[Strawbale] Air tightness and earth plastering

Derek Roff derek at unm...
Fri Apr 22 02:13:02 CEST 2011

I haven't seen any tests of the air-tightness of very thin or 
unfinished earthen plaster/render*.  Nor am I worried about it. 
Testing any component of a building in an improper or unfinished 
state is likely to show poor results.  On the other hand, I have seen 
enough tests of plastered strawbale houses to have complete 
confidence in the air barrier capabilities of earthen and other 

Lots of strawbale buildings do poorly in blower door tests, and the 
problem is not air leaking through the plaster, but through cracks 
between the plaster and other elements of the building.  Tiny cracks 
at the top and bottom of the walls, at doors and windows, and in the 
other places Andrew lists below all add up to lots of air leakage, 
relative to the standards that we are aiming for.

Most plaster of whatever material forms cracks as the layer cures.  A 
single layer is likely to have a lot of small cracks, cracks which 
are perhaps hard to see, but which will allow more air movement than 
the much larger area of plaster between the cracks.  In other words, 
it is not the plaster that is leaking air, it is the cracks in the 
plaster.  One reason for plastering in three layers is to avoid 
having cracks that extend through the full thickness of the 


*In my reading and experience, the use of terms like "plaster", 
"render", and even "stucco" vary widely between regions and 
individuals.  I haven't seen the consistency that Paul indicated.

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm...

--On Wednesday, April 20, 2011 2:25 PM -0700 Andrew Morrison 
<Andrew at StrawBale...> wrote:

Hi Sebastien. One thing for certain is that without a thick layer of 
render, preferably three coats, the blower door test is not likely to 
be successful. There is too much ability for air to move through the 
wall until the tender is complete and the tightness details at the 
floor/wall and wall/ceiling intersections has been installed. In 
addition, you'll want to make sure that your wall penetrations 
(plugs, plumbing lines if any, and other such areas) are well sealed. 
That's all for now as I'm writing on my phone's tiny screen.




Sent from my iPhone. Sorry about any typos or "auto words" that might 
be wrong.

On Apr 20, 2011, at 1:52 PM, Sebastien Hubert 
<sebastien.hubert at mc2000...> wrote:

> Hello everybody,
> Till yesterday I was pretty confident that it was possible to make 
a good air tightness using earth plastering.  Yesterday an other 
professional  called me regarding air tightness.  His neighbor and 
him did a blowerdoor test to verify the airtightness of their 2 
passive houses (should be passive).  The result was not good.  The 
n50 leakage rate is supposed to be smaller than 0.6. The building 
must not leak more air than 0.6 times the house volume per hour (n50 
? 0.6 / hour) at 50Pa (N/m?) as tested by a blower door.
> He obtained n50 = 1.2 and it was 0.95 for the second one.  They 
worked 2 days trying to find what could be the problem and didn't 
find any major leakage.  There was 1 point that could be a problem. 
A basement wall that is part of the living space get only 1 layer of 
earth plastering but no render.  The thickness is more or less 1.5cm. 
They decided to glue a airtightness sheet (1 square meter) on this 
wall.  They blew the air outside of the building.  Then they saw the 
sheet (that has been glued) inflating meaning that the earth 
plastering was not airtight.
> Of course, I have to say that it would be better to put the render 
on this wall.  Airtightness should be probably better.
> I'm building a straw bale (this is our house) and we really want a 
good airtightness.  For the wall, the earth plastering is supposed to 
do the airtightness.  We are going to make a blower door test before 
finishing the details.  This means the renders are not ready yet 
because we will put the last earth layer on the wall and ceiling 
at the same moment.  This will be done after the blowerdoor test. 
This means that the blower door test will perhaps not be successful.
> What do you think about it ?
> Does anybody already make a blowerdoor test with earth plastering 
airtightness ?
> I would really appreciate a feedback.
> Many thanks
> Cheers
> Sebastien

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