How Small Building Design Impacts Embodied Emissions
The sheer number of small buildings worldwide illuminates an opportunity to control the carbon destiny of this sector through the practice of informed design, a rather striking opportunity. Not only do residential dwellings comprise the largest share of small buildings, residences constitute the largest number of all buildings. The annual construction of new dwellings in the United States and the upgrade of inadequate shelters worldwide elucidate this reality, the two ends of the spectrum. At one end, approximately one million single-family homes are built in the United States annually with an average floor area in the vicinity of 2,500 square feet. At the other end, one and a half billion people live in temporary, minimal, or inadequate housing worldwide which will be replaced by more substantial structures through the alleviation of poverty and forced displacement. The production of construction materials for each scenario will release carbon emissions annually that far outweigh years of potential operating emissions reductions in the future. Both scenarios offer opportunities to decrease this buildup of embodied carbon over the next 10 to 15 years through conscientious building design and material choice.
Although a small building pales in carbon footprint relative to its urban high-rise counterpart, their relative numbers provide a different dynamic. This holds especially true when considering shelter for the billion-plus people who are impoverished or displaced. Initial upgrades in housing are typically from bamboo, wood, thatch, cardboard, fabric, and corrugated metal to cinder block and concrete, the worst material emitters on the list. To make matters worse, these upgraded replacements will be discarded and replaced as well, by multi-story structures on the march toward urbanization over the decades to come-more than doubling the emissions from this group, the largest of all. This first upgrade alone, though providing a minimally sized dwelling, will be responsible for releasing sizeable emissions.
For example, more than a million and a half dwellings in Vietnam's 2009 housing census were classified as "simple" structures, constructed with supporting columns, roof, and walls that were all classified "flimsy"-constructed with materials such as mud, leaves, bamboo, thatch, or tar paper. Replacing them all with minimal 320-square-foot dwellings-a mere four cinder-block walls and a corrugated-metal roof-would emit nearly 40 million metric tons of CO2 to produce the building materials alone. Building with brick would not fare much better, and cladding with brick over cinder block would be far worse. This 40-million metric tons covers just the upgrade of four walls and a roof. To provide minimally adequate shelter for the 1.6 billion people with such need, multiply that by 1,000. This hurdle for curbing emissions is created in good part by defaulting to cinder block and concrete, rather than low-carbon or carbon-sequestering building materials such as wood.
On the other end of the spectrum, single-and multi-family housing provides a different type of design challenge. In the United States, new buildings construction every year is equivalent to adding the entire building stock of New York City annually. Only 5% of the single-family homes built in 2018 were clad primarily in wood. Nearly 800,000 were clad with other materials, 26% vinyl siding, 25% stucco, 21% brick, 20% fiber cement, and 3% using other products. Unfortunately, most materials manufactured for cladding either contain cement or use a cementitious material such as mortar for mounting. This includes stucco, fiber cement, thin clay-brick, and cut stone cladding products. Cement production is responsible for 7% of all energy related emissions worldwide. Vinyl is the exception neither containing nor adhered with cement, but it too has a high-carbon footprint resulting from its carbon content. These cladding materials have average manufacturing emissions in the vicinity of 7 kgCO2/m2, which varies widely as a function of the cladding's thickness, finish, and the volume of mortar used for mounting, if any. Vinyl siding seems to be the lowest, at less than half the average in its simplest composition and minimum thickness. Facing with full-size clay bricks on the other hand, which is not included in the above average, in the vicinity of 30 kgCO2/m2 can be four times worse than the mean and 10 times vinyl.
Unlike any of these materials, wood provides a benefit. Wood shingles, shakes, or clapboards continue to sequester the carbon they have already removed from the atmosphere. If 20% of the houses in each non-wood cladding category were clad with wood, the net CO2 emissions saved annually would near 1 million metric tons in the United States alone, solely from the cladding materials, solely from U.S. single-family homes. This does not include the backboard materials that support the cladding nor waterproofing membranes, mounting systems, or insulation, all of which can be selected to minimize a wall's composite carbon. The same can be said for window-frame, door, flooring, and roofing materials; choices among wood, plastics, metals, and ceramic, petroleum, or cement-based products. Such gains may seem small for each individual residence, especially in regard to a minimal shelter, but the quantities built and renovated multiply the impact. Additionally, each material purchase activates a re-supply chain, initiating a release of carbon emissions in the "now" time frame. Those emissions not only negate many years of potential operating savings, the massive carbon they release immediately exacerbates the warming rate.
Single-family and small-multifamily dwellings are not the only structures comprising the Small Buildings category, let us not forget low-rise and mid-rise apartments, commercial and industrial buildings. Although significantly fewer than their one- to four-story residential counterparts, they are framed predominately with a combination of concrete and steel; often clad with fiber cement composites, concrete blocks, brick, clay tiles, aluminum, or steel-some claddings layered with a decorative or protective plastic-laminate finish. They too provide an opportunity to reduce embodied carbon through material selection and their structural methodology.
Yes, the design of small buildings and their material choices can significantly reduce carbon emission - now.
Adapted from "Thwart Climate Change Now: Reducing Embodied Carbon Brick by Brick", published by the Environmental Law Institute, Nov. 2021
