For some people, this is the most exciting part of the job. I personally love the plastering process and I think it helps the structure become itself. Before the plaster is applied, there is a construction job site whereas after the plaster is in place, there is a home. I know that not that much has actually changed, but the energy certainly shifts into something new for me. Many people think they want to do the plastering themselves and many actually follow through with that thought. I do believe though that most homeowners hand the job off to a professional. Quite honestly, I think that’s a good idea for most people. The reason is that the plaster is what people see when they first view your house. It is the one detail that they draw their entire conclusion about the quality of your home from. Plastering is not easy and plastering well is even less so. If you are wondering if you are right for the job, try it out at a workshop first so that you can make your decision with some hands-on experience under your belt.
We already spoke about the pre plaster details to some extent earlier in the E-course so you know the importance of a solid and tight substrate as well as the need for supported plaster across open spans of wood, etc. I even mentioned that my favorite plaster is Natural Hydraulic Lime (NHL) and that I think it is the best plaster for just about any straw bale home. Let’s take a quick look at some other options for plaster types so you can have a sense of what’s available. Keep in mind that no matter what choice you make, it is best to apply the plaster in at least three coats: the scratch coat, the brown coat, and the finish coat. Some plasters may require even more coats to achieve the finish you desire.
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 negatively affect the finish. 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 a mix of both art and science. This becomes important in the scratch coat for strength and in the finish coat for consistency of the finish appearance. I think that clay plasters are a great idea for interior applications more so than exterior applications. I love the feel of clay plaster and it greatly helps to moderate the interior moisture in the air due to its hygroscopic properties. If you use clay plaster on the interior and a less permeable plaster on the exterior, be sure to increase the thickness of the interior plaster to help regulate the moisture that passes through it. Doing so will help ensure that you do not trap moisture in the bales by mistake.
Lime plaster is one step up in strength from clay and is my top choice of all possible plasters. 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. As I have already mentioned, I prefer Natural Hydraulic Lime (NHL) as it is simple to use and provides a great finish. 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.
If budget constraints restrict you from being able to work with NHL, you can still work with lime by using a hydrated lime instead. The biggest issue with hydrated lime is that it needs to be slaked, mixed with water and then left to sit, for several months. I prefer a minimum of three months for proper slaking. Some locations, Australia for example, sell pre-slaked lime putty and that is a great product choice if available to you. Lime can burn you during the slaking process so be careful and understand the process fully before you get started.
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 straw bale homes because 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 manufacturing/production. It is a very environmentally impactful ingredient and 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.
SPRAYING VS. HAND APPLICATION
The good news is that either application process is acceptable. If you have a small crew, then spraying is probably the way to go. If you have a workshop scenario or a large plastering crew, then hand application is just as good and requires much less clean up. No matter which application method you choose, be sure to limit the amount of water you use in the mix. The more water you include the weaker the plaster will be. For a long time, I was under the impression that in order to spray a lime plaster, I would need to water down the mix and this was precisely why I did not like the idea of spraying plaster. I have learned over the years that this is not the case and that plaster can be sprayed at the same consistency as hand application mixes as long as you use quality equipment.
Even when using a sprayer, you still need to hand trowel in what you have sprayed so it does not completely eliminate the need for hand troweling. If you are like me, you will find a certain meditation and rhythm to hand troweling that is lost in the noise of the machinery when spraying. Although there may be speed advantages to using a sprayer, I still prefer the quiet and peaceful application of hand plastering with a hawk and trowel. Like I said above, either one is acceptable, so it’s just a personal decision you will have the opportunity to make when you get to that point of the job.
Thanks for Joining Me!
This concludes the 16 Day Straw Bale E-course. I hope you have learned a lot and have been inspired to build your own straw bale house. I so love this technique and am always pleased to see more and more people learn about it and continue to share it with their friends and family. No doubt there will be those who think you are crazy when you first mention the idea of straw bale construction to them; however, I am yet to find someone to stay skeptical once they have experienced a bale building hands-on…especially if the weather conditions outside are extreme in any way!
If you would like to learn more about straw bale construction, please visit www.StrawBale.com/Store to check out our instructional DVD series, hands-on workshop locations, construction plans, consulting packages and more. I hope to meet you one day in the future and continue to inspire you towards building your own dream straw bale home!
As an ECourse graduate, we would like to extend to you the opportunity to receive 25% OFF your purchase on any of our DVDs and book (sorry this discount doesn’t apply to workshops). Simply type in “ECourse” into the Coupon Field when checking out and the cart will automatically readjust.