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 plaster failures 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.

straw bale plaster 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 failures yourself. You can find out how to plaster step-by-step using the practices I’ve used with success for years with my “How to Plaster a Straw Bale House”. Click HERE for more information.

Want to learn more about straw bale houses and how to build one? Want to do so for FREE? Sign up for our totally free 16 Day Straw Bale eCourse! Find out more HERE.

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96 Responses to Too Many Plaster Failures

  1. Avatar
    ziggy Wed, April 24, 2013 at 5:54 am #

    Thanks for posting this. Advice to heed!

  2. Avatar
    Andrew Morrison Wed, April 24, 2013 at 6:04 am #

    You’re welcome Ziggy.

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    Craig Thu, April 25, 2013 at 5:38 pm #

    Hi Andrew I have read a lot about straw bale construction and considered myself well informed but just found your web site and the above article. This in all my reading and resaerch has never been mentioned before. This is important for me as I live in a climate with greatly varying temperature in 24 hour period so I need a large thermal mass as well as insulation. I was originally intending to build a combined perimeter wall of both straw and mud brick, build a structural straw bale wall first, then put first coat of mud then cement the mud bricks as normal but also bonding them to the first mud layer. (note no actual cement used just frase all mud) with rest of internal walls mud brick. This would give great insulation and thermal mass with a breathable structure. I did not consider this a problem until reading this article and now I am not so sure. Can you email me back with coments on this. Thank you in advance.

  4. Avatar
    Leila Whitlock-Hauth Sat, April 27, 2013 at 9:26 am #

    I would like to know if you have ever done just a thin lime wash on the outside of a home on an earthen plaster? What do you think of this if you have? Or if you have any ideas on it in general.
    Thanks, Leila

  5. Avatar
    Andrew Morrison Mon, April 29, 2013 at 4:05 pm #

    I’m not a fan of mixing materials in this way. I prefer to stick with either all lime (first choice) or all earth. I don’t trust the bond between two different materials.

  6. Avatar
    Andrew Morrison Mon, April 29, 2013 at 4:08 pm #

    Hi Craig. Personally, I think you would be better off separating the thermal mass from the insulation. I would prefer to see the mass away from the wall so that the sun can hit it and warm it up. Also, by having both sides of the wall exposed to the room (rather than one side up against the bales) you will get better thermal exchange and ultimately effciciency.

  7. Avatar
    Angela Fri, May 3, 2013 at 1:11 pm #

    Just to be certain, you’re saying it’s important that the foundation plaster coat be the strongest – just like it would be important to have a strong foundation for the actual building itself…

  8. Avatar
    Matt L Sun, May 5, 2013 at 3:49 pm #

    Mixing earthen and lime renders on a wall surface will always seperate/delaminate as they expand and contract at different rates no matter what order they are placed on the wall.Use one or the other.

  9. Avatar
    Andrew Morrison Tue, May 7, 2013 at 6:29 am #

    That is correct.

  10. Avatar
    Elaine Mon, May 13, 2013 at 10:53 pm #

    Hello, My husband and i just went to see a house made of straw it is a round house, beautiful inside, amazing to see, but the problem is that the outside looks like one of your pictures up there that the plaster is falling off and there are alot of cracks in the wall outside, now, is there is a fix to this? or is it too late and the moisture is in and can not be taken out? also we noticed the house smelt funny in the house, do straw houses usually smell different inside? because of all the straw?
    Thanks a bunch 😉

  11. Avatar
    Andrew Morrison Thu, May 30, 2013 at 12:09 pm #

    Hi Elaine. You should not notice a different smell in a bale house. Chances are that there is a moisture issue in the walls due to the cracks and failed plaster. All of that can likely be fixed, but it will take some work AND how much can be fixed will be dependent on the extent of the damage.

