Too Many Plaster Failures

delaminating plasterOver the years I have spoken many times about the importance of a quality plastering job. That importance has not waned, and I am unfortunately hearing more and more stories about failed plaster jobs around the world. A large percentage of the consulting work I do is helping clients deal with these plastering issues. There are two common themes, or dare I say causes for the failures. If you avoid these two approaches to plastering your home, your plaster should provide you with a very long life.

Failure #1: Mixing earthen and lime plasters on a wall surface. This is perhaps the most common mistake that I see over and over again. People choosing to use earthen plaster for the scratch and brown coats and a final, “durability coat” of lime. The problem here is that what you have is stronger plaster over weaker plaster when in reality, you want it the other way around: weaker plaster over stronger plaster.

If you consider all plaster work over the last say…thousand years, one thing holds true no matter what material you use. The second coat has more sand in it than the first coat and the third has more than the second. That makes the coats “weaker” as they move away from the wall. This is important because plaster moves, as do homes. If the weaker plaster beneath a strong lime finish coat can move more than the finish coat, you will ultimately get delamination between the two coats which will lead to eventual plaster failure. By laying weaker plaster over a stronger finish coat, it will always be able to move at least as much as the coat beneath it. This keeps the plasters well bonded and eliminates the high risks seen in the opposite application.

cracked cement plasterFailure #2: Trapping moisture in the wall. There are two main ways to create this problem. The first is to use a plaster than does not breathe well. For some reason, the use of cement in plaster is still celebrated by some builders. I do not understand this at all. We know that cement based plasters don’t breathe well and we know they are more prone to cracking than lime or earthen plasters. Sure they are stronger, but who cares when they will eventually cause your walls to rot. DO NOT USE CEMENT BASED PLASTERS on a bale home. That is as easy a fix as any.

The second way that moisture gets trapped in a wall is something I see all the time. People decide to use earthen plaster on the home’s interior and lime (or even worse: cement) on the exterior. Those materials all have a different rate of permeation. Let’s consider the most common application scenario: earthen on the interior and lime on the exterior. In any given hour, the earthen plaster will allow 10 units of moisture to enter the wall. During that same hour, the lime will allow 7 or 8 units to exit the wall. The remaining 2-3 units are stuck in the wall and will continue to build up in the straw, leading to moisture trapping issues which are the cause of bale decay.

I very often hear people complain about lime plaster being a bad choice because “they have heard” lime will cause rot in the bales. NO, THAT IS NOT TRUE. What is causing the rot is the overloading of moisture in the wall due to the uneven plaster moisture rates. Yes, there will be rot behind a plaster coat made of lime in this situation, but the lime is not to blame, it is the combination of materials that were used.

To prevent this problem you can either use the same material on both sides of the wall, or you can build up the interior coat to slow down the rate of movement through the wall. This is my favorite option. In the same example, we could simply build the interior earthen coat to 2” and leave the exterior lime coat at 1 1/4”. The added thickness on the interior will slow down the moisture movement through the wall such that the lime can release as much as the earthen will allow to enter in the same time period. Simple fix.

H. CurveAgain, remember that plaster plays a hugely important part of your home. It is not simply a coat of paint over siding. It is the protection your bales need, the beauty your neighbors see, and sometimes part of the structural system that holds your house up day after day. Don’t skimp out in this. What is the point of spending a bunch of money on the framing, plumbing, windows, or any other part of the house only to skimp on the plaster and watch it fail in 2, 5, or 10 years? Ensure that you use high grade plaster materials and that you apply it properly.

If I’m coming across a little strong here, it’s because I care. Want I want is to empower you to make the right decisions so that you don’t have to experience these falures yourself.

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60 Responses to Too Many Plaster Failures

  1. Lina Saleh Tue, February 7, 2017 at 3:58 am #

    Dear Andrew,
    We have designed and built a house from earth bags. We have tried earthen plaster twice and the last try was by the house owner and his contractor alone by partially maintain the interior of the house by using cement, lime and sand only on the cracked areas. Now, the new plaster has cracks and the connections between the old and new plasters have cracks as well.
    Do you have any suggestions? I think the contractor should have applied a layer of earthen plaster, (same as used in the earthbags themselves( then should of added the lime plaster layer. Also, do you think it is a good idea to partially maintain the plaster?

