Plaster is a great finish for someone who is chemically sensitive. I used clay plaster throughout my house but there are a few things I would have done differently.
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Should you use Clay or Lime Plaster?
Plaster over wallboard requires a textured substrate – a wheat paste primer was used under my clay plaster, which could become problematic (mould-wise) if the humidity level is not kept low.
The plaster was custom mixed, but American Clay Plaster is very similar (and comes in a variety of colours). (Note: the natural colour of my plaster turned out to be quite grey although I wanted it to be white).
A fibre needs to be used in the plaster which you must test before using. Alex used cattails found by the side of the road.
In hindsight, I would have used lime plaster throughout the living space and kitchen so that I could have done tadelakt in the kitchen (more on tadelakt below), while keeping the colour consistent throughout. Lime is naturally very mold-resistant though still requires a textured substrate (wheat paste or mesh?) (Lime plaster would have also been a lot whiter.)
Problems with Plaster over MgO Board
The MgO board and plaster in my tiny house cracked at every seam. We used joint tape and Murco joint compound at the seams but this did not stop it from cracking. (Turns out Murco is great for regular wallboard but not for MgO board.
For MgO board One Pass joint compound is recommended). Even where we waited for the boards to settle and crack – re-taped and re-Murco-ed the seams, the boards and plaster still cracked along those lines. Moving the house did not cause further problems, most of the cracking happened in the first move, the first winter when temperatures changed and when we were banging new insulation into the walls.
Clay Paint over MgO Board
Clay paint does not adhere to MgO board at all! We tried various times with various primers and in different thicknesses and it cracked and flaked off each time.
Lime paint/lime wash also does not adhere to MgO board. It has flaked off in every consistency. Whether there is a trick to getting clay paint or lime paint to adhere to the wall I don’t know what it is.
Problems with Wood Next to Plaster
Another challenge was that the hemp oil on the wood window/door frames seeped into the plaster slightly even though the wood had been oiled months before, was oiled before it was nailed to the wall, and seemed totally dry at the time we plastered. It is not a huge deal but I imagine if we had not waited so long for the oil to dry it would have been a much bigger problem.
Burnishing Plaster and Tadelakt Finishes
Tadelakt is an amazing waterproof finish for lime plaster. It is a non-toxic finish which involves burnishing with an olive oil soap for many hours.
I wanted to tadelakt the backsplash of my kitchen, but since we used clay in the living space and could not match the lime plaster to the clay plaster colour, I ended up using clay plaster in the kitchen.
Alex plastered my kitchen (pictured below) and burnished the plaster, which smoothed it a little, but did not make a huge difference.
Sealing Clay Plaster: My Results from Testing VOC-Free Sealers
I tested three different sealers for the clay plaster in my kitchen, which needed to hold up as a backsplash around the stove and sink. From left to right I tested: beeswax, AFM Safecoat WaterShield (2g VOC/Litre) and AFM Safecoat Penetrating Water Stop (zero-VOC) (one and two coats), on burnished clay plaster. I splashed Worcestershire sauce on the samples, waited a few seconds and then wiped it off with a sponge.
The Penetrating Waterstop was the only one that fully protected the plaster from the sauce. Two coats, however, turned white and blotchy. Note, the beeswax also changed the colour of the plaster a little yellow but it’s not possible to see that in the picture. So, it was an easy decision since the AFM Penetrating Waterstop had no discernable chemical smell to me at all.
This has been holding up fairly well over the last couple years but when oil splashes onto it around the stove it does not wipe off easily. It is not the ideal backsplash but it works well enough if you are careful.
Plaster of Paris for the Chemically Sensitive
Pure Plaster of Paris can be a very highly tolerable plaster for those with extreme sensitivities. One client recommended USG.
USG Veneer plaster is highly tolerable.
Corinne Segura is a Building Biologist with 6 years of experience helping others create healthy homes.
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