23 October 2013

Lime Over Clay...When is that ok?

There is a commonly held misconception that I would like to correct:  just because lime plaster works as a protective coating for cob buildings (with thick walls built from clay, sand, & straw), doesn't mean that lime plaster works as an exterior protective coating over thin clay plasters.

What am I talking about?  I am talking about the practice of applying a clay plaster base coat over exterior strawbale walls, and then finishing over the clay plaster with a more durable plaster made from lime putty.  The idea is that
          a) clay plaster is easier & more pleasant to apply
        and
          b) it must be ok to use lime over clay because lime works as a finish on adobe and cob walls.
However, this method of using lime plaster over clay plaster actually could lead to failure of your plaster and potentially also moisture build-up in your strawbale wall over time.

Exterior plaster is usually applied in 3 coats:

  1. First, the scratch coat - the purpose of this coat is to get a really good bond to your wall substrate (so for strawbale construction, this means your first coat is stuck really well to the surface of the bales)
  2. Second, the brown coat - this is mostly a shaping coat, to make the shape of the wall exactly as you would like it to look when finished
  3. Finally, the finish coat - this is the coat that gives you the desired texture and sometimes includes integral color

Basic rules for multi-coat exterior plaster...

When applying multiple coats of plaster on the exterior, you can use the same recipe for each coat.  In other words, there is no performance issue if all three layers of your plaster are clay or if all three are lime. However, if you are going to vary your recipe, then the following centuries-old rules must apply:
  1. the layers of plaster/stucco (if not all the same) need to progress from hardest to softest, with softest as the finish
  2. the layers of plaster/stucco (if not all the same) need to progress from least permeable to most permeable, with most permeable as the finish
Why is this?  Well, it has to do with how moisture moves through a wall...

Let's start with the properties of lime plaster/stucco...

Cured lime plaster, chemically speaking, is calcium carbonate...basically limestone.  It's softer & more permeable than cement, but harder and less permeable than clay.  Permeability refers to a material's ability to allow air-borne vapor, humidity, pass through it  Think Gortex...water-repellent and vapor permeable.  Lime plaster/stucco makes a beautiful, durable, breathable exterior finish.



Next, the properties of clay...

Clay plaster is a combination of sticky clay (the binder) and sand (the aggregate).  Clay particles are like sponges that can absorb moisture from the humidity in the air or liquid water from rain, snow, etc.  (The clay particles then release that moisture when the surroundings are dry.)  When clay absorbs water, the particles get fatter, literally increasing in volume (then shrinking as the clay dries).  If you have a fixed amount of moisture that is being absorbed, then thinner clay (ie, plaster) will swell more in volume than thicker clay (ie, solid cob wall).  This means that in the exact same weather conditions, clay plasters will expand & contract, whereas thick cob walls will stay relatively constant.  By itself, this is not a problem, because the clay is flexible enough to withstand the expansion & contraction.


So, what's the problem?

The short answer: it's an issue of thickness & water absorption.  As stated above, clay plasters expand when wet and contract when dry.  In short, they move by swelling and shrinking.  Not a problem if you only have clay.  However, lime plaster is rigid, like monolithic stone.  The way lime plaster deals with movement is that it forms a crack to allow independent pieces to move independently.  Sooooo, if you have a substrate of clay, that is expanding and contracting with the weather, and you have a finish surface of lime, that cannot handle movement....then the natural movement of the clay will result in cracking in the lime finish plaster (that you wouldn't have if you just used clay or just used lime).

The second problem is technical...when you have a soft substrate under a hard, less permeable finish, then over time moisture actually erodes the substrate (ie, the clay plaster) right out from under the finish (ie, the lime plaster).  This is seen dramatically in cases where adobe buildings were finished with hard, impermeable cement plasters, resulting in erosion of the clay right out from under the cement, leaving nothing for the cement to bond to, and whole chunks of the cement stucco just fall off the walls.  While lime is not as hard and as impermeable as cement, it is harder and less permeable than the clay, so the process may take longer but the same result is at risk of occurring.

How do I know?

