Saturday 29 March 2014

Rendering Resumes

In October last year we began to have some problems with the render. The render went on ok and looked fine when wet but dried very blotchy and with lots of cracks.

Freshly done render
More fresh render

Blotchy render

Lots of cracks
 At first we thought that the problem was that the render had been getting wetter and wetter as he weather got warmer and it had reached a point where it was too wet, However in retrospect this seems to be a wrong assumption. We are not sure what caused the problems with the render, it may have dried too fast or the weather been too warm. As other walls were done after the blotchy ones and they looked fine.

Subsequent walls looked fine
We tried to have a second coat put over the three blotchy walls but this did not go well and when it became clear that the second coat would not go on smoothly we had no choice but to chip off the whole wall of render and start again. This was done for three walls the two largest in the main room and one in the back bedroom.  Two of the walls were really soft even though it was about two months since they had been done and the render came off really easily. the other wall on the north side of the main room had some soft patches and some really hard patches.

Render chipped off bedroom wall
 The first wall we chipped off looked fine when first done, then started to dry blotchy again, but in a different way the second time.  Ironically another wall done on the same day and with the same mix turned out fine. When the second wall we had chipped the render off was redone it too looked fine to begin with but when it looked like it was starting to dry unevenly I wet it down several times and it ultimately dried with an acceptably even finish. We were not keen to redo the blotchy wall we had already chipped off once and tried to get some lime paint to match, but our test patch showed that the colour was not a close match and even two coats of the paint did not cover the dark and light patches underneath.
The first wall that was chipped off looked fine when first re rendered
Wall done the second time when drier
And drier again
Wall done on the same day and same mix turned out fine
Redone wall that was wet down after rendering

Test patch of lime paint
John, who had been doing our rendering, had a new baby and a full time job and no longer had the time to work with such a frustrating and inconsistent product. We looked for someone else to render the outside of the house. When Brett Smith from Shoalhaven Cement Rendering came out to give us a quote, he did not know how the product would go on so he picked up a trowel and did a small wall. The wall was wet down a few hours after it was done to slow the drying. The wall looked great and he go the job, which was later extended from just doing the outside of the house to finishing the inside as well.
Brett's test wall
It was pleasing to see work on the house resume. Worried that the earlier mixes were too wet Brett started with a slightly stiffer mix, however it proved hard to get a smooth finish and we went back to the wetter mix that John had used.

Halfway through the first wall
First external wall complete
One more picture because it looks so good
Work then progressed on a difficult wall that had been half done previously and had only half the window reveal done. Brett did an admirable job joining the new and old render and rendering the reveal on a very crumbly section of hemp walling around the window where a large chuck of hemp had fallen out.
Wall with join and freshly done window reveal
The window headers and reveals then threw up a new set of problems to be resolved. The render was not very sticky and did not like sticking to the window headers. On the next wall the decision was made to use a thin coat of a wetter mix on the window headers and to do the window reveals first before the rest of the wall. This way the window reveal could be done and the render stick then as the render stiffened any minor fixing up could be done. One thing about the hemp render it does not respond well to being fixed up or touched up once done as this tends to roughen the surface. It is much better to put the render on and leave it alone.

In the second week into the rendering 120mm of rain overnight at Culburra delayed further progress.  But when work resumed on the external walls Brett's decision to go around the corners when the first wall adjoining the corner was done proved to be a good one and the second wall adjoining the corner could be feathered in leaving a soft gently rounded corner.
Heavy rain cause the creation of a temporary lake
Nice softly rounded corner
Almost looks like the house is done
Most of the walls left to do on the inside were the first and second coats on the Magnesium Oxide board. The hemp lime render and the Magnesium Oxide board were not a good mix. The render needed to be done in two coats and even then the first coat had to be done with a slightly stiffer mix and barely stuck to the wall. It was very difficult for Brett to get a smooth finish of the MgO Board and one of the walls has started drying very patchy. We will have to wait a few weeks and see if this evens out as it dries or remains, in which case we may have to paint it as a 'feature' wall. In the entryway with a thin slightly wetter mix for the second coat the render went on a bit better.

Second coat of render on MgO Board
Entryway render going on second coat of MgO Board
Render still wet on MgO board


  1. Hi,
    We posted your Blog at The Hemp Exchange, I hope that is OK?
    Our website in – it’s free to use and it works. It will drive traffic to your website, guaranteed.
    I hope you share with our members often. We love your website.
    Thank you,
    James Kaufman

  2. Hey Kirstie. You may have already thought of this The blotchiness seems to be from uneven drying but perhaps its related to the dryness or porosity of the hempcrete wall behind rather than the weather/climate. As you said, walls rendered with the same mix at the same time turned out differently so not sure what other factor there is??
    Again, great blog, sooooo helpful for any owner builders, but especially those looking to use hempcrete.

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  4. How many hemp home have u rendered in Australia
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