Civil Engineering Reference
In-Depth Information
What we are looking for in a good earthen render
Trial and error
Trial and error testing is essential when establishing a good earthen render recipe. The
development of an earthen render is, to a large degree, a process of trial and error. No
matter how good your theory, the proposed render must always be checked on the wall. If
I am ever accused of nagging, this will be the subject. I cannot emphasise strongly enough
that there is no other way to truly know if your render is going to work. It is devastating
to work for days or even months applying render only to be told that it must be removed.
It might seem like a waste of time to mix test renders, but it is the best time investment
you will ever make. You don't have to mix a barrow full to test a recipe, simply use a jam
tin or jar so that the quantities added to the test can be accurately measured for
duplication when mixing for application to the wall.
The basic theory
The basic theory of mixing render is that if the render cracks you need to increase the
amount of sand; if it crumbles, increase the clay content. You will also find that the
inclusion of chaff, hair or other fibre will also help reduce cracking.
I recommend that you initially make up six test renders with small variations to what
you suspect will be the correct recipe. Make sure that your tests are of similar consistency to
that which will be applied to the wall. If the test is much drier it is likely to give you a false
result. If you will use a render pump to apply the render the mix will be significantly wetter
than if the render is applied by hand. We would all like to hit the perfect recipe first off
but this is rarely the case, as there are so many variables. Before you start this process be
prepared to test again and again. To simply choose the best of your first six tested recipes
may not be an option. You might need to do another six and even more. Remember, it is the
render that you are relying on to keep your bales dry and keep the weather outside. If you
are willing to live with the effects of the weather inside, it is a lot easier to erect a tent than
to build a straw bale house. There are no short cuts to producing a good render!
Test the render
When testing for the first coat of render, it is to be applied to naked bales of straw just as
it will be to the wall. If the render is to be applied to the wall by hand you will wet the
bales prior to spreading the render. If the render is to be applied with a pump you may
not have to pre-wet the bales. The test recipe for the second coat of render will be applied
over the approved first coat of render, and the third coat applied over the approved
second coat of render. If this is not done you are likely to gain a false reading and end up
with an unsatisfactory result. The first coat of render is to stick to the straw like poop to a
nappy. This is the key into the straw; the bond that the rest of the render will rely on.
When dry it is to be strong and not crumble, it will most likely crack, however this is of
little concern, as the cracks will actually assist the adherence of the second coat of render.
You can use the same recipe for the second and third coats. Each test should include a
section approximately 10 mm deep and another section 20-30 mm deep. I test this recipe
with what I call a hose test, which is simply to squirt a jet of water at the area with a