  12. Avatar
    Stephanie Geddings Tue, December 17, 2013 at 6:25 am #

    You mentioned about using different plasters on the interior vs. exterior and how using different thickness plaster can alleviate the different in moisture exchange. I am wondering does the same theory apply if you are using the same plaster on the interior vs. the exterior but they are different thickness. Say the interior is 1.5 inch and the exterior is 2 inches? Could you use that to your advantage to wick moisture out of the house? Say make the interior thinner than the exterior? Thanks, Andrew!

  13. Avatar
    Andrew Morrison Tue, January 7, 2014 at 1:37 pm #

    The same would be true; however, you would want the interior to be thicker than the exterior, not thinner. This way, it is harder for the walls to take in moisture than it is for them to release it. Keep in mind that homes are pressurized from the interior so moisture is driven into the walls from the interior. Rain and other moisture that is introduced from the exterior is better handled by design in terms of roof overhangs, etc.

  14. Avatar
    Sue Sun, August 17, 2014 at 5:33 pm #

    Hi there, this is a very late comment, but re passive solar & thermal mass: Personally not a fan of trombe walls etc. We built our strawbale house with low-e, timber-framed glazing, enough in the north wall (we’re in the southern hemisphere) to heat up a dark mocha coloured (oxides in top concrete layer), sealed, exposed concrete floor from ample sunlight penetration in winter. This is our primary heat-absorbing thermal mass. We have an interior brick wall at the back of our living area for additional thermal mass, and the lime plaster adds at least 20tonnes of thermal mass. Strawbale insulation quality I don’t have to explain here 😉 – all ceilings are superinsulated, plus sarking/insulation sandwich immediately below roofing iron.

    We’re only around 85% complete, and have a few bedrooms in which we have to do second and third coats of plaster yet, plus finish plaster over the N/E/W exterior to go.

    We have logged the temperature in this house for over a year, and it remains between 19 and 23 degC >95% of the time, summer or winter, with exterior temperatures as low as freezing in winter and as high as 42 degC in summer, without any artificial heating or cooling – and this is despite not having any curtains installed as yet. A ceiling fan suffices to keep us comfortable in summer, and we put the wood fire on perhaps one evening a week in winter, after a series of overcast days, just to bump the temperature back up towards the high end of its usual range. It amazes us how energy-efficient this house is – performing above expectations. Passive solar strawbale is a wonderful way to build. Taking the time to draw cross-sectional diagrams with your local sun angles at the solstices and equinoxes is really worth doing in your design process, and tweaking your glazing, eaves and thermal mass to suit your local climatic conditions.

  15. Avatar
    Stuart Cooper Mon, August 25, 2014 at 3:03 am #

    Andrew, I am building a SB house in Dunedin NZ. The external walls are about 25mm of earth plaster (Local clay, sand and chopped wheat straw)This is applied between battens (50mm) which has a flexible membrane, mesh and cement plaster over the top. (This makes a
    30mm air gap between the earth plaster and the membrane allowing the wall to breath freely as it should. The internal walls are intended to be earth plaster (Already under way) and a lime plaster outer layer. Reading the issues with mixing the two plasters I have a couple of questions.
    1. Since the external earth walls will remain isolated from the weather will the potential problem with delamination due to trapped moisture still be an issue?
    2. We intend to Scratch the internal earth coat with a cris-cross pattern to aid Keying of the lime coat. Is this OK?
    3.Lastly if we should be using earth finishing plaster on inernal walls what would you recommend to get a smooth finish (It needs to match other materials used on deviding walls.



  16. Avatar
    Andrew Morrison Mon, August 25, 2014 at 10:07 am #

    Hi Stuart. Thanks for your message. I am not sure how the external system will work as that is not something I have tried before. If I understand it right, you will have cement plaster over earthen plaster which is worrisome for me; however, the allotted “breathing” space makes it sound better. In terms of the interior, I would personally do an earthen plaster throughout and simply steel trowel it to get it as smooth as possible. I have seen some beautifully smooth earthen plasters. Best of success to you.

  17. Avatar
    Beth Black Wed, September 3, 2014 at 11:49 pm #

    Thanks for that interesting and easy-to-understand explanation. What effect does punching holes in the plaster have? Once your walls are done, should you never, for example, insert a nail to hang a painting? Wouldn’t that also change the way the plaster breathes? Thanks.

