Loading ...
Sorry, an error occurred while loading the content.
 

Re: [practical-sbc] Re: Earthen Plaster in Cold"ish" Climate

Expand Messages
  • Paul Park
    Thanks for the information. The engineer I am working with recommends something similar to what you have found. He suggests that I add a silicone based
    Message 1 of 7 , Feb 1, 2007
      Thanks for the information. The engineer I am working with recommends something similar to what you have found. He suggests that I add a silicone based sealer to the mix for the interior plaster to reduce the permeability. He also recommends an HRV unit governed by a humidistat and ongoing monitoring of bale moisture. He is not overly concerned about the exterior plaster, as we have designed a wrap-around porch.

      After I posted the question, someone mentioned Bruce Kings's "The Design of Straw Bale Buildings." I have since picked up a copy and I highly recommend it. He says: "The permeance of the exterior skin should be kept as high as practical without sacrificing structural or other performance requirements. For a typical residence the vapor permeance of the interior plaster skin should be kept as high as possible to encourage inward drying, but low enough to avoid a significant amount of cold weather diffusion wetting... In climates with significant amounts of cold weather (four months or more), the interior skin permeance (including paint layers) should have a permeance of less than 5 or 6 perms (300-350 ng/Pa s m2). Controlling the interior humidity during cold weather by good ventilation can allow the interior skin permeance to climb to ten perms in this type of climate." He defines a month of cold weather as one with an average daily temperature of less than 5 degrees celcius. For Creston there are five month with the average daily mean temperature below 5 celcius and four months with the average daily high below 5 celcius. King lists the vapor permeance of earthen plaster at 11 US perms.

      The CMHC study on moisture and plaster in straw bale building, http://www.cmhc-schl.gc.ca/odpub/pdf/62631.pdf states that calcium stearate reduced the permeability of the plasters tested (they did not produce a working earthen plaster).


      Paul Park


      sm_bamber wrote:

      "The conclusions from this analysis were that strawbale walls, if
      maintained intact and avoiding direct rain penetration, can avoid mould
      growth under the challenging conditions of a coastal climate. In order
      to achieve this however, certain conditions need to be met:

      * Internal relative humidities need to be managed such as to average
      50% or less. In a modern dwelling, this can be adequately achieved by
      means of extract fans or, with proper design, passive vents making use
      of stack and wind effects. In the case of a dwelling, there is no
      reason to believe that it is necessary to exceed the requirements of
      most typical building regulations or a similar standard. The use of
      high ventilation rates does imply some increase in winter energy costs
      and in cold climates controlled mechanical ventilation with heat
      recovery may be the most appropriate solution.

      * Bale mould risk is minimised by using plasters with high vapour
      diffusity, such as mud and lime plasters. Conventional cement or cement-
      lime plasters, while they may not result in mould growth, will tend to
      add to the risk because of their restriction of moisture flow."





      [Non-text portions of this message have been removed]
    • sm_bamber
      Hi Paul - that s great information - thanks so much for that. Sharon ... recommends something similar to what you have found. He suggests that I add a
      Message 2 of 7 , Feb 1, 2007
        Hi Paul - that's great information - thanks so much for that.
        Sharon

        --- In practical-sbc@yahoogroups.com, "Paul Park" <parkplace@...>
        wrote:
        >
        > Thanks for the information. The engineer I am working with
        recommends something similar to what you have found. He suggests
        that I add a silicone based sealer to the mix for the interior
        plaster to reduce the permeability. He also recommends an HRV unit
        governed by a humidistat and ongoing monitoring of bale moisture.
        He is not overly concerned about the exterior plaster, as we have
        designed a wrap-around porch.
        >
        > After I posted the question, someone mentioned Bruce Kings's "The
        Design of Straw Bale Buildings." I have since picked up a copy and I
        highly recommend it. He says: "The permeance of the exterior skin
        should be kept as high as practical without sacrificing structural or
        other performance requirements. For a typical residence the vapor
        permeance of the interior plaster skin should be kept as high as
        possible to encourage inward drying, but low enough to avoid a
        significant amount of cold weather diffusion wetting... In climates
        with significant amounts of cold weather (four months or more), the
        interior skin permeance (including paint layers) should have a
        permeance of less than 5 or 6 perms (300-350 ng/Pa s m2).
        Controlling the interior humidity during cold weather by good
        ventilation can allow the interior skin permeance to climb to ten
        perms in this type of climate." He defines a month of cold weather
        as one with an average daily temperature of less than 5 degrees
        celcius. For Creston there are five month with the average daily
        mean temperature below 5 celcius and four months with the average
        daily high below 5 celcius. King lists the vapor permeance of
        earthen plaster at 11 US perms.
        >
        > The CMHC study on moisture and plaster in straw bale building,
        http://www.cmhc-schl.gc.ca/odpub/pdf/62631.pdf states that calcium
        stearate reduced the permeability of the plasters tested (they did
        not produce a working earthen plaster).
        >
        >
        > Paul Park
        >
        >
        > sm_bamber wrote:
        >
        > "The conclusions from this analysis were that strawbale walls, if
        > maintained intact and avoiding direct rain penetration, can avoid
        mould
        > growth under the challenging conditions of a coastal climate. In
        order
        > to achieve this however, certain conditions need to be met:
        >
        > * Internal relative humidities need to be managed such as to
        average
        > 50% or less. In a modern dwelling, this can be adequately
        achieved by
        > means of extract fans or, with proper design, passive vents
        making use
        > of stack and wind effects. In the case of a dwelling, there is no
        > reason to believe that it is necessary to exceed the requirements
        of
        > most typical building regulations or a similar standard. The use
        of
        > high ventilation rates does imply some increase in winter energy
        costs
        > and in cold climates controlled mechanical ventilation with heat
        > recovery may be the most appropriate solution.
        >
        > * Bale mould risk is minimised by using plasters with high vapour
        > diffusity, such as mud and lime plasters. Conventional cement or
        cement-
        > lime plasters, while they may not result in mould growth, will
        tend to
        > add to the risk because of their restriction of moisture flow."
        >
        >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
      Your message has been successfully submitted and would be delivered to recipients shortly.