Clay Plaster Walls For New York Passive House

We are building a Passive House in a Manhattan brownstone. Instead of using more synthetic materials to air seal the brick walls (crucial for PH) we are using clay.

Here are some notes to a successful clay application over New York brownstone brick walls:

– Use reinforcment mesh (fiberglass is best, hemp is  mesh also usable, easy to work with if soaked in starch – more ridgid) wherever you have different materials that meet. On a flat brick wall, that is not moving, no mesh is necessary. but better save than sorry?! so use mesh anyways
– With clay, always make experimental surfaces and wait until dry (you can dry it with a heater, not necessary to wait for natural drying) Some surfaces (concrete, OSB) need a clay slick before applying regular clay plaster. The rule is more clay for the first layer, less clay for the next, least clay for the last. Never the other way round. Using mesh, maybe we can suffice with 2 layers? Let’s think about that.
– The first layer might crack, but that is normal and makes the second layer stick better. The last layer should be done in the same thickness and rather thin (3-4mm) over the whole surface, to prevent cracking
– You will get cracks between the plaster and for example wooden elements (ceiling or whatever). These cracks can be filled in later (just make them wet and add some clay), but any movement later will cause a minor hairline crack. If you have some hemp textile soaked in clay slick, press it to the wood with a small baton, then plaster over the whole. The soaked hemp is air tight, and any crack does not matter. Instead of clamping the clay hemp textile with wood the joist (cumbersome), we can drape the soaked cloth around the joist and then tape the edge of wood/hemp with 3m tape around the joist.

– Use reinforcment mesh in areas where possible settling or movement will occur. Fiberglass is great but has high embodied energy. Hemp works well too.  If you soak it in starch it is easy to work with and dries harder.  Wherever you have different materials that meet you especially need to put mesh. On a flat brick wall where you don’t anticipate moving no mesh is necessary.

– With clay always make experimental surfaces and wait until dry (you can dry it with a heater, not necessary to wait for natural drying) Some surfaces (concrete, OSB) need a clay slick before applying regular clay plaster.

– The rule is more clay for the first layer, less clay for the next, least clay for the last. Never the other way round. If you are ok with a rougher look or you are really good you can get away with two layers. We have even gotten away with just one!

– The first layer might crack, but that is normal and makes the second layer stick better. The last layer should be done in the same thickness and rather thin (1/16″) over the whole surface to prevent cracking.

– You will get cracks between the plaster and for example wooden elements (ceiling or whatever). These cracks can be filled in later (just make them wet and add some clay), but any movement later will cause a minor hairline crack. If you have some hemp textile soaked in clay slick, press it to the wood with a small baton, then plaster over the whole thing. The soaked hemp is air tight, and any crack does not matter.

About the author: Gennaro Brooks-Church

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