How to Choose a Plaster System, part 2

Earth Plaster Workshop in Siberia, 2005

This is the second article in a four part series explaining all of the issues surrounding the choice of a plaster system on straw bale walls…

In the first article of this series we were introduced to a list of parameters that we would use when choosing a plaster system.  This article will focus on the first three parameters in depth.  To refresh your memory, here is the list of all of the parameters:

• Cost
• Maintenance / Durability
• Strength
• Needs and expectations of the client
• Aesthetics
• Use of the structure.
• Local climate (micro and macro)
• Who is doing the work?
• Permeability
• Environmental Impact

COST

When considering any system in a building, cost will usually be the one factor that has the shortest but most potent impact on your life.  We tend to make decisions based on cost alone while excluding issues such as the return on our investment compared to other choices, as well as how convenient the choices are. Obviously the actual materials themselves will have a financial cost, but they will also have a relative environmental cost.  We are not concerned right now with the latter issue.  This parameter deals only with monetary costs to the owner of the project.

The plaster materials that are most commonly used on bale walls are earth, gypsum, lime and cement.  Many times a plaster will be a combination of these materials.  Earth is obviously the least expensive if it comes from your building site.  It can cost quite a bit if it comes from a long distance.  Most earth used for plaster that comes in raw form to your site (such as clay from a pit somewhere) will need to be processed.  It must be screened or soaked to break it up into a form usable as a plaster.  These processes can cost money if you are paying for labor.  Earth plaster can also come pre-processed in bags at a premium cost just like cement, lime or gypsum.

Cement and lime are similar in cost and can be purchased almost anywhere.  Of course there are different types of each, but we are referring to hydrated lime and typical Portland Cement.  Hydraulic lime can be very pricey, but is very uniform and great to work with.  Much of the labor cost will be in mixing and applying the material as opposed to the processing cost of earth.

Gypsum is also common but usually not available at any hardware store.  It can be found at plaster suppliers in the form of “diamond”, Gypsolite, Structolite, etc.  It is very uniform in nature and has a very fast set.  It is not appropriate for exterior use so it is usually used as a finish material on the interior.

While this article is supposed to be a guide, we are not going into much more detail in terms of relative cost of these materials.  That is the homework you will need to do.  For example, how much will it cost to plaster your building using each type of material if you are doing it yourself, with one other person, or by a professional plaster crew.  Cost obviously relates to the parameter “Who is doing the work?”  Basically, even though it relates to almost all of the other parameters in one way or another, the cost will be how much you will pay for what your choice is at the time of construction.  As any professional contractor does, multiple bids may need to be collected using different materials once the list has been narrowed to a couple choices.

Finally, the cost of ongoing maintenance should also be considered.  An earth plaster will require more maintenance over its lifespan than lime.  If you are a homeowner who is not interested in relatively more maintenance associated with the softer plaster materials, the financial cost should be part of the “cost” equation.

Cement plaster on 2-story wall

MAINTENANCE / DURABILITY

Maintenance can be a deciding factor alone for most people.  Some folks will want to maintain their plaster very little over the years while others will want to reapply their plaster every year or two.  Whatever material you choose, make sure you understand how much maintenance is involved.  Over the years stories have been told of earth plaster failures within relatively short time-periods (such as a couple years).  When a little research is done, it turns out that it is called a failure because the owners did not fundamentally understand that earth plaster requires relatively frequent attention and maintenance.  Make sure you understand how durable your finished plaster will be, and how often it will need to be maintained.

As mentioned above, the next obvious question regarding maintenance relates to cost.  How much will it cost to maintain your plaster.  If you will be performing the maintenance yourself the cost should be minimal if you are using earth plaster.  It will be the most if you are using cement or lime.  However, if you are a common homeowner with no clue as to the process of maintaining plaster, the cost is directly proportional to frequency of the maintenance.  For folks who will be paying for the labor of maintenance, earth plaster can become quite costly over time.  Cement and lime plaster require maintenance less frequently which is why they have been so wide-spread in use.  A vast majority of homeowners will not be able to maintain their plaster on an annual or bi-annual basis and therefore will require a more durable plaster.

With the advancement of earth plasters over the past few years, the frequency of maintenance can be decreased.  Whatever material you choose, make sure it matches your maintenance budget as well as the up-front cost.

STRENGTH

From the very first day each of us works with bale buildings it is easy tell they are very strong and solid in comparison to their framed counterparts.  An eight foot tall bale wall weighs roughly 500 plf (pounds per lineal foot), whereas a framed wall using 2×4’s with drywall and plywood weighs roughly 120 plf.  Just the weight alone increases the stability of the wall.  We know from research that the rigid plaster elements on each face of a bale wall resist all service loads imposed on the building.  If we wish to use bale walls as structural elements within our design we must pay attention to the plaster materials and what type of reinforcing is used.

It is easy for many folks to say to themselves, “I am going to use earth plaster and no mesh on my walls and save a bunch of money compared to cement plaster with 17 gauge plaster mesh.” But making this choice without designing another load-resisting system, such as x-bracing or internal shear walls, may prove to be problematic.  First, it could increase the maintenance over the lifetime of the building  This could happen by cracks developing from movement caused by wind.  This in turn could lead to premature failure of the building if cracks develop in soft plaster and allow moisture to soak the walls.

For all other types of construction, it is required by code to understand what systems will resist lateral and compressive loads such as wind and snow.  Most of the time, if plaster is used on the exterior of the building, it can also be used as a rigid structural element and quantified by either the building official or an engineer.  If we overlook this design step we potentially open ourselves up to future problems that are more expensive to fix than if a proper system was chosen in the first place.  In fact, it will still be many years before we actually see the effect of our decisions when it comes to strength because an event that could crack soft plaster may not occur for years after completion of the building.

Understanding the strength of your walls requires that you first determine what loads will be placed on your building.  This means understanding how fast the wind blows, how much snow falls, or how much force an earthquake would place on your structure.  If you live in a place with light winds, no snowfall, and no earthquake activity, you may be free of thinking about the strength of your plaster.  But there are few places on the planet that have all three conditions met.

The strength of the plaster system depends not only on the plaster material, but the type of reinforcing used.  In areas with earthquakes, heavy gauge mesh and cement plasters are more common.  In areas with moderate to heavy wind loads, cement and lime plasters with moderate reinforcing will be common, while earth plaster could also be used but may require additional structural systems to resist the larger loads.  In this last case, choosing earth plaster could end up costing more because the framing system would need to be strengthened and stiffened to resist the loads that earth plaster alone would be unable to provide.

These are real issues, and again relate to cost.  Most importantly, you do not want to under-design your building and have cracks develop in your plaster.  Make sure you are confident this will not happen with whatever system you choose.

Finally, when looking at these three parameters it becomes clear that they relate to other parameters and cannot be used in isolation of the others.  The process of eliminating materials and systems from your choices is iterative and may take a few rounds before you feel comfortable with your final decision.  In the next installment we will look at the next three parameters, throw them into the mix and hopefully narrow our choices down.  Ultimately, this exercise is about the process of elimination and getting to the best choice for your particular situation.

Thanks for reading and we will look forward to your return.

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