Too Many Plaster Failures

Written by Andrew Morrison

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.

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Thanks for posting this. Advice to heed!

You’re welcome Ziggy.

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… Read more »

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.

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

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.

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…

That is correct.

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.

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… Read more »

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.

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!
Stephanie

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.

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… Read more »

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… Read more »

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.

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.

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… Read more »

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.

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

thanks so much! big help!

Hi,

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?

Thanks,

Dawn

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,… Read more »

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… Read more »

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?… Read more »

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… Read more »

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.… Read more »

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… Read more »

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… Read more »

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… Read more »

Andrew,
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?

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)… Read more »

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… Read more »

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

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

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

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

why you dont answare ,and you close my questions

Hi Andrew,

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

Andrew,
I can’t seem to find any info on the use of expansion, construction, or control joints in the stucco/plaster walls. All the stucco contractors I’ve talked to want to know if I want any of these. I can see the benefit of construction joints. It gives the stuccoers a straight line to stop and start the next day, but I’m not sure about any of this. What can you recommend?

I am building a straw bale home. We have already stuccoed the exterior using a traditional cement based plaster. The house exterior was also wrapped in tar paper prior to stuccoing. The interior is not stuccoed yet. I’m considering using a structolite plaster. After reading this I understand I may have some issues. I live in a fairly wet climate.. south Mississippi. Could you also explain how the interior would absorb the same amount of moisture as the outside (given the same material and thickness exists on the two stuccos) if the inside is a dryer environment? There is much… Read more »

Hi Andrew. I’m in New Zealand and looking at doing a lime plaster on an earth floor which has a concrete base underneath it. The earth floor was originally years ago oiled with tung oil. Granted I have to scratch that up to create a key but my main question is what type of mix would you recommend and how thick to create a reasonably robust lime plastered floor?

Can I ask a question? Can I use an ordinary cement for the straw bale house? Is there any possibility that the wall will crack? Thanks Andrew.

Hello there, I’m just beginning my journey into natural earth building. I wanted to ask about using gypsum for interior plaster work, it’s been traditionally used in France for centuries and it has seemed to escape the natural building movement in the states. I am by no means an artist so I’m trying to find info out about different interior plasters that could be used to cover the stray bale that would allow me to use wood based mouldings such as baseboards and crown. Any info would be appreciated !

Hi Andrew, I have built a cob surround for my wood stove. The surround is cob on cement board and I then built a cob arch with a stone mantel and smeared the existing fireplace with a thin coat of cob to match. I now want to cover with a lime plaster and don’t want to wait weeks for a putty to set up. I have been thinking about use type S with sand and covering the cob with that for a finish coat. It is in the house so it will be dry and I would like it to… Read more »

I have a strawbale house in central Texas with lime plaster outside and clay plaster inside, clay being much thicker than the lime, as you suggested. I’ve had no issues, but some have told me I have to do a limewash. I love love love the finish plaster, a mix of 2 different sands, one yellowish and one pinkish. If I limewash, I’m afraid it will be more uniformly colored. Is it possible to do alternating layers of differently pigmented limewashes to get a similar effect? Do I really NEED to limewash if there isn’t any erosion? Any advice would… Read more »

Dear Andrew i am going to start a straw bale house in uruguay….i thinking to use lime for plastering the wals but there are not hidraulic lime in the country, there is only hidratated lime….it is possible tu use or wuath do you suggest
Thank you a lot for your time

Hi just looking for advice about adobe internal render over cement. Is this OK?

Dear Andrew, what do you think about the GREB straw bale technic where they use a straw bale wall and a mix of cement, hidratated lime, saw dust and sand stucco. This mean to be a breathable stucco
What is your opinion about it?
Thank you a lot

Hi Andrew, we are in the stage of prepping our walls for plaster in the next month. Mesh, lathe, burlap patches, stitching, etc. I’m curious as to the importance of a level or plumb wall. I realize that small inconsistencies in grade will be forgiven and made up for by the plaster and we are not looking for a “drywall finish” look. But what about a difference in bale depth of 2-4 inches. Will we be able to just load up the plaster inter scratch coat those spots? I’ve looked for images but all I can find is perfectly plumb… Read more »

Hi Andrew, We are getting reading to build a staw bale wall 4.5 foot tall around our cabin and garden in Colorado, high plains area. It appears that the lime based is preferred. Can you tell me the steps for covering and mixing the three coats I will need. I’m not sure if the lime based should go right over the chicken wire or if I need another layer of anything first. We have a few quarries near by for clay, sand and lime. Or, I can simply get the bagged lime at the store but, what they have is… Read more »

Hi Andrew, We are building our Strawbale house in south western Ontario (humid summers, snowy winters). Our plan is a clay slip over the straw (clay/water mix), earthern plaster inside (clay, concrete sand, straw) 1″ thick and a thin finish coat (clay, brick sand, cottontails) on the inside. On the outside, our plan is a clay slip over the straw (clay/water mix) and a lime/clay earthern plaster mix (for example, 1 part lime, 4 parts clay, 10 parts concrete sand, 10 parts straw). The lime will by type S bagged lime. We are planning on putting strapping on top of… Read more »

Hi Andrew. This may or may not be relevant to your experience, but working with cob building, doing a finish lime plaster over an earthen plaster, do the same rules apply in this situation? I’ve been building with cob and LSC in china for the past 3 years and still learning the challenges of plastering. I’m returning to finish a mixed cob / LSC building next month and would really appreciate any insights you have on this. I also don’t have any NHL available there, only type s builders lime and my general sense is that it’s a weak binder.

Hi Andrew I’m British but I now live in Malta. The houses are made of Maltese stone..it’s a type of Limestone. I’ve bought a shell aparment. It’s made of Maltese stone but on the outside walls there is gypsum render. It’s an apartment block so i can’t change the exterior gypsum render. In Malta the new trend is plastering over the Maltese stone on the interior side with gypsum plaster. It’s humid here all year around. I feel I will create condensation/mould damp problems if I use Gypusm plaster on the interior walls. The Maltese limestone absorbs the moisture in… Read more »

Andrew, We live in a passive strawbale home in Northern Virginia that was built in 2009. A lime plaster was used and I don’tt believe that wire mesh was used, meaning that the plaster coats were directly on the bales. Now we are experiencing some cracking/chipping on predominately the south side of the home at the southeast corner, under some of the first story windows, and a few other areas. These spots are in need of repair. I’m a capable handyman, but very unfamiliar when it comes to working with stucco. Can I just use a 1 part lime, 3… Read more »

Hi Andrew, We bought a post and beam straw bale house in NW Arkansas and have been living in it for a little over a year. The house is approximately four years old and we are seeing worsening problems with the plaster—cracks of varying depths and some crumbling at the base of the stucco on all exterior walls and some delamination on the extreme weather side. Regarding construction, we’ve learned that: — The exterior and interior scratch and brown coats are locally harvested clay and the final finish coat is hydrated lime. — The scratch coats were embedded in the… Read more »

Hey Andrew, hope this finds you well, very helpfull website btw. We are from Melbourne and about to inspect a straw bail house to buy. Is there any way of telling if earthen plaster was used under lime without chipping at it? Any hints what to look out for to know house was built with knowledge? It’s 12 years old. And if you have a chance to have a look, what do you think about this roof and it’s connection to the walls internally? (Also corrugated metal sheeting for ceiling inside and out, any problems that you see?) It’s at… Read more »