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Re: [Strawbale] Air tightness and earth plastering

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@unm...




--On Wednesday, April 20, 2011 2:25 PM -0700 Andrew Morrison
<Andrew@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@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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