Lime Plaster and Lime Wash on Cob or Straw Bale Wall

by ziggy on February 29, 2012 -- 3 comments -- Follow


Applying a lime wash in Gobcobatron

This year, I added a layer of lime plaster to the bottom half of the wall in my house. The idea was that adding some extra alkalinity to the surface would deter surface mold growth — although it’s been so mild this winter that I haven’t actually been able to determine if it makes a difference! (Remember that last year I was having terrible mold and condensation problems on the bottom of my walls during the coldest parts of year.)

Well, the thing that I actually want to talk about is — wow, I love the half earthen plaster and half lime plaster look! Having lived with it for several months now, I know that I want to try something like this in our new straw bale house. But next time it will be even easier than how I did it this time.

In the cob house, I opted for a bit of actual lime plaster for the above stated reasons. But from a strictly aesthetic standpoint, there’s no real reason to do lime and earthen plasters on the same wall. You can do a lime wash over earthen plaster.

What is Lime Wash?

Lime wash is nothing more than hydrated lime and water, stirred to a milky consistency. It’s like a paint, but thinner, and is typically used after your finish layer of lime plaster on a straw bale, cob, etc. wall. It brightens the surface, turning the dim gray lime plaster into a bright, vibrant white. You typically do at least two coats of lime wash for an even tone.

You can actually do a lime wash on top of earthen plaster, however. This can be used to great aesthetic effect. Want to lighten up your window reveals, or paint designs on your wall? Just mix up some lime wash and get to painting.

Making Designs with Lime Wash

All of the small little curves and flourishes that I did with April on our cob walls is lime wash painted directly on the earthen plaster. It is pretty seamless.


Small lime wash design next to window

Our plan is to do a similar design in the new timber frame & straw bale house. There’s no reason for us to do a lime plaster in the interior (and I prefer earthen plaster for several reasons, anyway), but it is nice to introduce the brightness of lime, and the neat contrast effect between the earth and lime. (Actually, the texture of lime plaster is different from earth, but that may not be worth the extra trouble.)

You can even introduce natural pigments into the lime wash and get a range of colors to work with. What we’ll do is a lime wash directly on top of the earthen plaster, probably two or three coats, depending on how much it takes to get a nice, solid white, and do a design similar to what you see above.

I’m a big fan of contrast, and the dark brown next to the vivid white of the lime is a very appealing look to me.


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  • Picking up on the problem of condensation, it would be interesting if adding lime to the cob mix would stop the mold. This is certainly the problem with cob- the straw is susceptible to forming mold. Of course to stop condensation involves the dew point temperature, indoor humidity, and would entail insulating the outside of the wall. We had this same problem on the north wall of an adobe house. Straw bale should prevent the problem with its high insulation value. This is a subject that is very complicated, hard to analyze, and often overlooked, but very important. The lime wash looks great.

  • Hey Ziggy,
    You say you use hydrated lime for the lime wash but everything I’ve read and learned is that hydrated lime is inferior to lime putty and natural hydraulic limes. What was your rationale for using hydrated lime?
    Keep up the good work!

  • Hey there: Well, it’s actually the stuff most easily available to us out here. I have never worked with hydraulic lime and am curious to hear more about how it’s different. (I am a lime novice.) The stuff we get can be seen here: – so-called Type S Miracle Lime.

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