[Strawbale] Air tightness and earth plastering

Sat Jun 11 07:19:05 CEST 2011

Dear all,

I can now give some feedback regarding air tightness of earth plastering.

I put some pictures of our work.

As you can see all the connections details were carefully made.

Today we did a blower door test.  The result we get is quite encouraging 
as we reach the value of 0.93 (each hour 0.93 air volume of the house is 
renewed under a pressure of 50Pa.  For a passive house the value should 
be 0.6 max.  Although we will not be passive we are quite close to it. 
A standard house reach a value of between 5 to 12, a low energy house is 
around 3.

Regarding the plaster, I can say that the air-tightness is not so good 
without the finish layer.  As a matter of fact, we could feel the air 
passing through the small cracks although earth plaster is 5cm thick.  I 
think thus that it will be really better with the finish layer.   We 
will finish the plastering we make a new test to see the evolution.

We only put 2 electrical boxes in the exterior straw bale walls.  Those 
are really 2 weak points although I really took care when placing them...

Have a nice weekend.

Cheers

Seb

On 22/04/2011 02:13, Derek Roff wrote:
> 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 plasters.
>
> 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 plaster/render.
>
> Derelict
>
> *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.
>
> :)
>
> Andrew
>
> www.StrawBale.com
>
> 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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