  18. Andrew
    Andrew Mon, September 8, 2014 at 12:51 pm #

    Hi beth. Small penetrations like picture hanging, etc. are fine for a straw bale wall. They can handle a little impact on the breathing like that.

  19. Avatar
    Matias and Gabriela Mon, November 3, 2014 at 8:23 am #

    Hello Andrew,
    How are you? We are really worried now hahah! We are building our SB house in the south of Buenos Aires, Argentina, 6 blocks from the Atlantic coast, so imagine the weather here, unpredictable, hard winds and lot’s of rain.
    We have a 2 floor house, wooden structure, SB walls, and we’ve done in the exterior walls a first layer of local clay, cut straw, sand and asfalt emulsifier (4%, except in 2 walls where we tried a 6% and it seems to be working better, because we could see some cracks on the walls that have a 4% after a strong strong storm). The interior walls are not finished yet (30% done) with cut straw, clay and sand. Our exterior plaster is thicker than the interior one.
    We notice that after strong storms (lot’s of water and winds, up to 70km/h), the plaster washes a little bit, doesn’t fall, but the straw and some cracks start to show.
    So, we are trying to find the correct finish plaster to have proper resistance and water proof, but keeping the right breathability and the straw bale healthy.
    We know that we put ourselves in a challenge, because of the 2 floor house… so the 2nd floor walls aren’t as protected as it should for this kind of weather (70cm roof overhangs).
    With this first plastering layer what do you recommend for the final plastering?

    Thank you very much, this is our first house and experience.
    We don’t have a website but we do have a facebook page with pictures.

    Have a wonderful day!
    Matias and Gabs

  20. Andrew
    Andrew Tue, November 4, 2014 at 6:40 pm #

    Hi Matias and Gabs. Great to hear your house is coming along! I am not a big fan of clay exterior plasters for the exact reason you are experiencing. Unfortunately, I have not seen good results for people who try to go to a more resistant plaster on top of clay (i.e. lime). In fact, lime on top of clay is perhaps my most common consulting job (fixing the problems associated with plastering that way). Even though others say it is fine, I have not seen it done successfully because the soft clay plaster moves underneath the more rigid lime plaster and tends to create delamination over time. I think you may be stuck with the clay plaster and simply adding some more asphalt emulsion to increase its weather proof properties. Unfortunately, I don’t have much experience with earthen plasters on the exterior as I am a proponent of lime plasters. Sorry I don’t have the magic wand for this one. Please let me know what you decide…

  21. Avatar
    melissa b. Tue, May 12, 2015 at 2:07 pm #

    hi andrew! love love love ur advice here. hubby and i are building a 600sf. strawbale next year or so. we live in the smokies..gatlinburg, tn. climate is very moist. if i use the same plaster inside and out, should i still make the interior coat thicker? we thought about cob wall interiors also, with lime plaster exteriors. can this be done safely? thanks so much for any help.

  22. Andrew Morrison
    Andrew Morrison Tue, May 12, 2015 at 2:25 pm #

    Hi Melissa. I would suggest adding a little extra thickness to your interior plaster, even if you use the same materials, just to be safe. If it is the same materials, then you won’t have to add much more, perhaps 1/4″ or 3/8″ is all. In terms of using cob, are you talking about making cob walls or using an earthen plaster? They are not the same thing. If you want to use earthen plaster on the interior wall surface and lime on the exterior, that is fine; however, this is where the need to thicken the earthen plaster is crucial.

