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High R-value CMU?

kloopster | Posted in Energy Efficiency and Durability on

They are claiming R 19.5 on an 8 inch CMU. I guess they are adding up inches of EPS and not deducting for the LARGE thermal bridging effects:

I’m not crazy am I? This is outlandishism in terms of r value performance?

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  1. GBA Editor
    Martin Holladay | | #1

    I agree with you that the manufacturer is playing fast and loose with the facts.

    If you read the laboratory report, you'll see that the testers measured that the blocks perform at R-13.5 when tested according to ASTM C-236. The manufacturer was evidently hoping for a different result, so the an unscientific note was added to "explain" these results: "Third-party guarantee programs commonly use R-25 as Omni Block's 8-inch wall system thermal performance value. This estimate includes thermal mass concepts, thermal lag, and wall tightness."

    It's safe to say that you can ignore the gibberish used to support the R-25 number. The lab measured the performance of the blocks and came up with R-13.5.

  2. hickhead00 | | #2

    I just came across this comment from 2014 after recently learning about omni block. I haven't yet used the product but i'm exploring it and have sent it to local architects we work with as well.

    Isn't it fair to say that "It's safe to say that you can ignore all the gibberish used to support the R-25 number" is only true if you recognize that R-Value is only part of the equation.

    Their marketing points on reduced thermal bridging, thermal mass benefits on the interior, etc... are real. It would be very very easy to build a stick wall with insulation that is a better insulating structure per the prescriptive method for IECC... right? Am i way off here?

    1. this_page_left_blank | | #4

      R-value is only part of the equation, but it is what it is. Claiming a different R-value based on other factors is flat out fraud.

  3. brendanalbano | | #3


    Things like thermal lag are certainly real, but seem like they are pretty context-specific, so converting them to an "equivalent" R-value seems pretty dubious.

    In a climate with hot sunny days, and cold clear nights, maybe a high mass wall facing south with a lot of thermal lag will nail that magic "absorb heat all day, release it all night" equilibrium. But maybe it won't. You're gonna have to do some modelling to figure it out!

    And in other climates it doesn't do you nearly as much good. Look at somewhere like Chicago where in the summer its hot at night and hotter during the day, while in the winter its cold during the day and colder at night. Thermal lag isn't going to help you out nearly as much in Chicago as it does in the desert.

    So yes, building a stick built structure with lots of insulation is far easier than trying to take advantage of more complicated thermal mass based approaches.

    1. hickhead00 | | #5

      I just did some back of the envelope for my are (pittsburgh) and there are really only 5 months out of the year where the thermal mass benefit would be helpful, so fair point there; however - shouldn't having a thermal mass to slow the pace of temperature change in your unit lower the variability in energy needs. At least it results in a more stable and comfortable environment, no?

      That said, the R13.5 for their product is for the entire wall assembly where as a R-20 (zone 5 IECC 2015) is the insulation for 75-80% of the non window/door wall assembly.

      Their argument (not mine yet as I"m still gathering facts) is that once you move onto more complex wall assemblies like exterior XPS that will provide a more continuous insulation and remove thermal breaks, you end up with a more expensive wall assembly then when you use their product.

      fwiw - we built homes in old historic neighborhoods and traditionally build at least part of the home with a brick facade... If i can replace my sheetrock, sticks, insulation, OSB Sheathing, XPS insulation, and brick facade with a masonry product that they claim is as costly to install as standard CMU... that feels worth it to me.

  4. Expert Member
    Dana Dorsett | | #6

    For a mass wall in zone 5 only R13 continuous insulation is required if half or more is on the exterior of the thermal mass, per TABLE N1102.1.2:

    Alternatively, a U-factor of U0.082 or less gets you there:

    The guarded hot-box of the assembly exhibited a "whole wall-R" of R13.57. (See page 5: ) That is a U-factor of (1/R13.57=) U 0.074.

    U0.074 is less than U0.082 so the wall section tested actually DOES meet code minimum for zone 5, but only by ~10% margin- nothing to write home about, and not exactly a high performance house.

    1. hickhead00 | | #7

      Yes Dana - it def does get you there, I've already confirmed that part. And also something else that is embedded in code that does give some credence to the idea of "equivalent r value" even if it's highly misleading... Mass Wall r-value/u-factor requirements are lower specifically because of thermal mass benefits which aren't accounted for in r-values.

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