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Best cold climate exterior insulation assembly

GJPehl | Posted in Green Building Techniques on

Hello GBAers,

Long time reader, first time poster here. I have huge gratitude for this resource, and I’ve finally become a member.

We started on a PassiveHouse retro-fit journey quite a while ago, and we really need to finish- I’m gearing up to do that now.

Our architect spec’d 6″ of polyiso on the exterior, fastened with 1×4 battens to which siding will be fastened, and which will also serve as a ventilated rain screen.

The house is in the Minneapolis area, and as I’ve been researching to identify the best way to get this 6″ assembly, I’m alarmed at the shortcomings of polyiso in cold weather.

I would really appreciate some firm guidance on this. At this point I’m getting close to just going with XPS, but we could also manage a mixed assembly as has been discussed elsewhere. Possibly the first 2″ with polyiso and the remaining 4″ with XPS?

Also, we don’t really have the option to go thicker than 6″- Tanner windows have already been installed and the sills were too expensive discard.

Thanks very much in advance for any insight that anyone can offer.

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Replies

  1. GJPehl | | #1

    Having read a bit further, I think I _don't_ want insulation that will lose R-value over time, so now I'm thinking about NeoPor or equivalent.

    Does anyone know where to source NeoPor or equivalent?

  2. user-228058 | | #2

    Somewhere on here there is a article from Martin on built up foam roof assemblies, if you read down in the comments from Dana there is great info on stacking layers of foam. Perhaps it is pertinent..

  3. charlie_sullivan | | #3

    I think your choice to go with NeoPor is excellent: low environmental impact, and and R-value that won't drop with time or temperature. With 6" thick, foil faced polyiso for the inside layer would be rational, as it would stay plenty warm, and its R-value would drop only very slowly with time, but it would drop whereas NeoPor won't

    It's likely that almost any regional EPS manufacturer could order Neopor beads and make you a batch of it, but as far as finding people who already do it, Insulfoam calls it "plantinum" and has a branch in Nebraska; Atlas calls it Neopor, Thermalstar Chrome, or Thermalstar GX, and is headquartered in Michigan.

    Thermalstar GX is a new product with a higher graphite content, and is advertised as R5/inch, vs. R4.5/inch for most neopor.

    Edit: there's also a list of Neopor sources on, of all places, the neopor web site:
    http://www.neopor.basf.us/contact

  4. GBA Editor
    Martin Holladay | | #4

    Gerald,
    If you can't find Neopor, you can spec a sandwich of polyiso and EPS, with the polyiso on the interior side of the EPS. Remember to stagger the seams of the foam layers to reduce air leakage.

    -- Martin Holladay

  5. GJPehl | | #5

    To Charlie Sullivan:

    Thanks very much for validating my impressions. I'm feeling very lucky that I stumbled on this information as I was researching whether there's a significant difference among polyiso manufacturers. I was shocked to read that at very cold temperatures polyiso thermal conductivity goes _up_, and by a lot. Particularly disconcerting since this insulation will cover the seem between the rim joist and the CMU basement wall. Our architect has done several Passive House designs, so of course I'm accepting his work, but on this detail, I've got to revisit with him.

    Regarding an inner layer of polyiso, I have to admit, that I really dislike the idea that the R-value changes over time. It's just a psychological thing, but still. On the other hand there could be a benefit in the summer, and most of all it's probably splitting hairs at this point. One issue that would be significant, though, is that the _bottom_ of the first layer _will_ be exposed to ambient outdoor temperature (in addition to contacting the rim joist), so that would mean that I'd have to use a strip of the EPS along the bottom of the inner layer, and then the rest would be polyiso.

    I'd really appreciate a gut reaction to that- worth it or not?

    Thanks again!

  6. GJPehl | | #6

    To Martin Holladay:

    Thanks very much Martin, and a special shout out- I've been reading you more than any other single building science writer ever since we started this long journey 2009, believe it or not. I tend to be a bit of an uber-skeptic, and GBA and you in particular have passed all my BS detector tests, and my gratitude is great. The only way I can tell who's an expert in anything is if I myself am an expert. Without that, I can only use other knowledge to estimate what's valid and not, I think that's why the world is crawling with people selling snake oil- most people don't realize how much they don't know, it seems.

    Anyway, to clarify your comment, are you saying that if I _can_ get Neopor, I should do the whole assembly with it?

    Do you have any thoughts about the possible assembly I described above- to get the benefit of polyiso on the inner layer re summer temperatures? This would require the additional effort of isolating the lower few inches of the polyiso with a strip of EPS so that it's not exposed, especially since it contacts the rim joist.

