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Re: OSB or Not?

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  • RT
    On Wed, 01 Dec 2010 12:10:00 -0500, Michel Van Mulders wrote: [ for the sake of brevity. Full text of message and thread
    Message 1 of 3 , Dec 1, 2010
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      On Wed, 01 Dec 2010 12:10:00 -0500, Michel Van Mulders
      <michelvanmulders@...> wrote:

      [<snipped> for the sake of brevity. Full text of message and thread can be
      seen at
      http://amper.ped.muni.cz/pipermail/strawbale/2010-December/002186.html
      ]

      > My feeling is that using OSB or similar large sheets with strawbale
      > building can be avoided.
      > Wouldn't the properties of strawbale walls be better, if just plastered
      > from both sides?


      My short answer would be "Not" (to the question re: OSB) and "Yes" (to the
      "just plastered both sides".

      Even with houses whose walls are not made of SB, I would not use OSB
      anywhere on the interior of the house inside the plane of the air barrier
      simply because OSB stinks. (Smell = off-gassing of likely unhealthy VOCs)

      Further, OSB is a poor substrate for plaster.

      Aside from the issues of off-gassing and unsuitability as a substrate for
      plaster, if the bales are not encapsulated by wet-applied plaster, the
      lack of direct contact between the straw and the OSB-as-air-barrier
      encourages convection to occur in the bales which results in a significant
      reduction in the effective thermal resistance of the SB wall assembly.


      > What would be your thoughts on this;
      >
      > The framing of the house on the inside is ideal for avoiding thermal
      > bridges.
      > On this framing (outside) I could nail thin strips of wood (30mmx15mm)
      > with spacing of about 50 mm or more, and stag the bales against the
      > strips.


      First of all to clarify some terminology.

      I understand "timber frame" (TF) to describe a framing system that uses
      large dimension timbers (ie smallest cross-sectional dimension typically
      greater than 150 mm) which uses mortise & tenon and dove-tail joinery to
      connect the framing members, joints locked with wooden pegs or wedges.

      Since this type of joinery requires skilled persons to execute properly
      and to execute properly requires careful attention to detail, all of which
      requires more time, the labour costs will be high.

      IMO, it would be a shame to bury this type of a frame inside of a SB wall
      and would be best left exposed to the interior so that it may be fully
      appreciated.


      I understand a "Post & Beam" (P&B) frame to be one in which most or all of
      the joints are simple butt or lap joints, usually relying upon metal
      mechanical fasteners for joint integrity. The framing members may be solid
      timbers or built-up from small dimension lumber. This sort of a framing
      system requires less skill to assemble and goes together much quicker than
      a TF and as such, the cost will be lower than that of a TF system.

      I understand a "Light Framing" (LF) system to be one in which all of the
      components are comprised of milled, small-dimension softwood lumber whose
      breadth is typically only about 38 mm, individual members usually closely
      spaced with centre-to-centre spacing typically being 400 to 600 mm ,
      usually with a thin (10 to 12 mm thick) sheet diaphragm (typically 1200 x
      4800 mm) applied to the faces of the assemblage to facilitate load-sharing
      and provide some in-plane shear resistance.



      I see little point in using a P&B frame for the exterior walls of a SB
      house from either a viewpoint of aesthetics or one of structural
      integrity. Both of these would (IMO) be best embedded in the SB walls to
      take advantage of the bales' potential to provide in-plane racking
      resistance and also to eliminate the problem of trying to maintain the
      integrity of the interior air barrier.

      Actually, I'm not particularly fond of the idea of using an
      exposed-to-the-interior TF for the exterior walls of a SB-walled house
      either because of these last two points (ie no contribution to racking
      resistance from bales, problems maintaining continuity of air barrier
      across framing members).

      If large dimension timbers are desiredfor their aesthetic value, I think
      that a hybrid of the two systems would be best ... embedded LF in the SB
      exterior walls, exposed-to-the-interior TF or P&B
      for interior columns and floor and ceiling framing.

      Ideally the LF members for the exterior wall framing would be spaced
      framing members (essentially a parallel chord truss or "Larsen" type
      truss) with each chord of the truss at each extremity of the SB wall.
      This truss configuration of relatively small dimension chord members
      (typically 38 x 64mm or 38 x 89mm ) not only makes for very strong
      (out-of-plane lateral resistance) framing members while using a minimum of
      wood, it also provides useful points of attachment at both interior and
      exterior surfaces of the wall.

      And since the LF members are embedded in the bales, the bales provide the
      otherwise-flimsy (in the plane of the wall) LF members with lateral
      support/buckling resistance so that the centre-to-centre spacing can be
      increased from the "normal" 600 mm C-C spacing that one would typically
      use in a non-SB LF wall.

      And since most exterior walls will include windows, doors and corners, the
      jambs and corner posts would be the logical locations for the LF hybrid
      "posts".