Maybe this all seems theoretical.  It did to me too.  Soooooo, I built a test...a small strawbale building in my mom's yard...to test the theory out and find out exactly what would happen.  (My poor mom!)
angled strawbale walls with deep roof overhang
(the drip line of the roof is over 42" from the base of the strawbales)
The walls are strawbale, with deep roof overhangs to protect from any direct rain.  So the moisture that interacts with the walls is almost entirely from humidity.  Over the straw, I applied 2 coats of clay plaster, totaling approximately 1-inch thick.  I let the clay dry completely and made sure the clay surface was crack-free.  Over the clay, I applied two coats of lime plaster.
2 coats of lime plaster over 1" of clay plaster
Then I waited a year.  My theory was that the thin clay base coat would expand and contract with changes in humidity, and that movement would cause the lime plaster to crack (since lime is not as flexible to movement as the clay is).
large cracks in lime plaster (over clay sub-plaster)
And here's what happened....in less than one year, with no direct moisture, rain, water anything getting on the walls, the clay sub-plaster had swelled & shrunk enough (from just humidity changes) to dramatically crack the lime plaster.  It's normal to see hairline cracks in lime plaster after several years, but not huge cracks like this and not within one year.  Hairline cracks will mostly self-heal, or can be lime washed.  But larger cracks like this need to be completely protected from rain or will require repair.


The bottom line...

My recommendation in regions with humid or wet weather, is to use an exterior plaster that is consistent in all three coats...either all three clay or all three lime.  Use all clay plaster if the walls are very protected from any weather or if you don't mind an annual replastering party.  Use all lime plaster if you want a more care-free, water repellent finish.

43 comments:

  1. Hi Sigi, I have recently found your blog pages (through FB) and am amazed by the amount of detail and explanation you include. I really appreciate your dedication to the cause. Thank you so much.
    Specifically to this posting, I have read about numerous natural additives that can be added to the final coating of a clay render to make it more durable and water repellent, what are your views on such additives. For example: the liquid extract from fermented maize leaves or other green matter, egg whites (though I can't imagine how many would be needed for a whole house), linseed oil, to name three specifics.

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    1. Thanks so much for your kind words. :-) There are definitely loads of great additives for clay plasters. (I'm going to write a post just on that topic!) I have not personally experimented with either of your first 2 examples, so I can't speak to either one directly. Linseed oil I usually use as a sealer after the wall is plastered, not added to the plaster, but I've heard of people adding it to the mix. My favorite additive by far is cow manure, followed in close second place by wheat paste. Bottom line is that I would encourage you to experiment! Go play in the mud! :-)

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  2. I have recently found your blog pages and I reallly appreciate your attitude to the subject. Thank you so much.

    As to the aboves entry topic:
    Have U conducted any experiments (or U are familiar with results of ) adding a different (increasing outwards) ammount of lime to the clay plaster leyers??

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    1. So glad you find these articles useful. That makes my day. For mixing clay & lime, you really need to experiment. There is a secondary reaction that can occur (I believe it has to do with magnesium content in the clay, but I'm not 100% sure), and that reaction can either help strengthen, or it can make your plaster weak. So experiment first. I never mix the two because I am using a different clay (usually site soil) on every project. So I use just clay, sand & straw OR high high high quality lime putty and sand. One or the other...

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  3. I'm plodding through a book (partially available at goole books) "Clay Materials Used in Construction" to understand the influence of Ca(OH)2 addition into clay on its chemical and phisical properities.
    But the topic is pretty complex even for a chemist. (a function of to many variables).

    I considered applying lime plaster on north west elevation (the most exposed one) of my future SB house, but I think I'll stick to clay and the "mayonaise" addition protection method of Tom Rijven.

    Hugs :)

    I'm getting back to study the dyes entries :)

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  4. Those tips are very enlightening, coming from an expert like you. Most people would just put up with any materials they find aethetically suitable without considering other factors such as the weather, which is important since you'll be using the stucco for your home's exterior.

