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Re: Perm rate of an earthen floor?

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  • RT
    On Sun, 07 Feb 2010 14:54:17 -0500, natural building ... There are a number of assumptions in the above that are questionable. Foremost is the assumption that
    Message 1 of 1 , Feb 7, 2010
      On Sun, 07 Feb 2010 14:54:17 -0500, natural building
      <naturalbuilding@...> wrote:

      > In my attempt to show that a house can be built using predominantly
      > natural, low embodied energy, un-synthesized and non-toxic materials,
      > I am hoping to avoid using a poly vapour barrier in my earthen floor.
      > My current proposed design involves a drain rock layer as moisture
      > barrier which is connected laterally to a contiguous rubble trench
      > (this will also serve as a Radon barrier, since the gas can escape
      > naturally to air through the rubble trench). Above this will be 6
      > inches of Perlite giving me R-16. This will be topped with a 6 inch
      > earthen floor sealed with Linseed Oil.
      > My question is whether this will provide an adequate moisture barrier
      > to satisfy the building inspectors?
      > Given that the ground under the building should remain almost
      > completely dry due to the fact that it is protected by the rubble
      > trench, logic suggests that there will not be enough (or any) moisture
      > in the sub soil to present a problem inside the building.

      There are a number of assumptions in the above that are questionable.

      Foremost is the assumption that the sub-floor drainage layer in
      conjunction with the rubble trench (RTF) will function as a Radon barrier.
      Assuming that the RTF is properly constructed in that _drained_ drainage
      tile will be installed in the trench, what will actually happen is that
      the drainage tile outlet will function as an air inlet to pressurise the
      higher sub-floor drainage layer to drive Radon and other soil gases
      upwards and through any discontinuities in and around the floor, (of which
      there will be many due to the nature of the materials used (as described
      by OP)) into the living space.

      Second is the assumption that a 6-inch layer of Perlite will provide R-16
      thermal resistance.
      The thermal resistivity of Perlite, like most other materials is dependent
      upon the density at which it will be configured in the assembly. In order
      to achieve a thermal resistivity of 2.67, the density of the Perlite layer
      would have to be ~ 7.8 lbs/ft^3, or about the same density as a dry,
      building-quality straw bale.

      Given that the substrate for an earthen floor needs to be well-compacted
      and that density would increase with compaction and by corollary, a
      decrease in R-value (since thermal conductivity increases with an increase
      in density)... well, you know.

      As for the effectiveness of an earthen floor as a moisture barrier, one
      might look at a terra cotta flower pot.
      The terra cotta is essentially the same basic material as an earthen floor
      but the pot is more impervious to moisture due to it's having been fired.
      (ie Not very).

      But to answer the question re: permeance values

      The ASHRAE Handbook of Fundamentals contains tables with vapour permeance
      values for building materials.
      Air permeance values of building materials are a little harder to come by
      but I think that it would be safe to assume that for 6" of compressed
      earth, it would be negligible.

      === * ===
      Rob Tom
      Kanata, Ontario, Canada
      <A r c h i L o g i c at Y a h o o dot c a >
      (manually winnow the chaff from my edress if you hit "Reply")
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