  2. Alice Mon, February 27, 2017 at 1:31 pm #

    Hi, I wrote in Nov 2015 about needing to redo my plaster. I know you’re not a fan of mixing earth and lime layers, but I don’t think it’s feasible for me to remove the earthen layers in addition to the lime layer that is coming off. Do you think a new lime layer would stick any better if i used a metal lath or Spiderlath between the earth and lime layers? What mixture of lime would you recommend for this? thanks

  3. Andrew Morrison
    Andrew Morrison Mon, March 6, 2017 at 8:52 am #

    Hi Alice. That’s a tough one. The best approach is to remove the failing plaster. If you cannot do that, then you could use a lime plaster over a spiderlath layer, but the lath will be hard to properly attach through the existing plaster. If the lath is loose, then the plaster will be able to move more than it should and future cracking may develop. Perhaps you can sew through the walls/plaster to secure the lath. Then you would be okay. I would start anew with the plaster (3 coats), or at least do two coats so that you can build up some strength in the new finish. I would do a 1.5:1 sand to lime for the first coat, a 2:1 sand to lime for the second, and a Finish lime plaster on top of all of that. Each coat would be about 1/2″ thick except for the final coat which would be about 2316″ thick. My preference is Natural Hydraulic Lime from TransMineralUSA (www.limes.us). Let them know you are working with me and they will give you my discount pricing.

  4. Andrew Morrison
    Andrew Morrison Mon, March 6, 2017 at 9:00 am #

    The problem is adding stronger plasters (lime or cement) over the top of weaker ones (earthen). The plaster underneath is able to move and flex, but the surface plaster is too rigid to move at the same rate, thus the cracks. The best approach is to remove the more rigid plasters and stick with the earthen plaster. If that is not an option then you can use a lime plaster, but you will need mesh reinforcement between the earthen layer and the lime layer. It’s a tough situation to fix without removing plaster. Sorry for the bad news.

  5. Colleen Tue, April 18, 2017 at 10:04 am #

    I’m building a straw bale home in the Utah desert. There are quite a few straw bale houses around here, and a lot of them are using earthen plaster both inside and outside, with silicate paint on the exterior. Do you think that would create too much of a disparity between permeability rates? Silicate paint is supposed to act sort of like Gore-Tex for the plaster, allowing vapor to escape but not allowing liquid in. In our dry forgiving climate maybe this isn’t as much of a concern?

  6. Andrew Morrison
    Andrew Morrison Wed, April 19, 2017 at 7:39 am #

    Hi Colleen. I don’t work with earthen plasters as I am a lime guy. That said, I have heard very good things about silicate paints and in your climate I assume that it would work well. If it were me, I would make the interior plaster thicker than the exterior plaster so that it slows down the moisture movement a bit. You should be all set…

  7. regit Sun, May 14, 2017 at 9:18 am #

    why you dont answare ,and you close my questions

  8. Andrew Morrison
    Andrew Morrison Mon, May 15, 2017 at 10:36 am #

    Hi Regit. I don’t see any other questions from you that are unanswered in this thread. Sorry about the confusion. Perhaps they didn’t post…? Feel free to ask them again and sorry that they didn’t come through.

  9. Benjamin Tue, May 23, 2017 at 5:34 am #

    Hi Andrew,

    Do you have any “easier” method for plastering a straw bale house than this method?
    I would do it without professional help and it seems very difficult for me to create a good plaster mixture for the first time with the same quality for inside and outside and also to apply it with the needed contingency. With little knowledge I though of closing the straw with wood or any other simplier to be applied material wouldnt work?

  10. Andrew Morrison
    Andrew Morrison Tue, June 27, 2017 at 8:27 am #

    Hi Benjamin. Plastering is indeed difficult work. You can clad the home with wood siding, or any other finish; however, the wall details would need to be different to make sure you have adequate ventilation between the siding and the bales and to also make sure there is enough backing and points of attachment for the siding.

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