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What would a weighted average R-value be for a Superior Wall Xi?

user-593033 | Posted in Green Products and Materials on

I am trying to determine what the weighted average R-value of a Superior Wall XI and XI plus wall is. Does anyone know? I can post cross sectional diagrams if someone can tell me how.

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  1. user-593033 | | #1

    Thank you

  2. user-626934 | | #2

    With no additional insulation and no drywall between the studs, Superior Wall Xi is about R-7....a little less when you factor in the poorly insulated header and uninsulated footer. See attached....I did these calcs in THERM a few years back. Note: my section drawing in THERM does not match the actual Xi wall section perfectly....but it's pretty darn close.

    Even though the parallel path method is the completely wrong approach to calculate whole-wall U-value and R-value for this kind of assembly, it's interesting to note that their calcs are incorrect. They should have taken the weighted average of the U-values, then converted to whole-wall R-values.

    I hope their structural engineering calcs are more thorough than their heat flow calcs...

  3. GBA Editor
    Martin Holladay | | #3

    Thanks. You're the first person I've heard of who did a THERM calculation on these walls. I'm grateful.

    Your answer is consistent with information we've published on GBA before. Bruce Harley figures that the Xi system has a whole-wall R-value of about R-8.1 -- not the R-12.1 to R-12.5 claimed by the manufacturer.

    Bruce provided information on his quick back-of-the-envelope calculation a while ago, when he wrote, "Although it wouldn’t be exactly right, a quick approximation would be to treat the stud sides and face all as exposed surface area with similar net R-value, so you’d have something like 14” at R-3.8 and 20” at R-12.5, for net of 34/(14/5.2+20/13.3) ~= R-8.1. A more detailed analysis would be a little worse than this because a good deal of that fin’s side has less than the 0.6 R-value of the 7.5-inch stud but it’s not going to add up to much."

  4. Reid Baldwin | | #4

    Classic marketing. By calculating something called a weighted average R-value, they give the false impression that such a quantity is meaningful.

  5. charlie_sullivan | | #5

    I would say that's beyond just misleading marketing. There are too many problems with their analysis to even be worth listing. And they don't just report it as a weighted average--they report it as "the R-value for the Superior Wall system".

  6. user-626934 | | #6

    You're welcome, Martin.

    For those who are interested, here's what things look like in THERM for this wall insulated with 6" of the fluffy stuff of your choice (@ R-3.7/inch) and 1/2" drywall....just a little under R-19 for the whole wall....again, probably more like R-18 once you account for the header and footer (and panel joints).

    That it would be this low is not intuitive until you look at the heat flow vectors in the image...

  7. user-626934 | | #7

    2nd try on the attachment...

  8. user-593033 | | #8

    Thanks for the therms. I am looking for the most accurate inputs for modeling in REM. Of course now I will have to model all my assemblies in therm to make it a fair comparison.

  9. AndyKosick | | #9

    Just in case anyone sees this, my thoughts at the moment,

    I stumbled on this while revisiting my Superior Xi R-values for HERS rating because I've never agreed with the company, and I've moved into Ekotrope software, so I'm rebuilding some libraries. Thank you John for running the THERM models. I've had these at about R-10.3 but that's using parallel heat path which I know doesn't account for the sides of the studs properly. I could go on but...

    The fact that Ekotrope allows you to check a "verified" box for assemblies that are approved within your organization has made me realize that RESNET (or somebody) needs to do exactly that at the national level. Create a "Verified" library of third party tested Assembly R-values so that individual Raters and Companies aren't all coming up with there own. Then build those right into the software. I know REMrate and Manual J software have had something like standard assemblies included but this would industry wide standard not platform specific. Superior Walls are a perfect example of a widely used standardized assembly that I'm sure, right now, different raters are using either wrong or different R-values for all over the country.

    I should probably take some action on this, but then they'll probably want me join the standards development committee or something.

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