Indoor and Outdoor air film

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    David Kaplan

    Herman,
    Best I can tell you is that you will ALWAYS be given the full components of the wall assembly if asked to calculate the R value of a wall assembly. Meaning, if the question does not include a vapor barrier or air and weather barrier in the components, you should not either. Don't assume anything. It will mess you up on a question like this.

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    Michael Ermann

    Herman,

    If I ask you how long it takes drive from Richmond to Boston, you have three options:

    1. You can respond with the approximate time it takes from interstate exit to interstate exit: 8 hours
    2. You can further add the extra time that you will spend in traffic working your way through cities like Baltimore, Washington, Philadelphia and New York: for a refined total time of 8.7 hours
    3. You can still further add the time it takes you to get from your door in Richmond to the interstate exit, and from the Boston interstate exit to your cousin’s door in Boston: for an even more refined time of 8.96 Hours

    Likewise, when someone asks you for the R-value of an assembly, there are three correct answers, each one a bit higher in number, and each one a bit more specific . . . but each one a bit more tedious to compute and with marginal additional usefulness when making design decisions. So I could ask you about the R-value of an assembly and you could correctly respond with three different values.

    1. By only including the insulation. At first approximation, an assembly’s thermal resistance is mostly included in the insulation: R-20
    2. By adding in the value of the structure, sheathing, interior finish material, and exterior cladding: R-21.5
    3. By further adding in the air films that form at the microscopic level in the craggy surfaces of the interior finish (+0.68) and exterior cladding (+0.17): R-22.35

    So on the exam, as David wrote, you’ll think about the answer based on the specificity of the information provided. If they are asking for a “door-to-door time of travel,” the exam will provide you with the air film numbers (no need to memorize something like air film R-values). If they want an “exit-to-exit time of travel” they’ll simply provide the values associated with the materials in the assembly.

    To your second question, the interior air films don’t vary that much, so we use 0.68 as a good average for an interior wall surface and R-0.61 for a ceiling surface. In practice, the values of the outside air films are always changing, primarily with the speed of the wind. In higher winds, the air molecules that hang out at the surface of the cladding are blown away quickly, so they provide a less-effective thermal insulator. We typically use the convention R-0.17 as an average for the wintertime air film, when it is often windier, and R-0.25 for the summer, when the air is calmer. I suppose that at a seaside site, where the wind blows harder, we could adjust the exterior air film R-value down accordingly, but that level of specificity isn’t appropriate in thermal calculations because when calculating conductive heat loss rates we are still making assumptions about what the exterior temperature is, how much thermal bridging there is, etc. I would argue that in the presence of an assembly with insulation, accounting for the air film at all is overly-specific: kind of like measuring the distance from Richmond to Boston in inches.—Michael Ermann, Amber Book creator.

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    Herman Nadal

    Got it. Thank you both for the responses.

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