There are so many things to consider when choosing a plaster for your straw bale home. I will try and give a quick outline here to get you all started on the path of inquiry. There are many decision to be made and many details to consider in those decisions. The plaster is the thing that most people will notice about your home, so make a wise and well informed decision. Below I will give some pros and cons to Clay, Lime and Cement based plasters. The decision is ultimately yours and I hope the information below will help you decide.
CLAY PLASTER (Earth plaster)
Clay plaster breathes well and can be inexpensive if the materials are available on site. The application of clay plaster is easier than conventional plasters for an inexperienced person. The plaster can be repaired easily without “burn marks” by simply misting down the affected area and reapplying the plaster. The downside of clay plaster is that it does not last as long as other plasters. Regular maintenance is a must as its durability is low. Direct water on the plaster can affect the finish negatively. It is not as strong as other plasters when considering shear and compressive strength which lessens the strength of the overall wall assembly. Finally, getting the right ratios of materials: clay and sand mostly, can be difficult and is somewhat of a science. This becomes important in the scratch coat for strength and in the finish coat for consistency of the finish appearance.
LIME PLASTER
Lime plaster is one step up in strength from clay and is my top choice. It is not quite as strong as cement based plasters for compressive and shear strength, yet it is considerably stronger than clay plaster. Depending on the type of lime you choose, it can be very easy to work with. I prefer Natural Hydraulic Lime (NHL) as it is simple to use and provides a great finish. You can buy plaster at www.limes.us (be sure to let them know you heard about them from www.strawbale.com).
Lime, especially NHL is relatively flexible and can actually heal cracks on its own in some cases. Lime is very durable and can be left alone once the plaster is complete. If you want to change the color or freshen it up, you can apply a lime wash to the finish coat at any time. The biggest drawback to lime (NHL) is the price. It is quite expensive and only available from a few distributors around the World. I use it on all my buildings and I believe it is worth the cost; however, if you are on a tight budget, it may be hard to fit in.
CEMENT BASED PLASTER
Although cement plaster is very strong and has great values for both compressive and shear strength, I would not recommend using it on your straw bale home. It is true that cement based plasters and stuccos are perhaps the most commonly used material for stucco crews and this means that the cost, therefore, is usually low when compared to lime or even clay if you hire the plaster out. It is also true that the materials are readily available in most markets as are skilled crews to apply them. All of that does not outweigh the negatives of using cement based plasters on bale homes.
They do not breathe well and are likely to trap moisture within the walls, causing rotting in the bales. Another down side is the environmental impact of cement. It is a very impactful ingredient and so that must be taken into consideration when making a choice. Finally, the material is very hard and as such, has limited flexibility. As a result, cracks are more common in cement based plasters and those cracks can allow water to make its way into the walls, again causing serious damage. In my opinion, cement based plasters, even when mixed with lime, should not be used on straw bale homes.
As you can see, there are a lot of options and a lot of details to consider within each option. My plaster of choice is NHL for the exterior and interior scratch and brown coat and then a gypsum, lime, plaster of Paris (Diamond Plaster or equivalent) finish on the interior (I did not even talk about this one!). I have done clay plaster, cement based stucco, and lime plasters and have had complaints and cheers about each. As with everything in my life, I shoot for a balance and try to stay in the center of that equation.
I was wondering if you(Andrew) or any one else has had any luck or experience with mixing the above plasters. There is a straw baler in this province (Alberta) who mixes lime and cement and I have always wondered if mixing lime with earthen plasters might mix the benefits of both – ease of use and durability. I have also read about mixing earth and cement but not as a plaster for a straw bale wall but actually as structure. This mixture reduces the amount of cement and lessens the environmental impact and I therefore thought that cement as a stabilizer for an earthen plaster might also be viable.
The statement about lime being self healing is a myth.
1. Autogenous heal only occurs in the presence of moisture if it happens at all. This amount of moisture moving through your wall will cause greater problems then the crack.
2 The level of autogenous healing that is discussed in literature applies to microscopic shrinkage cracks not visible to the eye. If you have a creack in you wall that are visible to the naked eye they will never be fill by autogenous healing.
Kelly,
I have mixed clay and lime with pretty good results. You can also use a scratch coat of earth plaster with a lime finish coat if you are looking to save money although I would recommend sticking with three coats of lime for best results. Anytime you start mixing materials, you have to work on the ratios to make sure your end product is of good quality. The more mixing, the more science comes into play and the more skilled you will need to be to keep the ratio of material consistent.
