How to read IECC R value table??

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    Michael Ermann

    While heat thermally bridges across wood studs because the insulative value of the glass fiber batts between the studs exceeds the insulative value of the wood studs themselves. . . the thermal bridging in metal construction far exceeds the modest thermal bridging in wood construction. Metal is just much more conductive than wood—so much so, that while structural bridging across wood studs may decrease the overall insulation effectiveness of the wall by 10% or 20%, the reduction across a metal stud wall may be 50% or 60%!

    When insulation runs outboard of the enclosure instead of (or in addition to) the insulation between the studs, the opportunity for thermal bridging evaporates because the insulation runs continuously and needn’t be interrupted by the structure.

    Typically, we use Styrofoam (XPS, EPS, Polyisocyanurate, or sprayfoam) when running insulation outboard of the enclosure, though we may sometimes use water-resistant forms of glass fiber or mineral wool.

    Rigid XPS continuous insulation outboard of the enclosure. . .

     

     

    Spray foam continuous insulation outboard of the enclosure. . .

     

     

    Mineral wool continuous insulation outboard of the enclosure. . .

     

     

     

     

    Because of the extreme thermal bridging in steel stud structures, we’d especially want to run continuous outboard insulation in steel buildings. The table  then translates the R-value of a wood-framed structure with batts in the cavities and no outboard continuous insulation to a thermally equivalent assembly for steel studs with at least some continuous outboard insulation. In this case to replace R-13 wood-frame cavity insulation we need . . . R-13 in the cavity and R-4.2 continuous insulation outboard of the enclosure; or R-21 in the cavity and R-2.8 continuous insulation outboard of the enclosure; or R-13 in the cavity and R-4.2 continuous insulation outboard of the enclosure; or R-0 in the cavity (no cavity insulation) and R-9.3 continuous insulation outboard of the enclosure (note then that it takes R-9.3 continuous insulation to replace R-13 insulation interrupted by wood studs); or R-15 in the cavity and R-3.8 continuous insulation outboard of the enclosure; or R-21 in the cavity and R-3.1 continuous insulation outboard of the enclosure (this last one feels redundant to me given that we were already told that R-21 in the cavity + R 2.8 is an acceptable substitute).

    As always, stop to think what these numbers really tell you. . . in this case, that R-13 cavity insulation in wood-framed construction is equivalent to the same cavity insulation in metal-framed construction only if we add an additional 1.25 inches of continuous insulation outboard of the enclosure.

    To watch a fun video I made on this problem,  dealing with enclosure, vapor barriers, airtightness, insulation, and thermal bridging, click <<here>>

     

     

    * There are other great (moisture-related) reasons to run continuous insulation outboard of the enclosure (see our Construction>Enclosure videos).

    *I know there’s a lot of thermal bridging content on these exams. That’s how it will be for you: random events still develop patterns. You may have a lot of acoustics, or earthquake, or some other technical test items. When they fail an exam, folks often make the mistake of “fighting the last war,” studying for that one over-tested technical area in their last attempt, when that is unlikely to be the over-tested subject in their next attempt.

    *Know that in cold climates, designing insufficient outboard R-value relative to the R-value between the studs can cause moisture to condense on the inside of non-breathing rain/vapor control layers because those rain/vapor control layers become cold. Think about that last statement for a minute until you understand it. . .if you do, that’s fantastic because this is a difficult concept.

    *You’ll need to be able to read all kinds of tables for these technical exams. No need to memorize them. . . just calmly interpret the relationship that the table illuminates.

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    Michael Ermann

    Whoops. . . I accidentally posted the same photo twice. . . this is the example I meant to share of mineral wool outboard continuous insulation. . . 

    Also, know that the cost content you need for these exams are not generally found in AHPP as someone else suggested on this thread, but rather, are scattered in dozens of places in the Fundamentals of Buiding Construction book. . .so there's no one place to find all (or even a lot) of the cost info you need. (That's why I summarized it for you, above). The good news is that you don't really need to know how to cost estimate for the ARE. Instead, cost-related questions come in three flavors: (1) insultingly-easy read-the-table straight-forward arithmetic-based questions where the exam provides you with a chart and asks you where the savings can be found (see an example in NCARB's official demonstration exam), (2) comparison questions asking for the least expensive alternative that's still appropriate for the given design constraints (that's where my list above shines), and (3) knowing the names of cost estimating procedural frameworks of escalating specificity. . . which method is used at which stage of design? As design progresses, you know more about the building and the estimate refines into something more specific to this building and less generally applicable to a typical dog kennel or ice skating rink or barn (that's where Thursday's Zoom call comes in).

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    Ya-Chu Hsu

    Hi Michael,

    The video is awesome. The content helps me a lot, especially the photos. Appreciate!!

     

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    Ya-Chu Hsu

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