  23. Avatar
    melissa b. Tue, May 12, 2015 at 4:54 pm #

    im not sure which materials would best sculpt interior walls. i havent researched it enough. im very artsy so i was going to incorporate some wall sculpting inside. as far as exteriors, what kind of exterior coating would you suggest? im not quite sure of the difference between earthen plaster and lime plaster. which one will be best suited to strawbale? i think i need to hire you to oversee this project! lol

  24. Andrew Morrison
    Andrew Morrison Tue, May 12, 2015 at 7:05 pm #

    Lime plaster is a combination of lime, water, and sand. Earthen plaster is a combination of clay, sand, and water. Lime is better suited for exterior uses than earthen plaster is due to the durability. Either material works on strawbale, so it’s more a question of durability. Earthen plaster is easier to sculpt with in terms of interior wall surfaces. If you want to add depth, you can use a cob like material (sand, clay, straw, and water) to build up the walls.

  25. Avatar
    melissa b. Tue, May 12, 2015 at 8:03 pm #

    thanks so much! big help!

  26. Avatar
    Dawn Kieninger Mon, June 29, 2015 at 2:27 am #


    We started a straw bale building last season and got one coat of lime plaster on to protect the structure before winter. I have never used lime plaster before. We have done a lot of “stucco” with Portland cement on renaissance festival booths. So the lime plaster at this point (just the base layer) is not falling off at all but is soft and bits can be rubbed away with pressure, not like a hardened cement coat. Does that sound about right or will new layers fail because the first one is too soft?



  27. Andrew Morrison
    Andrew Morrison Sun, July 5, 2015 at 12:57 pm #

    That sounds a bit soft to be honest. Lime does take a lot longer to cure than cement and does not reach full cure for a year, but it should be reasonably hard by now. It may be your mix ratio. A good lime plaster would start with a 1.5 to 1, sand to lime by volume for the scratch coat. You could go as high as a 2 to 1 sand to lime if need be. It also may be a result of the type of lime you used and how thick it went on the wall. If it is hydraulic, then you can apply a thicker coat and still get solid cure rates. If it is a lime putty, then the layering should be slightly thinner to allow for proper carbonation.

  28. Avatar
    Wynne G Sat, July 25, 2015 at 3:58 pm #

    Great info!

    What would your recommendation be if we’re building a straw bale greenhouse? Obviously, the inside will have plenty of moisture, as we will be watering. Most of the year, there will be more moisture indoors than out. With all the internal moisture, should we have lime plaster inside and out? Or do you still recommend earth inside, lime outside? How thick should the indoor/outdoor plasters be of the materials you suggest given the conditions this building will be subject to?

    You may wish to know our local climate. We live in the GENERALLY dry climate of northern Arizona, at 7000′ elevation in a pine forest. Temps may be as low as -20oF and as high as 95oF (these are absolute extremes). However, we can get very heavy monsoon rains in summer and several feet of snow in winter. The relative humidity typically ranges from 15% (dry) to 87% (very humid) over the course of the year, more often at the lower end of the range.

    We’re intending to build a stone masonry foundation that will rise a minimum of 1 foot above grade.

    Any special considerations we should be aware of?

    Thanks very much for your input!

  29. Andrew Morrison
    Andrew Morrison Sun, August 16, 2015 at 8:29 am #

    Hi Wynne. I don’t think you will need any special detailing to make this a viable option. Because your climate is relatively dry to start with, the extra moisture may actually be good for your home. You can go with either clay plaster or lime plaster on the interior. Clay plaster does a slightly better job at regulating moisture, so it could be a better option. I would do a 2″ coat on the interior and then a 1.25″ lime plaster on the exterior. Have fun!

  30. Avatar
    patrick Sat, October 10, 2015 at 5:01 am #

    Hi, i am building a straw bale extension to my house. I am already doing the plastering. Inside and outside with natural lime plaster. I have a question about this. I fill up the bigger holes between the bales with a thick straw/lime package. This can be some centimeters thick at some areas. It is autumn now and is it wise to continue with plastering now? Since the humidity goes up now. I also noticed that the humidity inside the bales goes up when i do plastering. This makes sense of course but how much may it go up and what are the chances that saw will rot when plaster needs to dry to long? Is it better to finish in spring or beginning of summer?

