The Material Ecology of a House: Alternative Wall Systems

For the past few blog posts, the material ecology of a house addressed various wall systems and the foundation system. This week, alternative wall systems will be reviewed to introduce some sustainable building materials that are less common. The material ecology considers the life-cycle of the building material, from sourcing, manufacturing, implementation, maintenance, and end-of-life alongside the reciprocal labor, health, and environmental impacts. The alternative wall systems that will be briefly reviewed include rammed earth, straw bale, hempcrete, bamboo, and 3D-printed concrete and earth. Similar to the wood frame or masonry construction types, each of these exterior wall systems provide their own structural integrity.

Rammed earth

The use of earth and soils for wall construction is one of the earliest techniques for building. While masonry unit construction is the most common form of using soils for building, rammed earth is an alternative method for making wall systems. This construction method does involve some specialized labor as well as wood formwork and reinforcing rebar, but it is typically made with earth sourced directly from the site for construction. Rammed earth is effectively the repeated tamping of layers of earthen soils stacked within formwork to reasonable heights for wall systems. There is no cement in the rammed earth system and typically little need for transport of the material, so the carbon footprint is minimized.

The wall system is quite thick, ranging from 18-inches to 24-inches in depth, and creates a solid earthen mass. This wall type is excellent for arid regions where the heat capacitance and thermal mass of the walls can help to balance out diurnal temperature swings in winter and summer because of the amount of heat that can be stored within the walls. In some cases, for non-arid regions, rammed earth can be combined with an internal layer of insulation material, resulting in earthen outer and inner faces of the walls while managing heat transfer with the insulating middle. These earthen walls are extremely stable and provide immense load-bearing capacity but are not very adept for seismic zones. There is no particular finish material required for rammed earth since the earthen layers create their own beautiful coloration and gradient of reds and browns based on the particular soil mix.

Straw bale

Straw bale construction is a method of tightly bundling hay into regular sized modules, or units called bales, and then stacking them side-by-side and in layers vertically to create building walls. These bales are quite thick, usually 24-inches to 36-inches in depth and provide immense insulating properties. Straw bale walls can be created with local materials in most regions and are relatively lightweight for installation and transport. Insect and fire-resistance treatments can be added to the bales, which are also covered with lath and plaster for finishing the exterior and interior faces. Straw bale construction is well-suited for cold climates because of its high insulation value. The material resource is renewable and biogenic, reducing the overall carbon footprint of construction.

Hempcrete

Hempcrete is a relatively new composite building module that is comprised of some portion of concrete mixture with a larger ratio of hemp agro-waste. The agricultural byproduct from hemp farming includes sheathes of hemp stalks and leaves that are dried and usable for binder materials in construction. The hempcrete block has become popular alongside the legalization of hemp growth in the US from the 2018 Farm Bill, resulting in farming industrialization at larger-scale production rates with greater amounts of byproduct waste material.

Hempcrete blocks are often composed into similar size modules that the concrete masonry unit (CMU) blocks are made, making the specialized labor practice translatable from CMU to hempcrete construction. These walls serve as load-bearing systems and have inherent fire-resistance because of the cement content. The overall carbon footprint of a hempcrete block is greatly reduced compared to a CMU block because of the amount of biogenic and renewable content from the hemp fibers. Similar to CMU wall construction, hempcrete walls will incorporate reinforcing rebar and the addition of some insulation and finish materials on both the exterior and interior faces.

Bamboo

Bamboo construction is common in many hot-humid regions throughout the world. This construction type makes use of rapidly growing bamboo stalks for structural framing components. The components are round in cross-section and smaller than the dimensional lumber that is used in wood frame construction, therefore the quantity of bamboo elements increases and the spacing between members decreases across the bamboo wall system. A single layer of bamboo members does not create the same wall thickness that can be achieved with dimensional lumber. Bamboo frame walls create thin and permeable systems that are useful in hot-humid conditions to encourage natural ventilation. In some cases, an outer bamboo frame can be complemented with an inner bamboo frame to create a thicker wall system that can be infilled with insulation. The bamboo frames can then be covered with lath and plaster or other sheathing or siding materials.

3D-printed concrete + earth

A newer trend in the housing construction market is the emergence of 3D-printed concrete and earth wall systems. The use of 3D-printing equipment for large-scale construction is taking place directly on-site with specialized concrete mixtures that are optimized for the additive printing process. 3D-printing, or additive manufacturing, reduces the need for formwork that is otherwise required for concrete or rammed earth walls. Reinforcing steel is still needed within these wall systems, but exterior and interior finishing can be minimized depending on the quality and material mixture from the printing process. While there are still some limitations with 3D-printing of wall systems, such as height limitations, curing time and conditions, and specialized labor trades, there are a handful of case study examples throughout the US that are successful.

Each of these alternative wall systems have major benefits for overall sustainability and can be discussed in greater detail in future blogs on healthy building materials. More immediate future blogs about the material ecology of a house will cover the roof systems, MEP (mechanical, electrical, plumbing) systems, and interiors. If you are planning for any new home construction or renovation and are interested in a healthy material ecology, reach out to AIDA, LLC today for a consultation. You can always find more information and healthy home resources at Aletheia Ida Design and Architecture, LLC (AIDA, LLC) at www.aletheiaida.com.