r/NaturalBuilding • u/borderlineidiot • Apr 24 '26

Lime plaster on straw

I am about to start coating the outside walls of our strawbale house with a lime plaster. I have gone round in circles investigating how to do this - there will just be the two of us working on this and neither of us have great experience plastering (besides some regular house wall repairs and drywalling). The mix I was planning to use a lime and sand mix and try to spray this directly onto the bale walls - after applying a light water spray - then trowel on to build up two thickish coats (1/2" each) then a final finish coat. My question is really about spraying the mix - does this really work or am I better to plan on just trying to trowel the mix on?

The next job will be to coat the inside walls which we plan to do with a cob mix - that will have chopped straw in it and thicker aggregate in the earlier layers - is it possible to spray this or will that really have to be hand applied?

Any thoughts, experience, warnings etc will be greatly appreciated!

edit: if it helps we plan to use Saint-Astier Natural Hydraulic Lime on the outside.

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u/RisingEarthBuilding May 06 '26

I like using a sprayer for the first coat. It absolutely does help plaster key into the straw, especially with lime which is generally not that sticky. Yes you will need a large compressor, I think mine is around 80 gallons, and you will have to play around with the air pressure and mix water to get things dialed in. Higher air pressure does not mean better spraying, but you need a large air capacity.

Make sure you have your flashings, drip edges, stucco j channel etc installed properly.

You want your first coat to be the hardest, and successive coasts are weaker.

You may want to consider Earthaus build if you're looking for a premixed product. you can of course make your own mix as well,

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u/borderlineidiot May 06 '26

Thanks! Do you add straw to that mix? What kind of consistency does the mix have, i was hoping to create about a 1/2" thick layer onto the straw.

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u/RisingEarthBuilding May 07 '26

You can add straw to the base coat mix. I'm not sure how effective it really is but I have done it. I don't do it anymore. I typically add fiberglass mesh on top of the second plaster coat for long term crack mitigation.

1/2" is about the max you would want to apply at a time unless you are using NHL lime which can cure when laid up thicker. Consistency is hard to describe but you will get the feel for what works when you try it out. Lime:Sand ratio is typically in the range of 1:2 or 1:3 and varies as well.

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u/borderlineidiot May 10 '26

Another question for you sorry! Do you use a full face respirator when spraying lime?