An Overview of Clay, Lime and Cement Based Plasters

There are so many things to consider when choosing a plaster for your straw bale home. I will try and give a quick outline here to get you all started on the path of inquiry. There are many decision to be made and many details to consider in those decisions. The plaster is the thing that most people will notice about your home, so make a wise and well informed decision. Below I will give some pros and cons to Clay, Lime and Cement based plasters. The decision is ultimately yours and I hope the information below will help you decide.

CLAY PLASTER (Earth plaster)

Clay plaster breathes well and can be inexpensive if the materials are available on site. The application of clay plaster is easier than conventional plasters for an inexperienced person. The plaster can be repaired easily without “burn marks” by simply misting down the affected area and reapplying the plaster. The downside of clay plaster is that it does not last as long as other plasters. Regular maintenance is a must as its durability is low. Direct water on the plaster can affect the finish negatively. It is not as strong as other plasters when considering shear and compressive strength which lessens the strength of the overall wall assembly. Finally, getting the right ratios of materials: clay and sand mostly, can be difficult and is somewhat of a science. This becomes important in the scratch coat for strength and in the finish coat for consistency of the finish appearance.

LIME PLASTER

bob-plastering.jpgLime plaster is one step up in strength from clay and is my top choice. It is not quite as strong as cement based plasters for compressive and shear strength, yet it is considerably stronger than clay plaster. Depending on the type of lime you choose, it can be very easy to work with. I prefer Natural Hydraulic Lime (NHL) as it is simple to use and provides a great finish. You can buy plaster at www.limes.us (be sure to let them know you heard about them from www.strawbale.com). Lime, especially NHL is relatively flexible and can actually heal cracks on its own in some cases. Lime is very durable and can be left alone once the plaster is complete. If you want to change the color or freshen it up, you can apply a lime wash to the finish coat at any time. The biggest drawback to lime (NHL) is the price. It is quite expensive and only available from a few distributors around the World. I use it on all my buildings and I believe it is worth the cost; however, if you are on a tight budget, it may be hard to fit in.

CEMENT BASED PLASTER

Although cement plaster is very strong and has great values for both compressive and shear strength, I would not recommend using it on your straw bale home.  It is true that cement based plasters and stuccos are perhaps the most commonly used material for stucco crews and this means that the cost, therefore, is usually low when compared to lime or even clay if you hire the plaster out. It is also true that the materials are readily available in most markets as are skilled crews to apply them. All of that does not outweigh the negatives of using cement based plasters on bale homes. They do not breathe well and are likely to trap moisture within the walls, causing rotting in the bales. Another down side is the environmental impact of cement. It is a very impactful ingredient and so that must be taken into consideration when making a choice. Finally, the material is very hard and as such, has limited flexibility. As a result, cracks are more common in cement based plasters and those cracks can allow water to make its way into the walls, again causing serious damage. In my opinion, cement based plasters, even when mixed with lime, should not be used on straw bale homes.

As you can see, there are a lot of options and a lot of details to consider within each option. My plaster of choice is NHL for the exterior and interior scratch and brown coat and then a gypsum, lime, plaster of Paris (Diamond Plaster or equivalent) finish on the interior (I did not even talk about this one!). I have done clay plaster, cement based stucco, and lime plasters and have had complaints and cheers about each. As with everything in my life, I shoot for a balance and try to stay in the center of that equation.

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70 Responses to An Overview of Clay, Lime and Cement Based Plasters

  1. Andrew Morrison Thu, April 21, 2011 at 7:44 am #

    Hi Ivan. I have to admit that this is not a place of expertise for me. I would imagine that the dusting is a result of the material not properly bonding. Lime does take a while to set up, but typically it is cured within 10 days, not fully, but enough to apply a second coat and that means any chalking or dusting has stopped. There are 2 processes for lime in terms of being “done”. The first is a chemical curing process and the second is drying. If those processes are completed within the 10 day cycle, you should not experience any other dusting. If you are, I think that the mix is not right. It could be that the wash is not taking to the clay pigments. Sorry I can be more helpful and give you a solid “this is what’s happening” answer.

  2. Steve Sun, April 24, 2011 at 11:29 am #

    Thank you Andrew. I couldn’t have asked for or expected anything better than that. Really GOOD!

  3. Ivan Welander Fri, April 29, 2011 at 3:56 pm #

    Thanks for the feedback Andrew! We also suspected the bonding might not be good (at least on the area of the house that has a cement block wall, but our dusting has been happening on the bale wall too). My wife has now modified the wash with casein to essentially make a milk paint, with good results so far… no more dusting.

