What is Light Straw Clay?

Light Straw Clay Construction (New Society Publishers, 2017) by Lydia Doleman is an essential how-to guide for those looking to start light straw clay projects. From the basics of LSC to project plans featuring LSC, Doleman gives readers everything needed to start out on their own successful projects. In the following excerpt, she explains what light straw clay is.

What is light straw clay? It is an infill system of loose straw lightly coated in clay that is packed into forms (temporary or permanent) to insulate and enclose a building. It is not load bearing, but it can be mixed to a variety of densities. The “light” in light straw clay refers to two things: the amount of clay (less than in traditional daub in wattle and daub) and the material’s weight (it is very light). Light straw clay is the English translation of the German word “leichtlehmbau.”

Light straw clay was developed in Europe mas an evolution of the wattle-and-daub infill system that filled in the spaces between structural members in half-timbered houses from the 12th century, on. According to Robert and Paula Laporte, North America’s preeminent light straw clay experts:

The light version was introduced after World War II, in Europe. The modern examples that Robert studied were infill systems like the wattle-and-daub predecessors, in which the light straw clay mixture was placed between a post-and-beam framework. In order to meet modern-day energy demands, this “lighter version” doubled the wall thickness and halved the density by introducing more straw, resulting in a radical increase in thermal performance.

There are a few other frequently used names for light straw clay: light clay, straw clay and slip straw are commonly used, and it has also been referred to as rammed straw (akin to rammed earth wall systems that have little-to-no fiber in them). For the sake of consistency, all of the above will be referred to in this book as light straw clay, or LSC.

Advantages of Light Straw Clay

As a light straw clay wall system is composed of more fiber (straw) than binder (clay), it is much lighter and much less dense than other natural wall systems like cob, adobe, or rammed earth.

Nontoxic

LSC is essentially nontoxic. Clay is known for its ability to absorb toxins due to its relatively large surface area and negative ionic charge. Because of this negative ionic charge, it absorbs positively charged ions which most bacteria, viruses, fungi, and toxic chemicals have an abundance of. Combined with straw (which has minimal pesticides and herbicides applied to it as a crop, with only two applications being applied per year and not near harvest), it creates a nearly nontoxic wall insulation and enclosure. Being made mostly of cellulose, it is a relatively benign material and, like clay, it does not off-gas any volatile organic compounds (VOCs), unlike many other manufactured types of wall insulation.

Low-Tech Method and Materials

In addition to being a building material that is not hazardous to your health (although working with straw and dry clay can be very dusty, so it is best to wear a respirator), it is also very “low tech.” The mixing and manufacture of light straw clay can be done without electricity: either manually or with a tumbler. Because the material is very lightweight, you don’t have to be a muscled, construction worker to participate in the installation. It can quite literally be fun for the whole family — including grandchildren and grandparents. It can be pleasant work; the job site can be quiet, the tools are very simple, and the material is all lightweight, nontoxic, and easy to install.

High Vapor Permeability

Light straw clay walls are highly vapor permeable. This is a benefit because vapor permeable walls allow moisture to exit a building instead of being trapped in wall cavities. Breathing, showering, cooking, etc., all generate water vapor in a building. Naturally “breathable” (aka vapor permeable) wall systems allow moisture, in the form of vapor, to move through the walls without the need for mechanical ventilation. Combined with clay-based binders and a clay-based plaster, an LSC wall system can help regulate humidity in a building, creating a much healthier environment in terms of air quality. Vapor barriers are discouraged for LSC wall systems because they trap moisture moving through the natural wall system, where it can condense and be trapped. If water in a wall system can’t escape, mold can form, which can be toxic for inhabitants. Clay’s hygroscopic capacity allows it to absorb high volumes of water vapor and then release it — “breathing” it out — without compromising the straw or wood that it is in contact with. So, LSC in general, thanks to clay’s properties, will regulate dampness, which also helps maintain its insulative qualities.

Place-based Architecture

The reason travelers find many types of vernacular architecture so appealing is partially due to their unique expression of people in a place. A building can be a reflection of place, and light straw clay buildings can embody place-based architecture by using materials from a structure’s own site. One of the most common materials to use from a site (besides wood from harvested trees) is the earth excavated to make room for the foundation of a building. The amount of excavated earth may provide an adequate amount of clay for both wall infill and plaster skins. What better way for a building to fit into its environment than being built from materials found on the site?

Carbon Sequestration

Light straw clay being predominantly made of straw has the capacity to sequester carbon. Straw is made of cellulose; the chemical formula for cellulose is C6H10O5, making straw approximately 45 percent carbon. If a building uses 100 2-string bales and each bale contains approximately 19 pounds (8.6 kg) of carbon, that is about 1 ton (860 kg) of stored, or sequestered, carbon. This, in conjunction with earthen plasters, energy efficient design, and low impact living can substantially lower the carbon impacts of building a new residence.