    Geoff Hull @ Gogo Contracting

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  5. Hi Sigi
    What is the best approach for installing lime paster over cob sculpting? For me applying the plaster is best with trowel but can't use the trowel around sculpting. Tried with my gloved hand but plaster is so sandy it just falls off.
    Thanks Sigi
    Tree

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    1. I use one of three things:
      1) very tiny, very flexible steel trowels that I order from www.landerland.com
      2) a small, flexible piece of smooth plastic - like a yogurt lid with the edges cut off
      3) a piece of heavy plastic sheeting (thick enough that it doesn't tear)

      You can also pre-smooth by using gentle sweeping strokes with a small, tight, sponge (I use a grout sponge from a tile supplier). This will smooth out any trowel marks, but will leave the surface slightly sandy. Then use one of the options above to push the sand into the surface and finish smoothing out the surface texture. Hope that helps!

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  6. Hi, great blog! It's the second time that I have ended up here as people tell me that I shouldn't have used lime over clay and then point me towards your experiment.
    Not that I disagree your thinking, but I thought you would be interested to know that lime on clay will and does work from our experience and we are now in our third year crack free straw bale house. We used clay from a brick factory and a bagged lime from the builders merchant, which I appreciate has a different composition, but so far, in a relatively wet cold and hot Polish climate, we have had no problems.

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    1. I appreciate that this is your experience. And without seeing exactly what you did, I can't say what the variables are to your success. However, what I have seen are many many cases where lime over clay is not successful, produces large cracks in the lime within the first 2 years. Glad yours worked out!

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  7. :-) Mr. Winko His Ubiquitousness. :)

    Some posts above I placed a comment with a link to a marvelous book about the natural construction materials.
    As a chemist I could appreciate its way of explaining tough subject of phisico-chemistry of substrates and processes.
    A friend of mine is right now conducting a whole house experiment with applying lime plaster over the clay one. So I'll be having my eye & ear open to the results.

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  8. Hi Sigi
    What a great blog! So many usefull information.
    I am building a cob hamam in middle of France. For the inside I thought of putting tadelak (do you know this? it's used in maroco inside hamams because it breaths but doesn't leave the water go through). To protect the cob hamam from rain I thought putting lime plaster on the outside. Now that I read your post I don't know what to do. Any suggestions? And what do you think of tadelak inside? Thank you, Olivia

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    1. Hi Olivia,
      Sounds like a great project. So to clarify the lime-over-clay issue...if the clay is thick (ie, the whole wall is made of clay), the lime as a finish plaster is perfectly appropriate. It is when the clay is thin (ie, a coat or two of clay plaster) that it will not work to put lime plaster on top. So if your walls are thick cob walls, then you absolutely can do lime plaster. If you do decide to do tadelakt, be sure to build up 1/2" of lime plaster as a base between the cob and the tadelakt finish.

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  9. Oh thank you so much and for the lime plaster base, i didn't know. Thanks a lot.

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  10. Hi, thanks for all the experience you are sharing.
    We are building our adobe house in Argentina, and we have (almost) finished the exterior and interior rough (scratch and brown coat) plaster.
    We have been using a sand and lime ratio of about 5 to 1. This recipe is widely accepted as a good practice in our region, but for the interior final coat (finishing), we are wandering if it will be possible to apply a clay plaster (as we love the texture and the fact that it can be hand applied), insted of using a lime plaster with sifted sand (as we will do for the exterior).
    What do you think about this? Do you anticipe some risk of cacking?
    Thabks a lot for your comments.
    Un abrazo grandee,


    Maia y Hernan

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    1. I'm not quite sure I understand. Can you list out exactly the layers you have over the cob now on the interior? (Is that what your question is, ie just related to the interior plaster?)

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    2. Yes, our question ss only about the interior plaster.
      We just have a one inch layer of sand and lime (5:1) applied over the adobes.
      We would like to apply a thin layer of clay plaster for the finishing over that lime base (for smoothness and color), but were afraid that cracks would appear.

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    3. You can put clay plaster over lime plaster...that's totally fine. Your lime plaster may be a little on the sandy side, so I would first make sure that your surface is not dusting (ie, when you brush your hand across the surface, you don't feel anything come off).