  4. Andrew Morrison Mon, May 2, 2011 at 11:00 am #

    Excellent. Glad it’s working well now.

  5. Michel Couvreux Thu, June 2, 2011 at 3:10 pm #

    Andrew,
    I should have participated in the blog earlier, but I was unaware of the existence of this discussion about lime.
    I would like to make some remarks, mainly related to Ken’s comments.

    2. “The statement about lime being self healing is a myth”
    Self healing properties of limes is not a myth but a reality. You can refer to the studies and reports done by the Getty Conservation Institute, Scottish Lime Centre, Israel Nature and Parks Authority, John Ashurst, etc to conclude that this a natural occuring phenomenon due to the presence of free lime in the mortar. However, we cannot expect large cracks (bigger than .03″) to self heal correctly. I must add also that this self healing, when it occurs, will fill the crack, but this crack will remain visible as the free lime is whitish.

    7 & 9. Here is what a conservator in Israel wrote: “I have been using St. Astier’s NHLs for the past 15 years in conservation of archaeological sites throughout our country in all climates and conditions. From fine wall paintings and mosaics to full reconstruction.
    So when we started to produce the CEB I naturally turned to NHL 5 as a stabiliser for our earth which usually has anywhere from 13% up to 20% clay content. 3% – 5% NHL 5 gives a very good stabilisation to the clay in the block, medium strength of 6-7 Mpascal, water proof and good thermal mass properties.” Earth should never be mixed with cement base products as the soluble sulphates present in earth material will react, in humid conditions, with the tricalcium aluminates contained in the cement, creating sulphate attacks and expansion of the material.

    14. “The lime we used in this contry for years was plain hydrated lime. Only in the last few years have we been using NHL and other foreign material. If you slake quicklime and use it as a plaster it is cheep. It takes a little time to understand the process and develop the skill to apply but it works very well. NHL was not used in this country in large quantities until the late 1990.”
    This is completely wrong and I do not understand how Ken can make such a statement. Hydrated Lime, as we know it, is a recent product issued from pretty pure limestone and burned in rotary kilns (which did not exist here until the 20th century). Until the end of the 19th century, the mortars used in construction were Hydraulic Limes, more or less hydraulic with great variation in qualities. Sometimes, the hydraulicity was so feeble that it could be used as a lime putty. Sometimes, the hydraulicity was so high that it was considered as a Natural Cement..
    What is new, since 1990, is that we have started to import high quality NHLs, reviving an industry (or art) which was common here, but had been forgotten, even if Riverton, in WV, continued to produce some NHL until 2004.
    Furthermore, I would not recommend anybody to slake quicklime in order to make its own exterior plaster. The process can be very dangerous (for example, pouring the water on the quicklime instead of adding to quicklime to the water). Then, what kind of ratio do you use? What kind of performance are you going to achieve? Nobody knows. You do not want to experiment on your house at the risk of loosing it in the future, due to a failure in the material.

    32 to 37. You can use the NHL on top of stabilized earth of adobe. The difficulty is the suction control when you need to wet the sustrate before applying the first coat of lime. The use of lime water for that operation works great. You will find more info on http://www.limes.us/pdf/plaster-cob.pdf

  6. Andrew Morrison Tue, June 7, 2011 at 5:11 pm #

    Thanks Michel. Can you tell me how the use of lime plaster over earthen plaster can work within the rule of plaster getting “weaker” as you move out from the wall. In other words, you don’t want to add a lime rich mix over a less rich mix; for example, a 2:1 sand:lime (3.5) should not be used as a finish coat over a 2.5:1 mix as the more rigid cover coat will hide issues that develop underneath it in the 2.5:1 coat. I’m curious as to how this is handled in the scenario you lay out in the link you gave.

    Also, are you truly able to get a good bond between the two different materials or is it just “acceptable?” I would think that the different materials, and their corresponding properties, would make the bond less than perfect. I understand people wanting to save money by using an earthen plaster for the scratch and brown coats, but that money savings is all lost if the coats fail. Furthermore, the science of the earthen plaster will be up in the air since it can and will change from one location to another. So you really won’t ever know exactly what you’re working with as your base coats. Not, at least, to the extent you would when working with measured and developed materials like sacked lime (or earthen plasters for that matter).

