[Strawbale] How to deal with code requiring vapor barrier

[Wayneapple at aol.com]

Hi All,
 
     And thanks to everyone who responded to my question concerning dealing 
with a code official that sighted a part of the IBC that required a vapor 
retarder in exterior walls.
    I figured I owed you an update.
This was a school project which meant it had to go thru the State 
Plan Review Dept. I had had it approved, but due to cost reductions it went 
back thru the review process a second time with a different reviewer, I think 
they gave it to the new guy as a test. He quoted 1403.3 of the Code that 
required the vapor barrier.
    I responded with the following:
 
1)      1403.3 Vapor retarder in Exterior walls:  
a.      This requires a vapor retarder (1 perm or less) be installed on the 
inside face of the wall in order to reduce the possibility of condensation in 
the walls causing degradations of wall materials and is based on standard wall 
systems. It does not vary depending upon the quantity of moisture generated in 
the building, type of wall construction, or climate. There has been much 
controversy in the building industry about how tight to make wall construction. 
Research over the last ten years has shown that vapor retarders and barriers can 
cause moisture problems and experts (including Joseph Lstiburek: Moisture 
Control Handbook, Buildingscience.com) now assume that moisture at some point in 
time will get into a wall, and it is important that there is a way for it to 
get back out. Lstiburek now has a more in depth set of rules that dictates the 
vapor retarder depending upon climate, outside vapor barrier, and first 
condensing surface(see attached). Lstiburek states that for the Flint climate a 
Class III vapor retarder (ie 1-10 perm) is adequate for walls with a greater than 
1 perm exterior sheathing (1 ½ “ of lime plaster is about 7.7).  
b.      Straw bale wall systems are quite different than standard walls. The 
large “storage capacity” of both the plasters and the straw bales allow it to 
accept moisture from vapor diffusion without reaching levels that support 
mold/bacteria growth or degradation of the straw. It becomes important that the 
walls are allowed to breath to allow moisture to get back out.   
c.      Most moisture problems in walls are a result of either water getting 
into the wall from outside (a leak) or indoor air leaking through the wall and 
depositing moisture at the dew point in the wall. The building has been 
designed to eliminate all openings in the wall. All electrical boxes are surface 
mounted to the exposed columns or are located on interior walls. There is no 
plumbing in the straw bale walls as well. Thus the major contributor to moisture 
in the wall has been eliminated. A continous 3’ overhang around the entire 
straw bale wall will insure that the wall is protected from water from the 
outside. 
d.      Lime plaster has been specified for the restrooms where a greater 
amount of moisture may be encountered (an automatic exhaust system will control 
moisture). 
e.       The straw bale itself has been rated at 2-4 perms. The interior 
finish of 1 ½” earth plaster with mineral paint finish is rated at about 10.5. The 
exterior finish coats of 1” of earth plaster and ½” of lime plaster have 
ratings of 16 and 23.2 respectively. 
f.        The following is an except from “Straw-bale Construction by Bruce 
King who has written two books on straw-bale construction and has engineered 
some of the largest straw-bale structures in the world 
(_www.ecobuildnetwork.org):”The_ (http://www.ecobuildnetwork.org):”The/)  overwhelming experience with 
straw-bale buildings is that moisture vapor intrusion ins not a problem if 
the wall can “breathe”; that is, if both surfaces are vapor permeable. There 
have certainly been leaks and degradation failures, but without exception they 
have been due to outright moisture intrusion, not vapor intrusion……..This 
leads to the most important, unusual(and seemingly counterintuitive) feature of 
accepted straw-bale construction: No moisture or vapor barriers should be used 
except possibly for the first few courses on the outside, windowsills and top 
of walls to provide protection during construction and backup against a roof 
leak. Building permit reviews have commonly generated the requirement to cover 
the bales with a barrier such as Tyvek or Grade D paper (as is done over 
studwall sheathing, though the evidence is emerging that even here moisture barriers 
can trap moisture where it can cause harm), and the argument has been waged 
many times in many jurisdictions over this subject. However, experience with 
straw-bale walls overwhelmingly shows that no barrier should separate the 
plaster and straw because the straw needs to breathe(transmit water vapor), mosture 
must not be trapped against the straw/plaster intrerface, and the structural 
system depends upon a thorough bonding of plaster into straw.”     
g.      The Architect has read extensively on the subject of permeability of 
finishes of straw-bale structures over the last ten years and has determined 
that wall composite of the lime and clay(earth) plaster finishes and the straw 
interior is the best installation for this climate. Although it does not 
necessarily meet the strict vapor retarder specified in the Code the composite 
meets the desired effect of the Code and that the installation of the full fledged 
, 1 perm vapor retarder may actually be detrimental to the wall composite by 
inhibiting drying of the wall. Under Section 104.11”Alternative materials, 
design and methods of construction and equipment” the code official has the 
capacity to permit other assemblies and I hereby request this assembly be allowed 
as designed. 
END OF QUOTE 
I then followed it up with a long phone conversation that led him to believe 
that I knew more than he did and the project was approved without a VT. 
Thanks all and I hope this might be of some help in the future. 
Wayne



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