Ken,
I have seen visible cracks heal in finish plaster when using Natural Hydraulic Lime (I should say “crack” as I only saw it happen once). I have been able to float out cracks in the brown coat a couple days after it was applied. Keeping the plaster moist for a few days helps with the curing process and allows for such floating adjustments. I think you are probably right for the most part and that my experience with the finish plaster crack was more luck than anything. I have always been amazed with that experience to say the least. On the other hand the brown coat fixes happen every time I do them.
Thanks for the overview Andrew.
I like the idea of the durability of cement plaster for the exterior wall, but I also like the “softer” finish of the clay plaster for an interior wall. Given the different breathability of the clay and cement would you anticipate condensation problems in the straw if I was to combine a clay interior wall plaster with a cement exterior wall?
I would be concerned about using a less permeable exterior than interior. You will be able to drive moisture in to the walls at a higher rate than you can lose it. That sounds like a bad idea to me. If I were to try this, I would stick with a lime exterior while using clay inside or I would use the cement based plaster exterior and a gypsum/cement/lime plaster inside. In addition, I might use a natural paint on the interior to increase the resistance to moisture. If you do that, be sure to have a heat recovery ventilation system or something similar to remove the moisture from the house.
Andrew
Andrew,
I agree with your point about the “science” but I wonder if there is not a compromise to be found between the expense of lime and the environmental issues of cement. I have read some material on compressed earth blocks in which they use cement as a stabilizer. The proportion is anywhere from 10% to 25% and it seems to work with good results. Have you ever tried this with an earthen plaster to increase its durability? Also, I have read a little online about “poured earth” which combines cement and earth(on site) to form structure for a building. The person uses a slip form and I was wondering is something like this could be used to lessen the difficulty of plastering.
I have heard about the poured earth (stabilized). I am not a huge fan of the idea as it lays the “mud” on in one thick layer. This is a recipe for telescoping cracks (those which go all the way through from the exterior to the straw). I prefer a three coat method. I am sure that a quality plaster can be made by combining materials, I just want people to understand that it takes some experience and experimenting to get the mix right.
Kelly
This would work better if you were using natural cement. The natural cement has a higher mode of elasticity then Portland and also has a much lower carbon footprint. The problem is it is expensive.
Ken
Topic: water resistant stucco finish.
I have been using El Ray accrylic stucco finish for my inside and outside walls, thus “helping” to elimate moisture movement through the walls. Cost is getting “too” high for the El Ray product.
Is there another way to control moisture movement through the walls?
hi, andar here, i just read your post. i like very much. agree to you, sir.
Andrew,
We have cement plaster walls (inside and out) on our strawbale house. I always believed that in order to change the color you could not use regular paint on them as it would affect the breathability, that you had to use stain and you’d better get it right the first time. That is not true?
Paint can cut down on a walls breathability; however, certain paints have better permeability than others. When dealing with a dry climate and a house already wrapped in cement which does not offer much breathability, the paint does not highly impact the overall permeability of the wall.
Kelly
Lime doesn’t have to be expensive. The lime we used in this contry for years was plain hydrated lime. Only in the last few years have we been using NHL and other foreign material. If you slake quicklime and use it as a plaster it is cheep. It takes a little time to understand the process and develop the skill to apply but it works very well. NHL was not used in this country in large quantities until the late 1990.
Ken Uracius
Ken,
Good point about the lime. I have gotten so used to using NHL that I did not even cover hydrated lime in any depth here. My biggest reason for using NHL over quicklime is the ease of mixing and application of NHL. I have seen way too many plasterers with burns on their arms from working with lime. If done the right way though, it can be a great option for plastering. Thanks for putting that in there.
The danger of hydrated lime is when it is in the quicklime state. After the lime has been slaked there is no difference between NHL and Lime as far as caustic goes. The slaking process can be amazing to someone who has never seen it done and very easy to do. The chances of getting burns are rare if you follow so common sense rules.The cost is about 1/10 the cost of NHL. The green factor is huge (no boat from Europe).
As always Ken, thanks for your experience and comments.
I am curious about the inside stucco – drywall interface and what works best. I assume it is easier to hang the drywall and stucco to it. If you want to get the bales covered before drywall, is it possible to drywall after stucco?
Then could the inside stucco finish coat be the same as the drywall finish when ready (gypsum plaster, clay plaster, etc.)?
Amazing resource here – thanks.
Matt
Thanks Matt. I usually hang the drywall before I plaster, but as long as it is up before the final coat, you will be fine. Use a wide fiberglass drywall tape on each joint and union with the bales. Then use the same finish coat material on all interior walls for a uniform look. I prefer a standard plaster of lime, sand, plaster of Paris and gypsum. You can use any finish plaster you like as long as the texture matches the bale walls. I do not recommend using a different finish on your drywall as it will be very obvious as to which walls are drywall and which are bale.
