[Strawbale] Air tightness and earth plastering

paul paul psheraton at hotmail...
Thu Apr 21 18:29:44 CEST 2011

In general terms, plasters are for the inside and renders (or stucco in the US) are for the exterior. You wouldn't say “I'm going to render the wall of this bedroom”, instead you would say you are going to plaster it. Likewise, you wouldn't say I'm going to plaster this exterior wall, you would say you are going to render it. If you were going to plaster an interior wall with a sand/cement render for an undercoat, then you might say “I'm going to render this wall”, so it can be confusing; But in general, render outside, plaster inside.

From: Andrew Morrison 
Sent: Thursday, April 21, 2011 5:46 PM
To: European strawbale building discussions 
Subject: Re: [Strawbale] Air tightness and earth plastering

As far as I know, they are the same. Here in the United States we say plaster. When I was in Australia teaching last month, they spoke of render. I don't know of a difference.


2011/4/21 Carina Simons <carina.simons at gmx...>

  Hallo to everybody,

  Since I am not a native English speaker I do not know the differents between a render and a plaster. Can anyone explain that to me?

  Many greetings,



  Von: strawbale-bounces at amper....muni.cz [mailto:strawbale-bounces at amper....muni.cz] Im Auftrag von Sebastien Hubert
  Gesendet: Mittwoch, 20. April 2011 22:53
  An: strawbale at amper....muni.cz
  Betreff: [Strawbale] Air tightness and earth plastering

  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/m²) 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



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