  31. Andrew Morrison
    Andrew Morrison Mon, October 12, 2015 at 1:44 pm #

    Hi Patrick. My preference would be to get the house plastered rather than leave it unprotected over the winter. You will likely add a lot of moisture to the wall, so once the initial cure periods are over (10 days each coat) you can introduce mechanical drying to the space to help remove excess moisture. You could do the first and second coats this year and then wait on the finish until next spring or summer. If you plan to use Natural Hydraulic Lime (the best lime in my opinion), be sure to contact either Michel at or Andy at (east coast of US) and let them know you were referred by me. Andy requires a vendor code (NHL 95501). They will give you a discount on your order.

  32. Avatar
    patrick Tue, October 13, 2015 at 12:29 am #

    Hi Andrew. Thanks for your reply and advise!.

    The thing is… i don’t have windows in yet so it will get cold inside anyway. (have to wait for new finances next spring) and winter is early this year 🙁 It started freezing already during the night. Won’t this be a problem when i spray the lime on during the day? During daytime it is not freezing yet. And probably will not for a while. Winter is unusually early this year.

    And what about the other question. Won;t straw rot because of lime plaster there where is stays wet to long? I heard that lime has an anti fungal reaction so this is why i am wondering.


  33. Andrew Morrison
    Andrew Morrison Wed, October 14, 2015 at 3:59 pm #

    Thanks for the added information patrick. In light of that, no, I would not suggest plastering this year. Do your best to wrap the house up in Tyvek (or similar material) that will allow it to breathe yet will keep water off. You cannot plaster with lime when the temperature will drop below freezing within 72 hours of application. Therefore, since you are already seeing freezing temps at night, you should not risk it. Bundle things up and wait until next year. The only other option is to install a plastic “tent” around the house so you can continue working. That tent can be heated. They do this in Colorado quite a lot; however, it is expensive. I worry a bit about leaving the house unfinished over the winter in terms of rodents, but I know people have done it with success. Good luck.

  34. Avatar
    patrick Wed, October 14, 2015 at 11:42 pm #

    Ok thanks. No worries about protection for the wals. I already have the roof up and it has at least 75cm overhang around so there won’t be water coming anywhere near the walls. I build it the french way. The Cut Under Tension from Tom Rijven. Did not notice any difference bye the way when cutting the strings. In my opinion this is not needed but did it anyway. The bales are so tight in between the wooden framing that there is no need to cut the strings. But it is an easy way to build and much cheaper than first placing an expensive wooden structure. Only nasty thing is that you can only start building the roof when you have all strawbales in place. Got it just in time for the bad weather 🙂 Well many thanks for the advise. And keep up the good work.

  35. Avatar
    Lakshmikant Fri, October 16, 2015 at 8:28 pm #

    Hi Andrew,

    We are re-doing our house in India that has existing concrete (Reinforced cement concrete) beams and columns. We intend to do lime plaster (lime+sand+water only) on the a red clay brick wall that’ll fill up the wall space between the columns and beams. We know lime plaster adheres very well to clay bricks and will give a fine texture. Our concern is whether the lime plaster will adhere to the concrete beam/column. Is there a way to solve this problem so that the entire wall including the beam and column have the same look? Please let us know.

    As an alternative, we were thinking of using a lime plaster on the walls with a stucco coat on the concrete parts (stucco= cement+lime+water+sand) and lime plaster (lime+water+sand only) on the walls. Can we use such a hybrid strategy, without compromising on the function and the look of the wall?


  36. Andrew Morrison
    Andrew Morrison Tue, October 20, 2015 at 8:20 am #

    I believe that the lime will adhere to the concrete beams and columns. That would be my preference. You may need to add a binder primer to the concrete before adding the lime. TO be sure, I suggest you contact Michel at He will know the best application for you.