Chadwick,
I’m not sure how, but I missed your comment back in July. I always try to ensure that the plaster on the inside of the house is less permeable than the plaster or stucco on the exterior. This means that if and when moisture gets into the wall, it will be easier for it to get out than it was for it to go in. This is not as crucial with lime plasters and clay based plasters as they do a good job of moving moisture anyway; however, here’s an example: Lime plaster exterior with lime interior base coats. The interior finish coat would be a basic interior plaster used in conventional construction like, Diamond Plaster, that has slightly less permeability than the pure lime. That will inhibit moisture from getting into the walls from the inside and will allow any moisture that does get in to be driven through the exterior skins.
Andrew cheers for the informative website, we are currently doing voluntary construction work in Liberia, as you can imagine after 16 yrs of war materials are scarce, but lime is available, we need to skim slightly damaged cement rendered internal walls,is it posible, can you reccomend a mix,should we P.V.A the walls first, can we mix cement and lime to use as a skim?????? Questions, questions, questions, would really appreciate your reply to this and am sure Mr Uracious will have something to say too!!!!! Thanks very much Lungs
I think Ken will likely be better to answer this as I do not work with cement rendered walls. My guess is that it is possible to do with success; however, I do not have any mix ratios that I can confidently give you. I hope Ken does. Thank you for doing such meaningful work in this world Lungs.
What is your weather like? I think you can use a lime and sand mortar mix without too much trouble. There are a lot of things to consider when designing a mortar mix.
I will be glad to e-mail you some information about mixes if you send me your e-mail address.
Good luck with your project.
Ken
kuracius@stoneandlime.com
Thanks Ken…again. Knew I could count on you!
I have been reading this with interest – our house is currently at a state of some plaster (cement/lime)on both inside and outside walls. What are benefits/problems of putting a lime plaster on over the inside as a finish coat? We live in Manitoba, where the climate is generally fairly dry.
That will be fine. Lime does well as a finish. My only issue with it is that it needs different size sands to hold together well. This can sometimes lead to shedding on the walls: sand coming off the walls when contacted. It is harder to get a smooth finish with lime, but it hides cracks better than most finishes. Like all things, there are trade offs. I have lime in 3 buildings on my land and after the initial shed, they have done very well.
Hi Andrew, my wife and I bought a straw bale home in south central NM in the adobe style (our elevation is 7300 feet). I know they used a cement stucco to finish the exterior due to the 5 gallon buckets in the storage room. The problem we are having is the walls that face the south are cracking (horizontal & vertical) and we have a leaking problem at the windows when the rain is driving against the walls (though this is not limited to only the south). I have eliminated the roof as being a point of entry for the water. This only applies when the rain is being pushed onto the walls. What is our course of action at sealing the exterior walls seeing that we don’t have a roof to protect the walls? Thank you for your time and excellent site.
That’s a tough one. Pueblo style roofs are not a good idea for bale homes because of the lack of overhang as noted. You could try identifying the cause of the leaks, probably the cracks. If those are indeed the issue, then a full resurfacing with lime plaster may be in order. I would avoid using the cement as it seems too brittle for the climate. The real risk is that the failures are in the structural element of the wire mesh and or bale backing. If that is the case, the cracking is likely to continue on future plaster coats. This is where a little lime could help because it is somewhat flexible and may be able to weather the movement with less surface cracking.
I am doing some work with adobe in Texas and the southwest has sme proBlems that are new to me but here goes. First could you get the name of the material you used for me of the buckets so I know waht I am dealing with. this will help me help to isloate what will work and what will not.I agree with Andrew that lime may help but i need to know a little more about you substrate.
Sorry I took so long to answer I am working at Ft Jefferson and e-mail has been down.
Ken Uracius
Ken Uracius
Hi Ken, sorry it has taken so long to answer. My brother in-law who is a painter is in town to help out said one of the chunks that came out of the wall has a first layer of concrete fiber, 2nd coat of mortar and a 3rd of (brand name El Ray out of Albuquerque) Perma Flex Elastomeric Synthetic Plaster. The product is 10 years old from when they built the place. Unfortunately I just found out this week that the builder who built this place had never built a strawbale house before. I hope this answers your question and helps you out.
I am not a big fan of Perma Flex Elastomeric Synthetic Plaster. Way too mutch goop. The buildings I saw at Ft Davis had this material on them and they had some problems. You may not have any choice but to use the same material. The cost of changing it out would be prohibited.
