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GBlist: Re: Strawbale Moisture Performance (origin GBList)

David Ewing wrote:
> DE: Some of us with Habitat's Dept. of Environment have been following with   interest the developments in straw bale construction. MOIST, Madison WI, etc.
    (forward to Howard Associates from Green Building List May 14, 1997)
               ------------------------------------------

I work with EEBA and am looking forward to establishing a stronger
relationship with HFH.  Perhaps HFH would like to post an inquiry to
the EEBA energy efficient building list, related to building science
questions concerning construction practices like strawbale.  To do so
send a message to: [ ee-building@crest.org ] (list has 200 participants)

I am also a user of MOIST, and now that you have said Gatley's study
indicates potential problems in a cold/moist climate it would be of
considerable interest to see if DOE would provide funding for further
study.  It would be useful to have the results replicated by other
researchers such as Florida Solar Energy Center, with whom I believe
HFH has a standing tech support agreement (no?).  MOIST appears to be
a good general tool for analysis of mass transfer in standard
construction
however, I would encourage HFH to contact the developer, Doug Burch, at
NIST (301) 957-6433 to see if he is comfortable with its use on the
strawbale walls.

.........................................................................
 Initial Professional Opinions -- By:  Bion Howard, Member ASHRAE, MEAS
  5-15-97 << Developmental and preliminary based on limited review >>
.........................................................................
Key to performance of strawbale walls are the following factors
I have gathered from literature and discussions with other building
scientists:

  1.) exclusion of liquid water penetration via openings (window,
      door R/O's and utility penetrations) needs excellent detailing
      of caulk and seal jobs (this also prevents moisture advection
      to the strawbale mass via excess convection). I have not seen 
      blower door tests being done on strawbale homes to prevent
      air-leakage borne moisture vapor convection via pressure
      differentials.  This should be encouraged ! 

      Discussion -- If a strawbale walled home in a moist climate
      had many through-wall leakage sites and then is operated under
      negative pressures, with dual inside & outside low-Perm coatings,
      moisture could build up in the bales to a critical level.

  2.) avoid use of exterior covering in combination with an interior
      covering having identical perm ratings -- this could lead to
      "damming" of moisture entering the wall via defects, and the
      inability of the wall to dry to a lower absolute humidity.
      A good general principal is to design "forgiving" assemblies
      that can dry to somewhere / as a function of climate and
      auxiliary conditioning design (DOE _Moisture Handbook_, c1993)

  3.) eve extensions (overhangs) should be encouraged to reduce the
      amount of wind/driven precipitation to the surface of the
      cladding (freq. stucco) since these claddings may have a
      tendency to conduct moisture inward if continually damp. The
      strawbale mass could provide a considerable gradient to moisture
      flow via physical principals, since constructors take pains
      to ensure the bales are very dry when covered.

  4.) a bead-breaker should be installed between the exterior stucco
      layer and the structural bale wall, to allow driven moisture to 
      intercept a capillary layer first.  (This is similar to a
      solution for EIFS problems proposed by Lstibruek and Petit)
      Such a layer could be an asphaltic building paper such as a
      typical 15# roofing "felt" or e.q. behind lath and stucco.

  5.) Annual inspections of strawbale walls for moisture content 
      may be useful in regions of the country where moisture is a 
      typical concern of _any_ type structural system.  The critical
      moisture level for decay in strawbale’s needs to be established
      since presence of micro-organisms, the food-source, and decay 
      operative temperature criteria are inevitably met over time.

      This critical moisture content level is likely _not_ to be the
      same as for wood-frame construction, since the strawbale wall
      may be much less able to ventilate via convection as would a
      conventionally insulated frame wall (mineral fiber batts).

I will be very interested in the further discussion of the building
science opportunities and challenges facing broad roll-out of strawbale
structural systems in the US climates.  Regards,  Bion Howard.

--------------- Contact Information------------------
        Mr. Bion D. Howard, Principal          
  Building Environmental Science & Technology   
  P. O. Box 1007, Upper Marlboro, Maryland 20773 USA
  bdhoward@ix.netcom.com <--> http://nrg-builder.com
     Voice#  410-867-8000  fax#  301-889-0889
-------------------------------------------------------

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