  37. Avatar
    Alice Fri, November 6, 2015 at 8:04 am #

    Hi, I’m in southern AZ and I’m having the same problem you’ve discussed, I did lime plaster (with a little bit of cement added) over earth on my exterior, and it’s separating. It did last for 10 years though, and it’s still fine on the side of the house that’s covered by a roof. The lime over earth on the interior is also fine. I need to redo the outside and am not sure what to do at this point. Someone suggested lime plaster with roof emulsion to make it stick, but does that breathe? Another suggestion was to mix sand with elastomeric, but I’m pretty sure that wouldn’t breathe at all. How much should I worry about the breathing issue anyway? We do get 17 inches of rain here, but it doesn’t usually stay very wet very long. It is humid during summer rains though. I just don’t know how to proceed from here!

  38. Andrew Morrison
    Andrew Morrison Fri, November 6, 2015 at 10:58 am #

    Hi Alice. Sorry to hear of your situation. I would personally take one of two approaches. 1) I would remove the failed plaster, including the base coats, and apply three coats of lime plaster. 2) I would remove the failed plaster and refinish the walls with all earthen plaster (3 coats minimum). I am not an earthen plaster expert, so I don’t know the details about how to make it weather proof; however, Bill and Athena Steen ( are total pros on that and they can surely point you in the right direction. I would not want to use either elastomeric or asphalt emulsion for the issues you point out. Good luck!

  39. Avatar
    Robert Dunn Sun, November 8, 2015 at 10:53 pm #

    Hi Andrew,
    I’m buidling two cruck-frame strawbale houses on the very wet west coast of Scotland. I’m copying a design already built in a wet part of England – Cumbria – so the removal of damp from the internal part of the building is important.

    the original design uses lime mortar on both the inside and outside of the straw (with a breathing gap on the outside and a clay tile roof over this). However a couple of Dutch builders are recommending that I use ‘mud’ (loam) to plaster the inside which will be more breathable and healthier for the occupants. What are your thoughts? (I understand that the rate of porosity of both inside and outside of the bales needs to be the same to avoid moisture build up in the straw).

    In this situation would you be minded to go for mud, clay or a lime mix?

    Many thanks, Robert.

  40. Andrew Morrison
    Andrew Morrison Mon, November 9, 2015 at 12:55 pm #

    Hi Robert. I’m a big fan of lime plaster inside and out; however, I know others prefer the feel of earthen plaster in the interior. That is fine as long as you thicken the coat up to slow the porosity down a bit, as you note. It’s really up to you as it is possible to make either situation work. Best of success to you.

  41. Avatar
    anne-sophie Mon, February 22, 2016 at 1:43 am #

    Hi Andrew, i have build a straw house in normandy (France). It’s clay plaster Inside, and there is already clay plaster outside ( it’s about 4 to 8 cm). This plaster is a mix of clay, sand, and many things as fresh herbs and other things that make it véry strong because of fermantation. Now, i have to do my decorating plaster outside, and i would like to use clay plaster. But everybody here is telling me i should use lime because of the rain. My roof is not very large, and yes my walls get wet when it’ s raining (and in Normandy it can rain often !) But, my last coat is not to protect the straw bales but only for décoration. What do you think ? Is there anything i could had to my last clay plaster to keep it stonger ? should i use lime ? Mix clay and lime ? Tanks (and sorry for my English…)

  42. Andrew Morrison
    Andrew Morrison Wed, February 24, 2016 at 11:39 am #

    First of all, your English is fantastic, so no need to apologize. I do not work with earthen plasters very much as I prefer lime, personally. As such, I am not the best person to ask about how to create the perfect earthen finish. My thought would be to stay with the clay plaster for your finish and then apply a silicate paint or other water resistant finish to the walls. The silicate paint is great because it allows the walls to “breathe”, releasing vapor while also protecting them from water by adding a protective layer. It is great stuff. I would not recommend adding lime to the clay or adding a lime finish. I have seen to many failures with those approaches to consider them an option. Other than these two tips, I would suggest you contact Bill and Athena Steen at the Canelo Project as they are clay plaster experts and will likely have even more advice for you. Bonne chance!

