation. Personally I think it's a much better product for this application. At R 4 /inch it's also good enough in thermal insulation to keep the dew pint off of the sheathing. And if not, the results are theoretically less catastrophic due to it's permeability.

    It seems the foam boards are always talked about but MW is left off the list...

  22. t6SUd3LsNZ | | #22

    I'd like to jump in here and ask a question to service my own selfish needs.

    2 years ago I had a house constructed for me with 2x6 walls, R24 fiberglass bat insulation, interior poly vapor barrier and a 1" polyiso foam sheet exterior cladding. Also the 1" exterior foam has felt paper underneath between the foam and OSB wall sheeting. I'm afraid, after much additional self education that my contractor didn't know as much as he led me to believe.

    My question is this.... How concerned should I be about the construction of my wall assembly wrt moisture? I know now that it is less that ideal.

    Should I pack my things and head for the hills?

    What type of monitoring could I do to head off any moisture problems?

    We burn wood as our main heat source wintertime and the moisture level never rises above 30%.

    I know the opinions are strong as they are varied but I would like some honest advice to either put my mind at ease or perhaps make it worse.


  23. GBA Editor
    Martin Holladay | | #23

    This is a question-and-answer forum. Unless I am mistaken, I don't remember anyone raising the question, "What types of insulation can be used on the exterior side of my wall sheathing?"

    The questions at hand concern the use of polyisocyanurate, and I have been trying to answer them. Your question is interesting, but it's not the topic of discussion here.

    If any GBA readers are interested (as Albert Rooks apparently is) in learning more about the use of mineral wool to insulate the exterior side of wall sheathing, here is an article on the topic: Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

  24. GBA Editor
    Martin Holladay | | #24

    Chris Elliott,
    To answer your question, we need to know your climate. Where do you live?

    You are correct that a wall with exterior foam sheathing shouldn't have interior polyethylene.

    There is another possible problem with your wall assembly: the rigid foam may be too thin to keep your wall assembly above the dew point in winter. However, we can't determine if this is a problem unless you tell us where you live.

    Even though your wall assembly isn't ideal, you should not panic. The walls on my own home include both errors: foam which is too thin, and interior poly. As far as I know, my walls are still OK.

    Not all errors are fatal -- fortunately.

  25. albertrooks | | #25


    "The questions at hand concern the use of polyisocyanurate, and I have been trying to answer them. Your question is interesting, but it's not the topic of discussion here."

    Yes of course your right on this. Your answers are clear, factual and helpful. Exterior foam needs to be done correctly and there are not many outlets for good instruction and/or details.

    Thanks for your indulgence and the Mineral Wool link. This area of GBA has so many readers and is often re-read much later by those contemplating projects and methods that it seems reasonable to remind readers that there are other ways to insulate.

    Greetings from Olympia WA on a warm sunny Sunday!

  26. t6SUd3LsNZ | | #26


    I live in Nova Scotia, East coast of Canada. Not sure what climate zone that falls under.

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