  7. Michel Couvreux Wed, June 8, 2011 at 1:45 pm #

    Andrew,
    The substrate and the plaster are two different issues. For the purpose of this discussion, I would consider only an exterior application where you want to provide a weather protection for the earth plaster. It is true that it is better to have a sustrate harder than the plaster. However, this is not always the case. I would take as an example the straw on which you are going to apply a base coat of lime plaster. All mortars, when drying, want to shrink. This is why a first coat of plaster needs to be stronger than the next one. This second coat is going to grab on a stronger first coat and is maintained in place without shrinkage cracks. If the second coat was stronger, the first one would not be able to hold the second one in place, and you will have cracking everywhere, with the second “pulling” and damaging the first.
    When you apply this first coat on a weak substrate, you can expect cracking, and this is normal. However, you need to try to reduce this by putting a material which is not too hard (this is why we do not recommend something stronger than a NHL 3.5). In the meantime, you should have stabilized the earth with lime water as described in
    http://www.limes.us/pdf/plaster-cob.pdf
    However, I would not recommend to try to protect the earth plaster with a single coat of lime as you have a high risk of cracking and may do some more damage by overworking the product.
    You would still need to do a three coat system. The savings, on a straw + earth building, would only be on the amount of lime needed (3/4″ to 7/8″ vs 1 3/8″).
    If the wall is prepared correctly, by somebody who knows and understands what is described in the document, then you have an excellent bonding, but I must say that this is the most difficult and important operation in the complete process of plastering.

  8. Andrew Morrison Tue, June 28, 2011 at 7:41 pm #

    Excellent explanation Michel. Thanks. Seems like the actual act of DOING this is the hardest part, not so much the details of how it works. In other words, human error can mess it up pretty easily, perhaps more so than it can do it well.

  9. Bob Fri, June 1, 2012 at 6:55 am #

    I’m still unclear on the use of type S lime instead of NHL for plaster. My understanding (limited as it is) is that when using type S you essentially get the same lime putty with a very short “slake time” as you would with slaking quick lime for weeks. Is this not the case?

    With NHL costing an order of magnitude more than type S lime, I’m looking seriously at using the type S if this will create a durable plaster. Thanks.

  10. Andrew Morrison Fri, June 1, 2012 at 9:55 am #

    Hi Bob. I have included two links here on type S hydrated lime. It is a quality material to use for plaster; however, I would suggest you slake it for at least 3 months. The longer it slakes, the better quality it is to use. I prefer NHL for several reasons over hydrated type S. First off, it is much more user friendly and less dangerous. Hydrated lime burns way more than hydraulic lime and that matters a lot when you get it in your eyes! Hydraulic lime is more durable over the long haul and provides better protection to your walls. The level of purity in NHL is high and this means that the plaster will always be pure and trustworthy whereas the less expensive type S lime can have some differences in quality from one batch to another. These differences can make for weak transitions in your plaster. Additionally, because of the drying process for hydrated lime, you cannot apply it as thick as you can hydraulic lime. This matters when applying 1/2″-3/4″ scratch and brown coats on a bale wall. As with anything else, I believe you get what you pay for with your plaster.

    So to answer your question directly, yes, you can use type S lime and have success; however, I believe that NHL is a better choice. That’s what will be on my walls when we build our next house!

    Here are the two links for type S: Link #1 Link #2 And here is a link for NHL. This link has a small discussion about hydrated versus NHL too, which is good.

  11. Thomas Tue, September 1, 2015 at 8:47 am #

    Hi Andrew,

    I have a strawbale building on the go right now and I am undergoing need of covering the bales with a plaster. I live in Northwestern BC, a very damp climate in the winter.
    I wish to cover the walls with mostly an earthen plaster, but I wonder the durability.
    It seems that because of the height of the walls there will be some rain blowing on two fo the walls (with the higher heights), about 4 feet high.
    I am wondering if I could mix lime with the clay plaster in order to make the clay plaster more stable, or in using a linseed oil on the outside to not let any blowing rain wick into the wall? Or would you recommend only a lime plaster on the outside for its’ durability, taking into consideration the need for a thicker clay plaster wall on the inside of the wall?

    Thanks for your help!
    Thomas

  12. Andrew Morrison
    Andrew Morrison Tue, September 1, 2015 at 9:36 am #

    Hi Thomas. I would not recommend mixing the earthen plaster with lime. I have not seen that done well in the past in terms of longevity and durability. I would not use linseed oil on the exterior because you don’t actually want to seal the plaster from moisture moving out and through the wall. I would highly recommend that you use lime plaster for the exterior in a three coat application. By thickening the interior earth plaster, you will slow down the movement of moisture through the wall so that it matches or is even slower than the exterior lime as noted in the article. I recommend you contact Michel at http://www.Limes.us and purchase Natural Hydraulic Lime plaster for the exterior. Because of your location, I imagine you will use an NHL 5, which is the strongest of the NHL plasters. Tell him that I sent you and he will give you a 5% discount on your order too, which is helpful. Good luck!

