[Strawbale] Air tightness and earth plastering

[RT]

On Thu, 21 Apr 2011 16:58:50 -0400, Andrew Morrison <Andrew at strawbale...>  
wrote:

>  On Thu, Apr 21, 2011 at 1:44 PM, Sebastien Hubert 
> sebastien.hubert at mc2000...> wrote:


>>
>> Thus to summarize, in our building the earth plastering is done but we
>> still need to do the last finish layer...  At the moment we have more or
>> less 5cm earth on the straw bale walls.
>>
>> Regarding the first house where they made the blower door test, on the
>> basement walls they only put 1.5cm earth plastering and the finish  
>> layer is
>> not done yet...
[BIG <snip> of extraneous, previously posted material]

Here in North America where buildings are more often than not made  
"cheaply" as compared to the traditional "Old World" methods that are  
still used throughout much of the world, there is an approach in  
conventional building systems called the "Air-tight Drywall Approach"  
(ADA) which as the name implies, utilises the gypsum board interior  
sheathing ("drywall") as the major component of the air barrier strategy  
(along with acoustic sealants and gaskets at the joints between panels and  
at penetrations for services) rather than the more typical approach of  
utilising a separate membrane (usually a polyethylene sheet) which is  
typically called up as a "Continuous Air/Vapour Barrier" installed over  
the framing and cavity insulation before the interior gypsum board  
sheathing is installed.

A couple of these ADA details may be seen at:

http://www.cmhc-schl.gc.ca/en/inpr/bude/himu/codemo/codemo_100.cfm

http://www.cmhc-schl.gc.ca/en/inpr/bude/himu/codemo/codemo_098.cfm


... the point being that it is not the thickness of a material that  
determines its effectiveness (drywall typically being a mere 12 mm thick)  
in an air barriers strategy but rather, it is how the various materials  
used in the air barrier strategy, are configured.

Wet-applied plaster is very effective as an air-barrier material simply  
because unlike sheet materials such a gypsum sheathing or wood panels,  
there are no joints between the individual panels that need to be sealed.

Control joint flashings (which should be used to break up the plaster into  
panels to eliminate the cracking that would occur as a result of  
temperature and shrinkage stresses or structural stresses like racking)  
are typically designed to ensure continuity so the weak points will  
typically be at the junctions between different planes (ie walls and  
ceilings, wall and floors) or different materials (ie wood and plaster) or  
at penetrations for services (ie electrical boxes, plumbing pipes, exhaust  
vents, chimneys).

These are best dealt with by the use of a flange underneath the plaster  
which overlaps the joint by 50mm or more on either side of the joint ,  
preferably using an integral geometry to lock the materials together  
rather than having to rely upon acoustic sealants (they off-gas/stink and  
are messy) and/or compressible gaskets which will remain flexible  
throughout the lifetime of the building.


-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
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