Try chasing the cracks and using a silcone to seal them and cover with the El Ray. I know this isn’t what you want to hear but i don’t think you can aford anything else. Some pictures would help.
Ken
Ken
We are aware that we can add 5% NHL lime to a clay base coat for greater stability (with the intent of having the outer coats all NHL) but after reading one of the comments above, it seems that more can be added. Do you have an approximate ration of how much clay to how much NHL 3.5 you would use? Would you also add a sharp, well-graded sand to the mix? How about chopped straw?
Hi Lynn. I know some people who are doing soil stabilization with lime and using it for plaster and or foundation work. That said, I am not an expert on it. I know that you want your plaster to get weaker as you move out from scratch to finish coat. If it gets harder, you run the risk of delamination and other problems. This is often what happens when clay plaster is used under lime plaster if very special considerations and precise recipes are not employed. I would hesitate to recommend the use of lime over clay unless the recipe is absolutely perfect. Even then, I would suggest sticking with the same concept: weaker as you go out. That would mean adding less NHL as you go out. I would definitely use the sand and perhaps the straw in the first two coats. No straw in the finish.
Why are you adding the NHL. Why are you using NHL in the first place? On what infomation did you base the design of your base coat? What do you expect to gain by adding NHL? If you can give me some more infomation I can help but it looks like you are headed for problems.
Ken
I agree Ken. If stabilization is the idea, I would imagine that lime putty would be a better option. I think the best thing is to either use lime or not, but to mix and match may cause problems.
Ken,
I wanted to add the lime to the base coat because I have heard of failure of walls using NHL over earthen base coats. It is my understanding that if you add some NHL to the base coat (5% is what I was told by someone with lots of NHL experience both here and internationally) that it would help stabilize the wall. I also thought that the lime added to the first coat would bond better with the lime in the next coat, providing more integrity to the whole wall from one coat to the next.
If we were wealthy, we would have all coats be NHL – we’re doing the best we can with the resources we have and, as such, we simply have to use an earthen base coat.
I have found several other references (including ancient ones) that mix lime with clay (earth) but I could find no specific ratios of lime to earth, sand, and fiber. That’s why I asked the question, since it appeared that Andrew, from a response above, had done this before. Andrew said above, “I have mixed clay with lime with pretty good results.” I was hoping he could tell me if he added more than 5%, less than 5%, whatever. I was just hoping for a second opinion from someone who has done this before.
Lynn
I don’t think the amount of NHL you are talking about using is going to do anything for you. The NHL is going to be too hard over the clay and NHL base and difficult to work with. It has to do with absortion the clay will absorb all the water out of you second coat and cause cracking.I have a good friend that works with adobe and plaster in the southwest. I will contact him and get some more infomation for you.
Ken
Ken
Hello There and thanks for the great resource.
I’ve just bought an unfinished straw bale house in Idaho. The exterior walls have 1 rough coat of cement stucco and the interior appear to be plywood with sheet rock finish. Three questions:
1) should we worry about water vapor issues with this setup? If so, how would we address the issue.
2) We don’t like the traditional look of drywall, can we plaster over it?
3)We would like to cut some windows in a couple of existing south facing walls. Can we do this?
Thanks a bunch. – George
Hi George. I’m a little concerned about the set up as is. Seems like a risky system. I’m sure you can make some changes to improve what you have there, but it will take some work for sure.
You can definitely plaster the interior, whether it be over the drywall or after removing it. That will depend on the solution to your first question.
You can cut windows into the wall for sure. The biggest concern is where the electrical is running. Be sure to turn off all the power when you cut into the wall. You’ll have to use battery tools or a generator to make sure you are safe. Another idea is to use a stud finder that can “see” electricity before you cut in. I would still recommend shutting off the power either way.
If you want help with this project, let me know. I am available as a consulting for such things. Good luck.
I had a question about the exterior. Would a rock facade have an impact on the ability of moisture to escape the walls? I am not a huge fan of the look of stucco, but I like the look of stone on the lower half of a wall with stucco above. Would that work, or would that cause problems with moisture? Also, does it matter which type of plaster to use in this instance?
I always recommend using lime plaster. It’s my favorite. What type of plaster will work for you will depend a lot on your climate. The stone facade will be fine. Again, use a lime plaster for the substrate (scratch coat) and use lime mortar in the joints. Be sure to install a water table above the stone at the transition to make sure no water leaks in behind the stone.
i have a 3 floor over basement terraced victorian home , built in 1860. the basement is undergoing electrostatic osmosis to treat rising damp. my architect suggests using lime plaster to reinstate the walls as before.the dampproofing professional recommends i use regular cement plaster because lime will just draw the moisture from below once again. what way to go???!am confused
That’s a tough one and a bit out of my field of expertise. There may be others here who can answer your question better than I can. Lime will move moisture through it better than cement, but I don’t know if it draws it out more. I hesitate to make a guess at this one as I don’t want to lead you astray. hopefully someone more qualified will chime in.