      (And I think that the lath strips in Michel's proposal would be
      susceptible to creating the same potential convection problems that would
      occur with OSB sheathed bales.)

      --
      === * ===
      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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    • David Neeley
      Rob gave a nicely detailed answer about various framing systems for exterior walls of strawbale constuction. Personally, though, if your local building code
      Message 2 of 3 , Dec 1, 2010
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        Rob gave a nicely detailed answer about various framing systems for
        exterior walls of strawbale constuction.

        Personally, though, if your local building code nazis permit, I would
        vastly prefer a load-bearing bale structure with no framing in the
        wall or next to it. Rendered bales are quite strong enough for single
        or two-story construction if designed with reasonably short runs.

        Of course, I quite agree that OSB is an unnecessary option, as the
        lathing would be.

        David

        On Wed, Dec 1, 2010 at 23:04, RT <archilogic@...> wrote:
        > On Wed, 01 Dec 2010 12:10:00 -0500, Michel Van Mulders
        > <michelvanmulders@...> wrote:
        >
        > [<snipped> for the sake of brevity. Full text of message and thread can be
        > seen at
        >    http://amper.ped.muni.cz/pipermail/strawbale/2010-December/002186.html
        > ]
        >
        >> My feeling is that using OSB or similar large sheets with strawbale
        >> building can be avoided.
        >> Wouldn't the properties of strawbale walls be better, if just plastered
        >> from both sides?
      • RT
        On Mon, 06 Dec 2010 01:41:35 -0500, Michel Van Mulders ... Just to clarify a few points: One needs to distinguish between air permeance and vapour
        Message 3 of 3 , Dec 6, 2010
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          On Mon, 06 Dec 2010 01:41:35 -0500, Michel Van Mulders
          <michelvanmulders@...> wrote:

          >
          > The question comes back to the basics; too much protection against
          > moisture movement sounds dangerous.
          >
          > The fact that the plaster started coming off with this man in Austria
          > could as well be of too dry....


          Just to clarify a few points:

          One needs to distinguish between "air permeance" and "vapour permeance"
          when talking about materials' ability to "breathe".

          With any well insulated building, the construction detailing needs to be
          such that air leakage through the building envelope materials is minimised.

          In heating situations, bulk moisture transport piggy-backed on air leakage
          from conditioned interiors will lead to large amounts of water vapour
          being deposited in the envelope materials where it will condense into
          liquid water at the dew point within the cross section.

          Once that moisture is in liquid form, heat loss through the well insulated
          building component will seldom be enough to turn that liquid back into
          vapour to be driven out through plaster finishes no matter how "vapour
          permeable" they may be. You may have witnessed this problem on the inside
          of Gore-tex garments in sub-zero temperatures.

          The air permeance of most plasters at the thicknesses at which they are
          typically applied to walls and ceilings is low enough that it may be
          considered to be "negligible". (ie less than 0.001 litres per second per
          square metre of surface area at 75 Pascals pressure difference).

          The minimum air change rate that is recommended by the American Society of
          Refrigeration & Air-conditioning Engineers (ASHRAE) is 0.35 complete
          changes of the total air volume of the building every hour (ie 0.35 ACH) .
          Baubiologists prefer a higher 1 ACH.

          These are the minimums below which are considered unhealthy.

          I'll leave it to you to do the arithmetic but it should be obvious from
          the above that if one relies upon the air permeance of a wall or ceiling
          assembly to allow sufficient air movement to provide the required air
          changes, one would be seriously mistaken.


          The only other means that natural air movement through the envelope
          materials would occur is though air leaks at discontinuities in the air
          barrier. Since those locations would also be the same place that moisture
          is being leaked into the envelope materials, there is almost a certainty
          that there will be moulds etc. at those spots so that unhealthy and
          potentially deadly microbes will be transferred to the ventilation
          airstream (such as it is).

          That is to say, the air exchange rate would be too low to be considered
          healthy (ie polluted) and what little air exchange that does occur would
          be contaminated by mould spores, crud and if combustion appliances are
          used for heating, quite likely combustion by-products (ie combustion
          spillage).
          *
          Another point that bears mentioning re: OSB or Not ?

          About 15 or more years ago, blower door testing was carried out on a
          number of SB houses built in maritime provinces Atlantic)here in Canada.
          They were all wet-applied plaster on SB.

          The tested air leakage rate was low enough to be considered "air tight"
          for the purposes of the R-2000 performance standard. Houses built to the
          R-2000 standard were/are some the most energy-efficient, heathiest homes
          on the planet.

          In the buildings which did not meet the required air-tightness standard,
          the weak points in the envelope were the "usual" suspects ... penetrations
          in the ceiling air barrier for electrical boxes. Once they were sealed and
          the building was blower door tested again, they passed.

          --
          === * ===
          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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