  43. Avatar
    A.K. Harrison Thu, June 2, 2016 at 5:28 am #

    I am in the construction process of my strawbale home and will have a 10 foot porch wrap around the structure. I live in south Mississippi and have an abundance of clay available but not sure if it will be more maintenance than I care to endure. The hydraulic lime seems to be very expensive (shipping). Are there other options similar to hydraulic lime that can be used ie: hydrated lime?

  44. Andrew Morrison
    Andrew Morrison Fri, August 12, 2016 at 1:27 am #

    Hi A.K. I would suggest you go with a quality earthen plaster over a hydrated lime plaster since you have the wrap around porch. I think it will give you a better finish and will be relatively maintenance free due to the protection. I am no earthen plaster master though. For that, I suggest you contact Athena Steen at the Canelo Project. Good luck. Send pictures my friend!!!

  45. Avatar
    Jenna Sun, August 14, 2016 at 9:10 am #

    Thank you so much for sharing this beautiful, helpful & clear information.
    I’d mega-appreciate some of your insights/guidance with my situation…?

    We live in Eastern Canada (zone 4), and the timing for our bale installation will be Autumn- when the temperatures will change.
    We intend to clay plaster the exterior walls & install wood siding with a vented rainscreen. Our interior walls will also be clay plaster.

    I’m wondering:
    1) Should we do all 3 regular clay plaster layers for the exterior, (given it will also be sided), or is there a different technique for plastering in this case?

    2) Given the upcoming cold, are there risks/limitations in completing the exterior plaster before winter?

    3) If we can get the exterior done in autumn, would it be possible to then do the interior plaster as winter sets in- with a woodstove as our heatsource? (Or might wood heat be too drying and mess up the curing process)?

    Appreciate your time, if you’re able.

  46. Avatar
    John Plumridge Sun, August 14, 2016 at 5:36 pm #

    Good point Andrew about delimitation when laying lime over earth for exteriors.
    A couple of things:
    1. I’m nor sure what you mean by less sand in the base layers top form a weaker mixture. In line render systems, sand decreases for outer layers, and the strength of a render with sharper sand can be much stronger than an outer layer with less sand or different sand.

    So, would you clarify exactly what case you mean?

    2. Concerning not mixing different materials (applying lime over earth for example), what about the se of having an earth clay plus lime mix for render – do you have experience of that?

  47. Andrew Morrison
    Andrew Morrison Mon, September 5, 2016 at 5:41 pm #

    Hi Jenna. Sounds like fun! I would suggest a heavy initial clay coat only behind the rainscreen. No need to make it beautiful, just get it as flat as you can and make sure you have adequate cover of the bales to prevent flame spread. You should be okay with the winter temps as long as it doesn’t drop below freezing for a day or so after plastering. If it were lime plaster, it would need to stay above freezing for 72 hours after application. No problems with completing the inside plaster with wood stove heat. You are simply dealing with drying an earthen plaster, no chemical curing takes place.

    Have fun!

  48. Andrew Morrison
    Andrew Morrison Mon, September 5, 2016 at 5:45 pm #

    Hi John. Thanks for your questions.

    1. Plaster actually gets weaker as it moves out from the walls. The first coat is, for example, 2:1 sand to lime. The second coat would be 2.5:1 sand to lime. As you can see, the sand content is increased, not the lime. That makes the plaster “weaker” as it goes out. This allows the under layers to move and the coats on top can handle all of the movement. If the strongest layer were on top, it would have a hard time moving under the same conditions and delamination thus occurs.

    2. I have mixed lime and clay in the past, but not a lot. I did not like the way it came out. I am a fan of either pure lime plaster for all coats or all earthen plasters. I don’t like mixing. That’s my personal preference. 🙂

  49. Avatar
    Dan Kelly Sat, December 17, 2016 at 3:02 pm #

    My plaster on brick is failing by bubbling and cracking. Any chance I can send a picture?

  50. Andrew Morrison
    Andrew Morrison Tue, December 20, 2016 at 6:27 pm #

    Hi Dan. Sure. Send a photo to It may take me a while to get to it as I’m on vacation, but I’ll check it out as soon as I can.

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