Wednesday, July 2nd, 2008 | Popularity: 30%
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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. 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
Cement plaster is very strong and has great values for both compressive and shear strength. If used as part of an engineering system for your home, cement based plasters are the best option. They are also the most well known for plastering crews and stucco crews which means it is the medium they will most want to work in. The cost, therefore, is usually low when compared to lime or even clay if you hire the plaster out. The materials are readily available in most markets as are skilled crews to apply them. Colors are many and paint can be applied directly over the plaster when you want a change of view. The down side to cement based plaster is that it does not breathe well so should not be used in moist climates. In dry areas, I am confident that it will work fine especially if a heat recovery ventilation system or something similar is used in the home to remove moisture. 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.
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.
Popularity: 30%
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Andrew Morison is a licensed contractor specializing in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs. You can check these out at http://www.LearnStrawBale.com.
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July 2nd, 2008 at 10:10 am
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.
July 2nd, 2008 at 4:35 pm
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.
July 2nd, 2008 at 6:23 pm
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.
July 2nd, 2008 at 6:41 pm
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.
July 3rd, 2008 at 12:19 am
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?
July 3rd, 2008 at 8:02 am
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
July 5th, 2008 at 8:41 am
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.
July 5th, 2008 at 11:33 am
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.
July 6th, 2008 at 8:06 am
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
August 10th, 2008 at 2:39 pm
hi, andar here, i just read your post. i like very much. agree to you, sir.
August 11th, 2008 at 1:10 pm
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?
August 11th, 2008 at 6:56 pm
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.
August 15th, 2008 at 1:56 pm
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
August 15th, 2008 at 8:59 pm
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.
August 16th, 2008 at 6:32 am
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).
August 18th, 2008 at 7:25 am
As always Ken, thanks for your experience and comments.