    Thanks again for any thoughts.

  7. Expert Member
    Dana Dorsett | | #7

    It's the average temperature through the foam that matters. When you have a very thick layer compared to the R value of the cavity insulation (say a nominal R36 foam with R13 cavity fill) the performance polyiso in a cold climate is dramatically higher than when you have only only a modest amount, such as 1.5"/R9 polyiso over R13 cavity fill, where the average temp through the foam is much much colder.

    With 6" of polyiso on the exterior of a 2x4/R13 structure, when it's +15F outside (the binned hourly mean temperature for January in Minneapolis), with a 70F interior the warm edge of the foam is going to be ~55F, and the cold edge ~+15F, for a mean temperature of ~35F. At a mean temp of 35F all polyiso tested is going to be performing north of R5/nch, and most will be performing north of R5.5/inch, according to this set of tested samples:

    https://www.greenbuildingadvisor.com/sites/default/files/images/Polyiso%20performance%20-%20NRCA%20testing%20-%20graph.preview.jpg

    Even at the 99% outside design temperature of -11F for Minneapolis the warm side of the polyiso would be ~50F, and the mean temp through the foam would be ~30F where all samples in the graph were still north of R5/inch.

    A worst-case derating of polyiso in your climate and stackup would then be R30 for the 6" of foam, and most of the heating season it would be performing even better.

    If you split the difference and used EPS/Neopor on the exterior layer it would perform only marginally better than that, and it will likely still outperform Neopor, or XPS, not enough to be paying extra for.

    But ff you're putting 6" of foam on the exterior of a fat double-studwall or something that shifts the mean temp of the foam layer down to where some vendors' goods may be under R5/inch.

  8. GJPehl | | #8

    To Scott Mangold:

    Thank you- I'll try to find that- I'll need all the help I can get. I've devised a couple of strategies, but I'm still working through some of the details. One challenge will be how to tape the under layers- Zip paneling is the air barrier, so I'd rather not screw into it temporarily.

    Right now I'm thinking I'll do all the layers at once, but stagger them at the corners, which will leave enough exposed in the field for taping. That then leaves the corners to deal with, so maybe I wrap the corners as the _first_ step, so I can tape each corner layer one at a time.

  9. GJPehl | | #9

    To Dana Dorsett:

    Wow, thanks for the input- I feel like I'm getting the star treatment here.

    So the cavity is conventional 2x4 16OC construction with tar-paper wrapped 'balsam wool'- built in 1957. R-value negligible? 2-5 or so?

    You wrote:
    >>If you split the difference and used EPS/Neopor on the exterior layer it would perform only marginally better than that

    So that really nails the question- thank you _very_ much for such a thorough and insightful answer.

    A couple of follow up questions, if I could:

    1) Where would you expect the R-value of polyiso to 'settle'? Enjoying 20 years of superior performance is definitely a good choice if the out-gassing does _not_ result in inferior performance beyond that point.

    2) Is there a significant difference among manufacturers and specific SKU's of polyiso product? Naming names as you feel comfortable would be greatly appreciated.

    3) Can you point me toward an accessible discussion of temperature 'binning'. I think I've got a gut feel, but I'd like to understand the concept and application better.

    Thanks again for all the help.

  10. GJPehl | | #10

    To Dana Dorsett and Martin Holladay:

    Above Scott Mangold mentioned an article by Martin on foam roof assemblies with comments by Dana on stacking layers of foam. I can't find it, though- can either of you direct me?

    Thanks again!

  11. GBA Editor
    Martin Holladay | | #11

    Gerald,
    My best guess is that Scott Mangold was referring to Comment #3 by Dana Dorsett on this page: How to Install Rigid Foam On Top of Roof Sheathing.

    -- Martin Holladay

  12. charlie_sullivan | | #12

    Follow up question answers:

    1) Martin lists the long-term settled R-value of polyiso as 4.2 per inch.
    https://www.greenbuildingadvisor.com/articles/dept/musings/comparing-properties-insulation-materials
    That might be a little low--it might actually be more like 5. I would be expect it to end up similar to Neopor; Martin is saying it will be a little worse. In any case, in the long term, Neopor is no worse, and I think Neopor will be cheaper.

    2) There apparently is variation in polyiso cold-weather performance. But I doubt you'll find anyone able to recommend a specific manufacturer as better or worse. They are tight-lipped about their process and may make the same SKU in different facilities or at different times with slightly different recipes.

