The Material Ecology of a House: Masonry Walls

The material ecology of a house, (which addresses the interrelationships between the materials utilized in construction and the processes, labor, and environmental effects), can be a complex puzzle. This blog post focuses on the material ecology of masonry walls that may be utilized for residential construction. Masonry construction systems consist of non-technical ceramics, which are earth-based soils and silicates that are blended with water and other agents to cure with certain strength and performance properties.

Masonry systems are also unitized based on the size of a single brick or block and can be combined to create various patterns and overall wall dimensions when stacked together. The bonding of masonry units is created with mortar joints acting like a ceramic glue between bricks or blocks. Masonry wall systems are typically designed as load-bearing walls for the structure of a house in order to carry the span of the roof framing. Because of this, designs for masonry-based wall systems have specific structural integrity that is necessitated and often requires the addition of steel reinforcing within the walls to account for some tensile forces that will naturally occur.

Some of the earliest examples of masonry construction include sun-dried adobe earth blocks. In some regions of the world, the earth bricks also integrated local plant matter to create tensile properties within the masonry. The adobe blocks are made with selecting an appropriate soil mix from the local earth most often at the site of construction. Ideal soil mixtures incorporate fine-grain clay-like soil and a certain percentage of medium granulation sand-like soil. The unitization of masonry systems is derived from the human hand, which allowed for construction workers to fabricate the blocks by hand as well as stack them into wythes one-by-one via hand placement. Adobe is rarely utilized today as it has both structural limitations and weatherization challenges (although both of these aspects can be addressed with certain techniques and treatments).

Another form of early masonry construction was the use of local stones to create stone houses and structures. The compilation of medium-size stones that could be held in one’s arms and placed into a wall stack without additional equipment or machinery is a building artform that is uncommon today. This was one of the most sustainable and regionally appropriate building techniques for centuries across all parts of the globe. This construction type also utilizes a mortar to bond the stones together, and different forms were invented for structural integrity – such as the arch, dome, and vaults. Stone and masonry construction are employed for ceilings and floors and retaining walls in addition to exterior walls. Masons are skilled in selecting and cutting stones into shapes and sizes that will facilitate the composition of walls and other elements. Because this building technique is so specialized, it is often much more expensive for contemporary structures to be made from stone. Often times stone will be utilized as façade materials for modern houses, but in a non-load-bearing fashion and just providing decorative effect as the exterior finish to wood frame construction.

Brick masonry construction became prevalent during the Industrial Revolution when the kilns and processes for mass producing and firing the clay units was available as a technological development. The expanse of the railroad throughout the US in the nineteenth and early twentieth centuries proliferated brick masonry construction for housing across many regions, regardless of climate or other conditions. Brick refractories were established in numerous cities and the brick became a primary building material beyond single-family houses. Brick-laying was an established construction trade and specialization, which has declined over time in the US. Brick has also suffered from its limitations in seismic zones, where height and use of masonry construction is restricted by codes that have become more stringent to prevent major failure.

As a cousin of the brick, the concrete masonry unit, or CMU block, is sometimes utilized for single-family residential homes, though more often employed in multi-family housing and small commercial construction. CMU blocks are pre-manufactured units with typical nominal dimensions of an 8” height by 16” length, while the depth comes in 2” increment sizing from 4” through 12” wide. The depth determines the thickness of the wythe of a CMU wall. The actual dimensions of the CMU face are 3/8” less than the nominal number to account for the mortar joints between units. CMU is manufactured in kiln plants just like brick, but because of the concrete mixture including cement these units create a little more of a carbon footprint compared to clay-bricks. CMU walls also require additional concrete to be poured into the open cells on-site during construction. The open cells exist within the center portions of the units and allow for vertical rebar (reinforcing steel members) to be placed before the wet concrete is poured into the cells. The rebar provides some of the tensile strength that is otherwise missing from masonry systems.

Insulated concrete forms, or ICF units, are a relatively newer technology that combines a concrete form of similar dimensions to the CMU block but made with a polystyrene insulation board instead of concrete. The concrete of ICF units is then poured into the middle cells and the insulation remains on the outside faces. As far as energy performance design of exterior walls, ICF construction performs very well since having insulation on the outside face in hot seasons and on the inside face in cold seasons provides an optimal heat transfer control when compared to insulation in the middle of a wall system. However, this inversion of insulation prevents the opportunity for thermal mass design strategies, which make use of the heat capacitance of concrete or masonry materials. Thermal mass strategies allow for heat to be absorbed into the walls during the day and then released at night, since the heat capacitance delays or creates a lag of heat transfer.

Sometimes, the design for energy performance takes precedence over the considerations of embodied energy and healthy material selections. The polystyrene insulation used to make most ICF blocks is problematic for the health of the laborers making the units, since persistent exposure to styrene can irritate eyes and breathing passages and negatively impact nervous systems. Concrete and mortar also have health and safety impacts to the laborers’ lungs and respiratory system due to the cement dust inhalation that can occur. Silica is common in most sands and cements and can cause scar tissue or cell necrosis to form inside the lungs if inhaled. Concrete also contains the known human carcinogen – hexavalent chromium. Overall, masonry construction provides longer lasting structures compared to wood framing, giving a greater life to the materials employed and contributing positively to sustainable design in that regard. However, the negative impacts on health of the labor force need to be addressed and considered – ensuring that material selection occurs from well monitored and controlled production plants with the highest of health, safety and welfare standards and protocols.  

One challenge with masonry wall construction is the end-of-life recycling potential. Most masonry units are not easily recyclable and will end up in construction waste landfills. Numerous efforts are being made to incorporate certain percentages of post-waste agregate into new concrete mixtures. Some brick materials can also be recycled into post-waste powders for new clay, but the implementation of this is not widely adapted. Rebar and steel components from masonry wall systems can be recycled for re-use but need to be separated from the demolition waste. The overall material ecology of masonry wall systems can be regionally appropriate since the soils can be extracted locally and the masonry units produced within reasonable distances to construction sites, minimizing transport of materials. Masonry is also a sensible design material for longevity and adaptable for many climate types, depending on the wall system layers for heat transfer and moisture control design.

Future blogs about the material ecology of a house will discuss other types of exterior wall systems as well as roof systems. 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..