      In terms of cracks, they are only a guarantee if your lime is cracked...then you need to repair those cracks prior to plastering otherwise they come right through. To prevent cracks in the clay plaster, be sure to dampen the surface before you begin, and do some test mixes to make sure that your proportions are good in your clay plaster (too much clay in the mix will cause cracking).

      Hope that helps get you started.

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    4. Thanks a lot for all the useful tips!!! We are still getting a place where to collect some nice the clay (cause buying it here is not an option), but will let you know how it goes. Abrazo grande y muchas gracias de vuelta !!!

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  11. Hello!! I'm starting a construction with the Cobb method in Argentina .. the truth that this blog and website have been very helpful for me, are really excellent !!!
    I recently saw a video where lime and clay were mixed to make the plaster. Beyond the complication when color mixing, it is advisable or beneficial to do this?
    Sorry for writing, I'm using a translator :(

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    1. I'm so glad this information is useful to you! :)

      For your plaster question...the short answer is yes, you can mix clay and lime. However, unless you are just using a small amount of clay added to lime to pigment, you need to do tests of the plaster mix. The reason is that all clay has a bit different mineral content, and that can impact the chemistry of the lime curing process. So just make some test plasters first, let them cure for a while, and be sure the resulting plaster is strong and has the look you want to achieve.

      Hope that helps! And good luck with your construction project!

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  12. Hi Sigi, I have come back to this post several times. It describes perfectly the issues that can arise when you put lime over clay render. It also has relevance for my work on old Scottish mudwall (cob) where I have experienced problems with the lime/mudwall interface breaking down. This is not gritty well graded cob but a very silty fine mix that is soft and friable to touch but, at the same time, remarkably resilient to weather (250 year old exposed material with little roof overhang doing well). I think the problem with this material is just the same as with the clay renders you describe - it moves with changes in humidity and is not stable enough for the rigid coating of lime which cracks and eventually falls off. Thank you for giving such clear and generous information at all times. Earth builders in all corners of the world appreciate it.

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    1. Hi Becky! Thanks so much for sharing your experience. It totally makes sense to me and sounds like those walls want clay plaster instead of lime. Silty clay isn't as strong, typically (because the particles are spherical instead of angular)...so I wonder if that's part of what contributes to the issue.

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  13. what base sizes are recommended per oven size ? Like 22.5 would be a what size base, 31 would be ?... If using cinder blocks is it ok to put in ground a little ways instead of pouring a slab? Slab or no slab? Thanks!

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    1. I'm not totally understanding your question...
      If the interior of your oven is 24", for example, then you add the 4" mass layer (x2) and the 6" insulation layer (x2) and you will get 44" total diameter for the oven. The base should be at least that big. Does that answer your question? If not, please try asking another way.

      Also, you wouldn't need a slab at all for an oven

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    2. Ok, for a 27" oven how big should my base be? Thanks!

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    3. draw a circle representing your inside diameter. Then draw a circle around it representing your thermal mass layer. Then draw one last circle around it all representing your insulation. It will be clear how to add it all up for your conditions...and this is the size of your base.

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  14. Yes! I've definitely experienced this plaster swelling in humidity. Even during our Turkish summers when there is no rain at all, my plaster expands and contracts almost daily depending on the humidity level. How do I know? My doors suddenly jam tight, and then mysteriously open again over night.

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  15. Hi! You advised me to use 3 layers of lime plaster on the exterior strawbale insulation of our planned house in Hungary. Could you suggest your lime plaster "recipe"? I highly appreciate your experience, your words enhance our confidence in starting our project.

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    1. I use a high quality calcium hydroxide putty and clean, gritty sand in a 1:3 proportion (lime to sand). You have to put it on in thin coats (3/8" maximum) and allow each coat to cure for at least 7 to 10 days. Protect the plaster from wind & direct sunlight while it is curing and do not apply when the temperature will be below 40F for the entire curing time. I also highly recommend the book Building with Lime by Stafford & Wingate...it explains everything you ever might want to know about lime...there's lots more to it than what I can describe in this context.