    3) Binning is as intuitive as it sounds, but I'm not sure exactly where to point you. One useful resource is the free "climate consultant" software that will give you some nice plots of the distribution of temperature and humidity. http://www.energy-design-tools.aud.ucla.edu/climate-consultant/request-climate-consultant.php

  13. Expert Member
    Dana Dorsett | | #13

    There isn't enough public data to say for sure, but I'd hazard that most polyiso would settle at about R5/inch @ 75F mean temperature, but when fully depleted the temperature derating artifact also disappears. The blowing agents & co-agents responsible for the higher initial-R are also responsible for the loss of performance as those gases condense inside the cells, giving it higher thermal conductivity.

    Temperature binning has long been used to analyze energy use, heating/cooling loads, etc. that starts by logging the temperature every hour, and tallying the number of hours in a year the measurement was at each temperature. By counting up the number of hours in a month or year that the outdoor temperature in a location one can make useful estimates of peak & average heating loads and designing the mechanical systems accordingly, without extreme oversizing, which can be bad for both comfort & efficiency. eg The 99% outside design temperature is the temperature bin where 99% hourly bins over the past 25 years were that temperature or warmer. It's also useful in assessing the average performance of temperature varying materials, such as foam insulation.

    Adding up the hours in each bin for a month and dividing by the hours in that month to calculate the mean temperature is one easy way to come up with an average temperature for that month, but not the only method. One could also find the temperature bin at which half the hours were warmer, half were colder, which would be the median bin temperature. For analyzing the energy impact of that variable-R the mean temperature would be more relevant than the median, but in most locations the median & mean for the month probably won't differ by very much. My WAG on the binned hourly mean for January in Minneapolis was cruder than crude- I eyeballed the temperature data graph on Weatherspark:

    https://weatherspark.com/averages/30956/Minneapolis-Minnesota-United-States

    So, if I'm off by a couple of degrees, just shoot me, OK? :-)

    Pulling the actual bin data for the crunch would be un-necessarily accurate for this discussion. The error in the guesstimated R-value at the estimated mean temp is probably greater than the WAG at the mean temp itself, since there's no way to know which of the dozen or so curves match the material that you eventually install. The purpose was to demonstrate the range of performance to expect in your stackup and location, and that it's probably going to be as-good or better than the alternatives.

    One underappreciated aspect of polyiso relative to polystyrene on walls is that in the event of a fire it does not melt, but rather chars in place. Building in a rainscreen gap between the foam & siding it can make a real difference in the flame-spread characteristics. A fire that starts on an upper story won't spread downward via gravity with a flow of burning liquid polymer.

  14. GJPehl | | #14

    Thanks to Martin!

  15. GJPehl | | #15

    Charlie Sullivan wrote:

    >>I would be expect it to end up similar to Neopor; Martin is saying it will be a little worse. In any case, in the long term, Neopor is no worse, and I think Neopor will be cheaper.

    This is great- thanks very much for the input. I'm getting comfortable making a choice.

  16. GJPehl | | #16

    Again, thanks Dana.

    At this point I'm not sure I have clear leaning, but the good news is that it appears that the choices are converging a bit.

    1) If it takes 20 years for polyiso performance to fall to that of EPS, then that's a pure benefit, setting cost differences aside.

    2) If polyiso performance eventually falls below the level of graphite EPS, then that's a penalty going forward.

    3) Fire characteristics of the two are _definitely_ food for thought. Maybe this skews me toward hybrid assembly or all polyiso.

  17. GBA Editor
    Martin Holladay | | #17

    Gerald,
    I'm not sure which GBA articles you've seen. Here are some links (in case you missed them):

    Cold-Weather Performance of Polyisocyanurate

    In Cold Climates, R-5 Foam Beats R-6

    Thermal Drift of Polyiso and XPS

    Choosing Rigid Foam

    -- Martin Holladay

  18. GJPehl | | #18

    Funny, I read all but one- 'Choosing Rigid Foam'.

    Maybe I wouldn't have bothered everyone if I had found that one first :-)

    Thanks again to all!

  19. JIM BAERG | | #19

    Gerald,
    Just to throw a curve in the discussion, have you considered exterior dense packed cellulose? It is often rated at R-4/inch and you can avoid all the issues associated with foam.
    On new homes, we typically do an 8" thick exterior assembly using gusseted exterior studs at 24", cellulose, Insulweb, building wrap, 1x4 furring for venting and then horiz. siding. I don't see why you couldn't do this on an existing building. The higher the R-values, the more the cost savings of cellulose come into play.

  20. GJPehl | | #20

    To Jim Baerg:

    If I could do it again, that's what I'd do- we only have 6".

    Thanks :-)

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