What a great resource here! We have a home in houston under construction. It is not straw bale, but what struck my interest to read your forum is we would like to finish the interior walls in plaster…the question is what plaster would be the best in our humid Houston climate? Looking at either American Clay or Diamond Plaster. I really like the “green” earthy element of the clay; but after reading this blog, the moisture absorbtion has me concerned. Can you pls guide me with an expert opinion?
This is an interesting question and as I don’t have the same level of expertise in conventional plaster as I do straw bale construction, I may not be your best referral. That said, I have used both products with success in conventional homes. If you are looking for a more green product, then American Clay is the way to go. It will be perfect and moisture should not be a problem at all. In fact, clay will absorb moisture, store it and then release it back into the air keeping your house in a state of equilibrium in relation to moisture levels. A very nice feature.
I love USG’s Diamond Plasters. However, gypsum’s properties of being slightly water soluble creates the problem of the future possibility of leaching efflorescence. That’s another reason to stay away from concrete stucco–portland cement is composed of -/+10% gypsum.
Anyways, here’s a little secret for my favorite finish:
1. Brush on the limewash (integrally colored, of course!)
2. Keep the walls wet with water mists
3. Brush on another limewash coat, and then immmediately take a 7 1/2 angle grinder and attach a round black brush used for drywall dusting, put some tape around the grinder bit so the brush stays. Turn it up to 1000 rpm.
Venetian Plaster!
Very cool Tim. Thanks for the tip. I love little secret tips like this!
To piggyback on Tim’s comment, I have some questions about lime washes…
We live in northern Wisconsin, where we’ve been working on our bale house for quite a while, but we’re finally getting around to our interior finish plasters.
In our bedroom we dyed the plaster with American Clay pigments, which worked well. We’re planning on using an iron oxide integrated in the finish plaster in another portion of the house. But for several different colors we were planning to do a lime wash with American Clay pigments.
We tried some test batches a few weeks ago and they’re still dusting off to the touch. I know lime takes a while to set up, but so far it seems like this will continually be a problem… will it? Or do you have a recommended mix/method of doing lime washes that will make it more durable? Does adding clay pigments change the mix? What would be the maximum ratio of pigment to lime? It also occurred to us that we could put some sort of clear coat or oil over it, but we’d like to avoid that.
As a background, our base coats are
1: native clay, sand, and straw
2: clay (native and/or bagged), hydrated lime, sand, chopped straw
3: a finish layer of hydrated lime and bagged silica sand, with or without a clay dye added for color
(4: the lime washes we tested used hydrated type S mason’s lime – Western “Miracle” brand – unslaked for any extended period, with various American Clay pigments added)
…I realize our plaster layers violate the “weaker as you go out” concept described to Linda above, but like her, we felt budget constraints, and we like having the bulk of our plaster be native materials. Our bedroom has been finished like this for nearly a year with generally good results.
Thanks for the great resource and for any help in advance!
We have built a super adobe cabin in South East NM, that has a conventional roof but no overhang. We have a lot of sand on site and no clay. It is hot, dry, cold and wet when it rains. We get a lot of winds in the spring. We live off the grid and plan to keep it that way. I need a recipe for the exterior plaster using the sand we have. I don’t have clay and want to use a cement, lime, sand mix or just lime and sand. Its impossible to find the recipes for the 3 coats. Please help thank you. Steve
Hi Steve. I use several mixes on my bale projects. They are below.
1) Natural Hydraulic Lime
Scratch (S) Coat – 2:1 Sand:Natural Hydraulic Lime and water to proper consistency (Varies)
Brown (B) Coat – 2.5:1
Finish (F) Coat – Either 3:1 or use the premixed Ecomortar which already has the sand:lime ratio in the bag
2) Non Natural Hydraulic Lime (add 3-5% brick dust to each mix)
S – 2:1 Sand:Quicklime (slake a minimum of 3 months)
B – 2.5:1
F – 3:1
3) Traditional Lime Plaster
S – 2:1 Sand:Lime Putty (If you can find it) or Slaked Quicklime (Basically making your own lime putty) Slake for at least 3 months.
B – 2.5:1
F – 3:1
4) Lime:Cement:Sand (You can try this one if you like, but I don’t like cement mixes personally.)
S – 6:3:1 Sand:Hydrated Lime:Cement
B – 7.5:3:1
F – 9:3:1