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.

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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    2. I appreciate this conversation, exploring the nitty gritty of substrate Compatibility. I'd be very interested in hearing from others in different situations on this topic of layering lime plaster over clay plaster. In my 25 years of professional natural building I have been doing just that on many builds without any problems. Perhaps Mendocino, California is an approproite climate region, though it can be very moist for months. I have also inspected many of the old Missions up and down the Cali coast, where lime was used over adobe apparently with great success as its held up (though occasionally renewed) for well over a century. Adobe is, of course, very thick , but often was uncoverd and directly exposed to heavy rains. Cracking has occured in some places for a multitude of reasons, though usually not detrimental.
      I have many times repaired areas of cracking on builds I've led (common behind beginning trowelers) where lime plaster is over clay plaster, over straw bales, and, after dissecting , have usually attributed it to:

      pour adhesion (for few reasons),
      abrupt changes in thickness,
      inadequate aggregate or fiber in the mix,
      the substrate beneath the plaster has cracks that "telegraph" through.

      After remedying the apparent cause, these repairs have worked very well, seemingly ruling out incompatibility of lime and clay for those locations. I can think of three Straw bale houses and several outbuildings I've finished this way. I have come to instruct plasterers to tap the plaster behind them to listen/feel for pour adhesion (a common problem) and I have to wonder if this or some other issue is at play instead of incompatibility from swelling? Over the years I have come to more appreciate high fiber clay plaster (and plenty of sand) as it generally seems to not swell and contract much and creates a cohesive mat over irregularities (also a common cause of cracking). Hope to hear from more experiences and further our understanding of good practices :)
      Happy Troweling,
      Colin
      @polecrafts
      www.polecraftsolutions.com

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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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    2. Additionally why not mix lime with the cob? It adds durability, in Morocco and Southern Spain lime itself was sometimes added to the blend for rammed earth or adobe/ clay based architecture. The old Andalusian term is Tapia Militar or Tapia Royale.

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    3. not all types of clay mix well together with lime

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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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  20. Hi Sigi. Greetings from Lithuania. I guess we all have the same problems all over the world and when the lime is different everywhere and you have no idea how high or how low it is in calcium level you scrach your head and.... Do you know any test for the lime putty to decide the quality of it? (I slacked lime and dug it in the pit for at least a year to mature, but I have now idea of the quality of it). Also another question. I plastered my straw bale house with clay plaster having high content of straw in it. It came out 3-4 inches thick. Only lately I found out about advantages of lime plaster on the straw bales on the outside walls. But my outside walls have mud on them already... So I am thinking about few options: 1) to put lime wash over outside walls; 2) to put few thin layers of lime+hair (a large amount for to make plaster elastic); 3) to put few thin layers of lime+sand plaster; 4) to put few layers of 1lime+1clay+ 4sand plaster (for some people it worked well (but I strugle with this one in my mind). Thank you for your service to clay minded people.

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    1. It's very difficult for me to speak to quality of materials from the other side of the ocean... I would try the following: a) find someone local that you can confer with that understands traditional lime plaster and b) make some test plasters and let them cure fully to inspect for flexibility, cracking, hardness, etc.

      What I wouldn't do, is lime over your clay. Limewash may be ok, but I wouldn't put lime plaster over. You are nearly guaranteed to have a future failure if you do this. The point is not that you need some kind of lime bridge between the materials. The point is that you have something soft, and pliable, and prone to erosion, and that swells under something hard and less pliable. And the result will be slow erosion of the clay until eventually the lime is not bonded to anything and it will fall off in chunks.

      I would stick to clay plaster from here forward. Perhaps you can do a limewash only, and reapply every few years as needed. Or decide that you are ok with the look of erosion of your clay plaster. The clay erosion is only an issue if/when it gets so thin that you see the substrate below.

      Hope this helps.

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    2. Thank you Sigi for your advice.

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  21. Hello Sigi! I found very useful information in here and in the discussions as well, however I need to ask a clarifying question. In a wet temperate climate, with a lot of high atmospheric moisture and fog for days on end in spring and autumn, would the bales in a SB house get moist because of the atmospheric moisture with either clay or lime plaster? Let's say I would like to do a clay plaster, I am not worried about doing some annual maintance, there are in the aceea some traditional clay plastered wood houses which need some small repairwork and lime recoating every few years. But I worry about the bales getting wet and about giving a bad example, either in terms of failure or unjustified costs, that could discourage others in raising SB homes. Thank you!

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  22. Hello Sigi! I found very useful information in here and in the discussions as well, however I need to ask a clarifying question. In a wet temperate climate, with a lot of high atmospheric moisture and fog for days on end in spring and autumn, would the bales in a SB house get moist because of the atmospheric moisture with either clay or lime plaster? Let's say I would like to do a clay plaster, I am not worried about doing some annual maintance, there are in the aceea some traditional clay plastered wood houses which need some small repairwork and lime recoating every few years. But I worry about the bales getting wet and about giving a bad example, either in terms of failure or unjustified costs, that could discourage others in raising SB homes. Thank you!

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    1. As long as you build your wall soundly, get the bales up off the ground, build deep roof overhangs, and use breathable finishes, your strawbale walls can withstand air-borne humidity. I have a whole article explaining this here: http://www.buildnaturally.com/EDucate/Articles/DryStrawbale.htm
      Hope that helps.

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    2. Thank you. Yes, it gives me courage. I have never dealt with a SB construction and therefore find it difficult to predict the behaviour of moisture. I will definetly go with good detailing and construction and so then with clay plaster. Thank you again for your patience and aduce!

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  23. Hi Sigi...i really really appreciate all the information and resources you have on your website. It is tremendously helpful. I have a question about this topic regarding interior walls - does the same logic apply? Shall the layers of plaster be consistently all clay or all lime? Obviously the inside does not experience the same weather exposure, so I am curious if lime plaster(for kitchen) needs to applied for all layers? Thanks!

