Clay plaster – The many benefits of

clay plaster in a straw bale house
earthen floors and clay plaster
clay plaster and an earthen floor – work and image by Jeffrey Hart

What are the benefits of clay plaster?

I sat down to write this article on the benefits of clay plaster. In searching for a suitable source image, found this article from Paul Fitzpatrick which far exceeds anything I could write!

Below is a quick reference of the benefits of clay plasters!


Environmental benefits of clay plaster

clay plaster in a straw bale house

  • if you make your own base plaster, material can be sourced locally, with lower or no transport costs / emissions
  • unlike gypsum (and lime, although lime re-absorbs CO2 on setting), no firing required, so lower energy use / emissions
  • totally biodegradable
  • clay doesn’t set chemically like gypsum does – it just dries; so at least for the base coat, if it dries, you can wet it and knock it back up again, which means less waste
  • no nasty chemicals


Benefits for the building

earthen floors and clay plaster

  • breathability: clay plasters prevent excess moisture in vulnerable buildings. Clays are hygroscopic – they absorb moisture if humidity is high, hold it without being damaged, and when humidity falls, release moisture back to the air. So they maintain a consistent humidity, and minimise damp, and moulds caused by condensation
  • gypsum on the other hand absorbs moisture, but it’s not breathable, as it doesn’t let moisture out again, and it gets damaged in the process – it moulds, and / or the plaster can blow, and fall off
  • clays can be used with / over other breathable materials without taking away their breathability
  • flexibility: clay is soft and pliable, and can move, like lime, without cracking; it can cope with situations where you get minor movements, and so is excellent in old houses
  • gypsum on the other hand is not flexible, and cracks with any minor movements
  • protection of timbers: lime absorbs moisture through capillary action, but with clay, there is a chemical attraction that draws moisture in – and because clay absorbs moisture at a faster rate than lime or timber, it takes moisture away from the timbers of the building, protecting them from damp, mould, rot and insects
  • plus as clay absorbs moisture, it seals and prevents water from passing right through into the structure of the building. Moisture is held and released when the air around it dries. This is why clay can be used to line a pond – clay allows so much moisture in and then provides a barrier
  • aesthetic: clay plasters are beautiful in their texture and the way they reflect light, and they have an organic, natural feel.


Health benefits from using clay plaster

clay plasters applied to loft conversion

  • clay plasters maintain a consistent relative humidity of around 50-60%, which is beneficial for people with asthma or other respiratory problems, but detrimental to dust mites, which reduces problems associated with allergies
  • they contain no nasty chemicals, and are able to absorb toxins and thus improve indoor air quality. Of course, you’ll try not to use toxic cleaners or buy toxic househould products, but in this day and age, it’s difficult to avoid toxins completely, and so clay plasters can help
  • you have to be in a room with a clay plaster to understand how beautiful, sensuous, natural, earthy (add your own epithet here) they are. They just make you feel good

Embodied Energy In A Building

embodied energy in a building

What is embodied Energy?

When we are talking about our design and material choices in a natural home, we often discuss the embodied energy. This can be defined as:

Embodied energy is the energy consumed by all of the processes associated with the production of a building, from the mining and processing of natural resources to manufacturing, transport and product delivery. Embodied energy does not include the operation and disposal of the building material, which would be considered in a life cycle approach. Embodied energy is the ‘upstream’ or ‘front-end’ component of the life cycle impact of a home.

Why does Embodied Energy Matter?

I once wrote to a well-known insulation manufacturer asking for their calculations on the embodied energy in their product. The reply came back that insulation doesn’t have an embodied energy because it saves you energy. Alarming no?

So if I build a house with the most energy intensive insulation available, and I mean really fill the house with the stuff, way over code. The fact that the building may cost the earth practically nothing to heat and cool over its lifetime… is massively overshadowed by the enormous energy debt the house is in to start with. Even if the house exists for 200 years it might never pay off its initial construction debt.

Others come to similar conclusions; in one study, A comprehensive assessment of the life cycle energy demand of passive houses, the authors concluded that a family living in a Passive House, a very high standard that demands a lot of insulation, will, over the life of the house, only use a tiny bit less energy than those who live in a conventional house.

Like this author, they conclude that the only way to seriously reduce our life cycle energy cost is to live in smaller apartments downtown, but that is another slideshow.

But when you look at the list of materials in the passive house they studied (on TreeHugger here) you see it is full of 135 cubic meters of polyurethane foam insulation, sixty cubic meters of concrete, clay brick cladding and clay tile roof. There is a seriously high amount of embodied energy and carbon in this house.


Read more in this great slideshow from Lloyd Alter @ treehugger –

This is a series where I take my lectures presented as adjunct professor teaching sustainable design at Ryerson University School of Interior Design in Toronto, and distill them down to a sort of Pecha Kucha slide show.