[Strawbale] Code Required Vapour Barrier

[Matt Chase]

Hi There

To help you out in your case for not using a vapour barrier, I would 
recommend reading John Straube's reports on moisture related 
properties of plasters.  The most recent one can be found at 
http://www.ecobuildnetwork.org/ or 
http://www.ecobuildnetwork.org/pdfs/Straube_Moisture_Tests.pdf.  
This report references a previous one, which is available as a 
research summary from Canadian Housing and Mortgage Corporation.  
The full version is available by request (talk to Don Fugler) if you 
are interested in the details.  I have a copy too, let me know if 
you're interested.

Dr Straube has tested the permeability and water uptake (absorption) 
of various types of plasters with various additives that are 
typically used by SB builders and reports on how these factors will 
influence moisture control in a wall system.  

In discussing the issue with your building inspector, you can also 
mention that both earthen and lime plasters are hygroscopic (meaning 
that they are good absorbers of moisture) so, although they may have 
a high permeability because of the way permeability is measured (and 
would thus not qualify as a vapour barrier), they will perform well 
in terms of overall moisture management.  Permeability is measured 
once a sample has reached a steady state of moisture flow, but until 
that point, a hygroscopic material will store moisture, thus 
retarding the transmission of moisture.  In a continuosly changing 
moisture environment, hygroscopic materials have a large capacity to 
deal with moisture: they can store and release it.  Only in 
situations where adverse conditions persist (several humid/rainy 
days in a row) would the ultimate moisture storage capacity of the 
material be reached, and then moisture would have little resistance 
to flow.  However, since both lime and earthen plasters are so 
permeable, they will dry relatively quickly as well.  All of this 
informaiton is explained quite clearly in Dr. Straube's report.

Good luck,


Matthew Chase
MSc. Candidate
Biosystems Engineering,
University of Manitoba
Winnipeg
(905) 428-8378
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