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    2. Is it really 3/8=0.375 inches, or 3 to 8 inches?

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    3. yes, 3/8" = .375"
      if you are metric, that's just about 1 cm (.95cm)

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  16. Thanks for the detailed article! What do you consider a clay plaster layer thick enough not to have issues with lime coat? We have about 2-3 inches of clay plaster over the bales.
    Also, not sure if it matters, but the straw bale wall is not a part of a house, so both sides of the straw bale wall are outside, under a small roof with a 2" overhang on both sides.

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    1. really thick...probably at least 4". And the thinner it is, the more straw I would want the thick plaster to have, because the straw mitigates the expansion movement of the clay when any moisture is present. And also I would be sure your lime is applied in 3 plaster coats.

      But this is just a guess based on varying experiences, not with actually testing it out. I don't know the absolute answer.

      The thing that would concern me the most is a 2-inch roof overhang on a strawbale wall...unless you meant 2-feet?

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  17. yes, two feet :)
    Why do you feel that 3 coats of lime plaster would be better? Thx!

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    1. because the thicker plaster is less likely to crack with movement of the oven, and that means less chance of water getting under your plaster and into the clay below

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  18. This article is so informative! Thank you. One question I have. Would you fill in the cracks and corners of the straw bale wall with a lite straw clay mix before your 3 layers of lime or would you mix a lime and straw mix to build your wall out before your discovery coat? I would think after reading this you should go all lime but wanted to hear your thoughts. Obviously lime is more expensive than site soil but the longevity and durability is more important for me

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    1. Hi there Matteo, and thanks for the great question. So I am hesitant to put lime over any exterior veneer clay materials. If your mix has too much clay, then you will get the same issues of expansion & contraction underneath your lime plaster (as described in the article above)...and that could lead to cracking in your lime plaster. Adding straw to your lime would be ok, but you still cannot apply lime thickly, because it cures by reacting with carbon dioxide in the air...which means you need to keep your plaster coats to 3/8" or less to ensure proper curing.

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  19. Blessings to you for providing such detail! I want to make sure I have this right so we can apply it to our project properly: We are building a "bale-cob" home wherein the bales are "mortared" together with cob. The final result will be approximate 2' columns of traditional cob with runs of about 9' (linear) of straw bales, 3 bales high between them that are pinned with stakes, clay slipped, then "mortared" with cob. On either side (inside and outside) of the bales will be thick (3 and 5 inches respectively) layers of cob. Our plan was to finish the inside using clay plaster and lime render on the outside. We live in central Oklahoma, which is classified as "humid sub-tropical" with infamous tornadoes and droughts. We will have a 2' overhang on a one story house. We rarely get "driving rains" even with our intense storms. After reading a previous comment, I gather that the 5" cob on the outside of the walls qualifies our project for a lime render use. Am I hearing you right? The thought of messing something this important up after all our hard work makes my stomach turn... Also, our quest for high quality, high calcium lime has been, er, difficult. Do you have any suggestions for finding good lime to use? Again, blessings to you, I truly appreciate your expertise!!

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    1. So I would first try to talk you out of putting cob between your strawbales. You are creating a thermal bridge that has no insulation around every single strawbale, and that will seriously reduce the energy efficiency of your building envelope. (And if you need a building permit, it likely will prevent you from meeting energy efficiency codes.) Then "cob" refers to a monolithic wall system made from the same materials as adobe. Finishes attached to another wall system are plasters. I'm not sure what you gain by adding 5" of clay tothe outside of the strawbale walls, other than lots of work. Thermal mass provides the biggest benefit to the interior of your insulation. So I would use only clay plaster inside and then only lime plaster/render on the outside. The lime plaster/render should be applied in 3 coats, minimum, each not more than 3/8" thick, with curing time between. I use a pure, high calcium lime from Mississippi Lime and then soak it in water for months. You can also use a high quality, fresh type S lime, such as Graymont, which you can usually find at a good masonry supplier.

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