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    1. It's still always stronger to follow these plaster layer rules...however, inside is definitely less risky than outside in terms of potential failure. In theory, the same rules apply, but in practice, there are fewer variables/elements that could cause failure if you do apply lime over clay. If it will be a lime plaster, I would still recommend at least 2 coats.

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  24. I exterior rendered our cottage with two coats of clay render and two coats of lime, over straw bale. I put a 10x10mm fibreglass mesh in the first coat of lime.

    Eight years later I pulled off some parts in order to build an extension.

    The wall had been directly exposed to the extreme southerly weather. The layers of render were perfectly intact, and the strawbale looked as if I'd placed it yesterday.

    Rendering is only as good as the person who applies it, and I did notice some small sections of lime that had delaminated, but were held in situ by the mesh. Also, one large section of the wall had largely delaminated the top coat of lime due to water ingress from a lack of roofing. The bottom coat of lime on this same section however was almost impossible to remove cleanly from the clay when we built our extension.

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    1. @ Pol
      Thanks for your feedback.Do you happen to have taken some photos of this? It would be a huge positive indicator that lime over clay is feasible.
      How thick were the the clay layers?
      What kind of clay you used? any additions to it?

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  25. Hi Sigi! Thank you so much for your useful information and sharing them.
    I just have a question, so is it the same for Lime Plaster(all 3 layers) over a Clay BASE Wall? I mean is it okay to apply Lime Plaster over a clay wall? if it's not, is there any solution to make it work?

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    1. yes! A clay wall is thick enough that it does not swell & shrink with any changes in moisture. So that works just fine. (It's when you do thin clay plaster layers and then try to put lime on top that trouble happens.)

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  26. Hi Sigi! How about clay and lime plaster? 50/50 Have you heared of or experienced? Thanks!

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    1. yes, I've heard of this. However, adding clay to lime can be super tricky. There are impurities in the clay that can cause the lime to act funky. And each material can actually get in the way of the other one being it's best self. So personally, I pick which plaster I will use, and then stick with that one start to finish. If you do go with a mix of clay & lime, be sure to do test areas first and see if they cure strong before you commit to a whole wall. Then do all of your layers with that same mix.

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  27. Hi Sigi thank you for sharing so much of your knowledge. I wondered see if you may be able to help me. Do you happen to know if there is any paint I could apply to tadelakt please? We have a huge brown tadelakt Wall we’re hoping we could paint with something breathable, perhaps limewash? Thank you in advance

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    1. not that I know of...sorry! You can apply a wax finish, and the wax can be pigmented. But that will hinder the permeability.

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  28. Hi Sigi, huge thanks for all the amazing information you share. I am buildinga "raised from ground" Geodesic Dome, by raised I mean that it is similar to a tree house, it is 3 meters above the ground, I already have the timber structure completed and I am trying to decide how to "cover" the dome and walls. I live in the Colombia, by the caribean sea, which means is pretty hot all year round and have high humidity in the air, Also we have to rain season during the year which can be very heavy. As I understand from this reading is that I should use either clay or lime, I would rather go with clay since it is a resource I have, but I am highly concerned about protecting it from rain. Lime could be an option too. I want to cover my geodesic dome with clay so it is entirely permeable (Breathable???), and hopefully will keep a cool indoor environment. Could you give me some advice in this? Thanks and congratulations!

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    1. Hi Carlos, no plaster is appropriate to use as a roof material. So unfortunately neither clay nor lime will protect your dome on the top. You can use a proper roofing material over the dome and then use either plaster on the vertical sides (under the roof overhang)

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  29. Very helpful information.
    However I want to point out that you should never expose anyone in mold intentionally.
    Probably you already know that it can be very toxic and even leave permanent damage to the exposed one and overall it degrades one's health

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    1. First of all, there are many many species of mold, including ones that are edible (blue cheese, for example) or medicinal (penicillin for example). You are thinking of black mold, which commonly grows in damp, dark places and surface. That mold is indeed a neurotoxin and I would never recommend any detailing that would risk the propagation of black mold.

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  30. Hello Sigi,

    Thanks for your really informative website. There is a group (as in a collection rather than a corporate group) of builders in New Zealand who use lime over clay as a matter of course on external clay walls. As you know, NZ can be very wet and humid. The first coat is a clay slip, the second coat is a very straw heavy clay coat which is pushed into the bales and which has an aggressive key. The lime goes over that. The thinking is that if a wall is exposed to, for example, driving rain, the lime may delaminate but, in doing so, it saves the clay and also the straw bale underneath. In NZ, eaves are usually wide on earth houses. This method has been used for upwards of 10 years without any problems (as far as I am aware).

    I would be interested in your thoughts on this method. In your opinion, is it the aggressive key which is making this method effective? I can see there have been some other comments advocating for lime over clay.

    Thanks in advance.

    Bjorny

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    1. That method breaks the rules of exterior plaster. #1: if the layers vary in hardness, the hardest layer should be applied first. And #2: if the layers vary in permeability, the least permeable should be applied first. The lime over clay is reverse of that. And eventually it will begin to fail. It can take a matter of a few years (if other detailing is not done well, like deep roof overhangs) or it can take a couple decades. But this method is not appropriate if you are looking for longevity. I explain the "why" in the article above, but briefly, in the longterm, any moisture that penetrates the outer layer (and it will at some point) will slowly begin to erode the surface of the soft clay underneath. And the point when it will fail is when enough clay has eroded that the lime plaster has nothing left to stick to. You can test if this failure is imminent by nocking on the wall...if it sounds hollow in places, it is very close to failure

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