Sustainability 10: Density

A green friend
A green friend

When most people envision a ‘green’ world, they picture high-tech energy-efficient homes in environmentally sensitive gardens. Conversely, when most imagine a ‘gray’ world, they visualize a gritty urban wasteland, perhaps similar to a crowded concrete canyon of Manhattan. In fact, that dense and hectic slice of New York City is probably far ‘greener’ and more sustainable than any idyll in a park can ever be — the answer lies in density.

Simply put, denser communities are almost always inherently greener and more sustainable than spreading, sprawling communities. Think of it as economy of scale in sustainability.

To illustrate, consider two examples: A) a conventionally-designed single-family detached home of 2,400 sf on a quarter-acre lot on a typical suburban residential street, housing a family of 3 in a partial-two-story Colonial-style structure, with a typical driveway, apron, walks, patio and deck, and B) a ten-story condo building of 85 dwelling units averaging about 1,950 sf each, housing families averaging 2.4 persons per dwelling unit in a high-rise structure occupying 80% of its quarter-acre site, with the remainder of the site landscaped, and with a roof garden covering half the structure’s roof.

• Population density: Using reasonable assumptions of required suburban roadways, collectors, parks, schools, etc., Example A, if extended across the countryside, will tend to max out at an overall population density of no more than about 4 persons per acre. Applying similar reasonable assumptions of required city streets and city amenities, Example B can achieve overall population densities of up to 250 persons per acre. Example B is thus far less wasteful in land use.

• Conditioned and maintained surface area: To comfortably heat and cool its occupants, Example A must insulate and condition a total exposed building surface area of over 1,700 sf per person. To achieve the same level of comfort, Example B must insulate and condition less than 350 sf of exposed building surface area per person. Example B is clearly cheaper to heat and cool and maintain.

• Required infrastructure: If Example A is repeated across a suburban landscape in the typical patterns prevalent today, each housed resident of the community may require as much as 35 feet of extension of such municipal facilities and services as access roadways, sidewalks, public utilities, mail service, road maintenance, trash collection, and police and fire services. If Example B is repeated across an urbanized landscape, each housed resident may require as little as a 0.5 foot extension of such municipal facilities and services. It is thus far easier and far cheaper for a municipality to provide essential facilities and services to Example B.

• Access to jobs, shopping, entertainment, and recreation: Assume for a moment that, to be viable, a firm or store or bowling alley or restaurant or cinema might require access to a market pool of either 1000, 5000, or 10000 workers or customers. Within what radius can an establishment reach such a market pool? With Example A, 1000 people would be available within a radius of about 3300 feet, or 5/8 mile; 5000 people would be available within a radius of about 1.4 miles; and 10,000 people would be available within a radius of about 2 miles. With Example B, the effective market radii shrink to about 420 feet for 1000 people, about 1300 feet or ¼ mile for 5000 people, and under 1,000 feet for 10,000 people. Obviously, Example A would thus require auto trips for even the simplest errands, while Example B could offer almost all amenities within a reasonable walking, biking or public transit distance.

• Green space: With a likely average of open green space of about 1,670 sf per occupant, Example A bests Example B, which might command only 60 sf per occupant. However, in most typical conditions, Example A would provide 70% or more of that green space as simply monoculture lawn to be watered, mowed, and fertilized — all practices that tend to contribute to fuel use, water use, fertilizer runoff, and water purification demand. Conversely, in most typical conditions, Example B would concentrate its green space with multiple diverse species, shade trees and decorative plantings, and may capture and manage rainfall. The density of Example B also frees up other land for open green space and recreational use by all.

In our striving for sustainability we must not shy away from increased density. In fact, increased density, wisely planned and artfully designed, may be our best path to sustainability.

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