  13. Tracy Bays Sat, November 5, 2016 at 4:00 pm #

    I know almost nothing. I think based on my research that I would choose NHL. What about color? Can you use house paint? Would latex paint destroy breathability? What are the options, or limitations of coloring my NHL walls for interior, or exterior?

  14. Andrew Morrison
    Andrew Morrison Mon, November 7, 2016 at 1:45 pm #

    Hi Tracy. You can add color into the final coat (or even the last two coats if you want). The colors are available through the distributor. I would suggest not using latex paint, especially on the exterior as that will mess up the breathability of the walls.

  15. patrick Mon, July 10, 2017 at 9:50 am #

    Hi Andrew, I build an extension to my house of straw bales and did everything with lime. Interior and exterior. It looks like it works well. I have had some problems in one area where I was to late in the year to still add lime so I had to wait for this year to finish. But because of some bad luck some areas became over a 22% moisture level.

    I fixed the problem asap and shielded the bales more properly. They became less wet soon. BUT as soon as I applied a lime coating on, in april, the bales dried even more. They are now even below 14% and in some places 11%. It looks like the lime acts as a sponge and removes all exes moisture in the bales.

    I do want to make the outer wall stronger as just lime is not to strong. I did not use any additives. I have an overhang of at least 75cm everywhere so there was no need for this.

    Someone mentioned Sodium silicate to strengthen the outer walls and after applying this I could also finish it with a hydrating chalk paint. What is your opinion about this? Do yo think Sodium silicate breaths enough? It is said it does but I can’t find anyone with enough experience.

    Thanks.

  16. Andrew Morrison
    Andrew Morrison Mon, July 10, 2017 at 10:07 pm #

    Hi Patrick. Thanks for your message. I have not personally used Sodium Silicate, so I don’t have any advice for it’s use. That said, a lime plaster (lime/sand/water mix) is a very strong plaster and should not require any additional strengthening. This is especially true if you used hydraulic lime. The plaster will continue to strengthen over a year period, reaching it’s full strength at a year. Perhaps you simply need to wait a bit longer for the strength to form. Hydrated lime is not as strong as hydraulic lime and it won’t fully harden if it is applied thicker than 1/2″ at a time as it cannot cure without exposure to oxygen. Hydraulic lime can cure under water and as such will harden no matter how thickly it is applied.

  17. patrick Tue, July 11, 2017 at 12:15 am #

    thanks for your quick respond. I used the lime from tierrafino http://www.tierrafino.com/ Don’t know if it is hydraulic or hydrated? It is easy to use in any case. We will be building a complete house in France (hopefuly next year) and will be using the same lime I think. But I would like to do less overhang on the building. Like 35cm instead of 75 cm so it will catch more rain. What should i then do to protect the outside layers of my house? Any idea?

    I wanted to paint it with hydraulic chalk paint and the gy who sold me this said that then I had to first spray/paint it with Sodium Silicate to make it rock hard so it wont chip or crumple anywhere and the paint would stick on better.

    Do you have a better idea to make the outer lime wall better water resistant? Hydraulic chalk is way more expensive to buy in these quantities this is why i’d like to stick with lime plaster but I don’t like the loss of light when I make the overhang so large on the building.

    Thanks

  18. Andrew Morrison
    Andrew Morrison Tue, July 11, 2017 at 11:41 pm #

    Hi Patrick. I would suggest silicate paint for the exterior. It is very water resistant but does not cut back on the ability for vapor to “breathe” through the wall from the inside. This means you won’t trap moisture in the bales. I’m not sure where to get it as I have not worked with it in many years (I teach now and am no longer building). I imagine a quick search on line will get you there…

  19. Cecilia Sun, October 1, 2017 at 7:29 am #

    I would like to lime plaster the interior/exterior of an old adobe home. The exterior currently has a cement stucco and the interior looks like an oil based paint. Can I plaster directly over this or do I need to remove cement stucco and oil based paint? And if so, how can this be done? Or is there a primer I can apply that would work? Thanks.

  20. Andrew Morrison
    Andrew Morrison Wed, October 18, 2017 at 10:59 am #

    That is beyond my level of plaster expertise. If it were me, I’d want to remove as many layers of differing materials as possible. Negotiating earth (adobe), cement, and lime sounds like trouble to me.

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