[Strawbale] Air tightness and earth plastering

Sebastien Hubert sebastien.hubert at mc2000...
Wed Apr 20 22:52:54 CEST 2011

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 n_50 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 (n_50 ? 0.6 / hour) at 50Pa 
(N/m²) as tested by a blower door.

He obtained n_50 = 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


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