How to Choose a Plaster System, part 3

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

In the previous article we began by exploring the first three parameters in depth.  We will continue by exploring the next three parameters and look at the in the context of the list.  We will begin this article with a discussion of “Need and expectations of the client.”  To refresh your memory, here is the list of all of the parameters:

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

AESTHETICS

Lime plaster with a fresco finish

During the design process the appearance of the finished project sits in our “minds eye” and moves us to make revisions and refine what we are thinking.  We have a desire to end up with something that is either pleasing and balanced, out-place and provocative, or somewhere in-between.  Our goals may be different, but whatever they are plaster will play a role in the final aesthetic.  Each type of material has a range of colors, textures, depth and applicability when it comes to aesthetics.  We will explore some of these characteristics.

Earth plaster is unique in that your base color will be the color of the earth, or clay, you choose to use.  Clays can be found in countless colors across the planet.  What is important to understand is that earth plaster alone is difficult to color.  You may decide to cover the earth-tone base coats with a colored finish coat, such as a clay aliz or limewash.  Many people enjoy the earth-tone color of the clay in the plaster and decide to just expose it.

In contrast, lime, gypsum and cement are lighter in color to begin with and can be colored more easily.  They can be colored to match earth-tones or be very bright.  Coloring is almost limitless by using paint pigments, clay pigments, mineral oxides, etc.  However, the darker the color, the more pigment needed.  Making a lime plaster dark can sometimes compromise the quality of the plaster due to the amount of pigment necessary to make such a dark color.  Do some research with a plaster consultant before deciding to make a light material dark.

Texture is also important.  All of the materials can be finished with various textures.  The type of texture will depend on how fast the material sets, or hardens, which determines how long it can be worked after being applied to the wall.  Smooth textures usually require more working time than rough textures.

USE OF THE STRUCTURE

How will the structure be used?  This is a question that is either implied by the design, such as a residence or an office building.  But it can also not be as clear, such as an industrial or commercial building that could see many different uses throughout it’s useful life.  A mechanic’s shop or garage in the backyard my require extra structural elements for lifting motors and machinery.  An indoor mushroom farm will require high humidity and constant temperatures.  We need to be aware of the limitation of each plaster material and system when when choosing one to match the use of the finished building.

We could come up with any number of hypothetical uses of a bale building, but the main issues that the plaster will influence are moisture and structure.  How permeable is the plaster and how stiff and strong is it?  These parameters are part of our list.  Understanding what the needs of the occupants will be throughout the life of the building will help inform our choice of plaster due to the different characteristics of each material and reinforcing system.

LOCAL CLIMATE (MICRO / MACRO)

Siding over bale walls on a second story

The climate at your building site will influence many components of the building, including the plaster.  How much wind-driven rain will hit the building?  How many dry times are there throughout the year?  How fast will the wind blow, or how much force will it place on your building?  How humid is the climate throughout the year?  Is there shade from trees or protection from wind?

In terms of durability from rain and wind, earth plasters will be the least durable, as discussed in the first article.  Lime will be more durable than earth, while cement will be the most durable.  If your project is in a wet climate with high humidity throughout the year you may consider not finishing your building with plaster at all!  You have the choice of finishing the building with a rainscreen assembly, or some form of siding.  In this case you will still need at least one coat of plaster behind the rainscreen assembly, but it will not be subjected to direct contact with moisture so your choice of materials may change a bit due to this fact.

Permeability of your plaster will also play a role with the climate.  This is a topic that we will cover in more depth in a later article in this series since it is on our list of parameters.  How easily water vapor can move through your plaster may play an important role due to your local climate.

To wrap up our discussion on these three parameters, how the building will look, who will be using the building and what your climate is like all play important roles in determining your plaster system.  There is not one material that will be the solution for all situations in all locations on the planet.  We must make honest, logical decisions based on facts, not our emotions.  We will discuss the final three parameters in the next article in this series and hope you will return for that installment.  Thanks again for reading and return soon for the final installment.

Popularity: 25% [?]