Ecological Building Network - The Art and Science of Building Well

Larry Strain, Siegel & Strain Architects

While it may be obvious that smaller buildings use fewer resources and energy than larger buildings - they're smaller after all - it's not so obvious that smaller buildings use less than larger buildings per square foot.

Why is this? When buildings reach a certain size; the way they are built changes; the materials they are built with changes; and the way they operate changes.

And when those things change, environmental impacts change, and they tend to go up.

During construction, embodied energy and associated green house gas (GHG) emissions increase on a per square foot basis as building size, particularly height, increases. The most obvious reason for this is the change in materials. We don't use low impact materials such as wood and earth to build tall buildings, we use steel and concrete when we get over about 5 stories.  But materials don't account for all the increase - tall concrete and steel buildings have higher embodied energy than short concrete and steel buildings. In one study of embodied energy that looked at the impact of different structural systems and also tracked height, building height had a bigger affect on embodied energy than what the buildings were made of. ¹

Operating energy also goes up with building size. According to the CBECS database (Commercial Buildings Energy Consumption Survey), for most building types, energy use intensity (unit of energy/sq. ft.) goes up with building size. There are a couple of reasons for this - mechanical and lighting systems are more complex, and passive approaches such as daylighting and natural ventilation are harder to do in very large buildings.

This is not an argument against large buildings; there are plenty of reasons buildings need to be large - building program, type of services offered, area and population served. Taller buildings also occupy land more efficiently and contribute to lower site and transportation impacts. But if size is part of the sustainable equation, we at least need to understand the relationship between size, efficiency and environmental impacts.

In the natural world, size clearly affects structure and form.

J.B.S. Haldane wrote an essay in 1928 entitled "On Being the Right Size"² in which he explains how size affects form and function in the natural world and why changes in size always produce changes in form. It begins:

"The most obvious differences between different animals are differences of size, but for some reason the zoologists have paid singularly little attention to them. In a large textbook of zoology before me I find no indication that the eagle is larger than the sparrow, or the hippopotamus bigger than the hare, though some grudging admissions are made in the case of the mouse and the whale. But yet it is easy to show that a hare could not be as large as a hippopotamus, or a whale as small as a herring. For every type of animal there is a most convenient size, and a large change in size inevitably carries with it a change of form."

Gravity has the most obvious affect on size and form.

"Let us take the most obvious of possible cases, and consider a giant man sixty feet high-about the height of Giant Pope and Giant Pagan in the illustrated Pilgrim's Progress of my childhood. These monsters were not only ten times as high as Christian, but ten times as wide and ten times as thick, so that their total weight was a thousand times his, or about eighty to ninety tons. Unfortunately the cross sections of their bones were only a hundred times those of Christian, so that every square inch of giant bone had to support ten times the weight borne by a square inch of human bone. As the human thigh-bone breaks under about ten times the human weight, Pope and Pagan would have broken their thighs every time they took a step. This was doubtless why they were sitting down in the picture I remember. But it lessens one's respect for Christian and Jack the Giant Killer."

The same is true for the buildings we build. We can avoid massive, thick structure through engineering ingenuity and technology, but those systems and materials are more complicated and energy intensive.

Haladane also goes on to talk about how size affects how organisms function.

"A typical small animal, say a microscopic worm or rotifer, has a smooth skin through which all the oxygen it requires can soak in, a straight gut with sufficient surface to absorb its food, and a single kidney. Increase its dimensions tenfold in every direction, and its weight is increased a thousand times, so that if it is to use its muscles as efficiently as its miniature counterpart, it will need a thousand times as much food and oxygen per day and will excrete a thousand times as much of waste products. "

"Now if its shape is unaltered its surface will be increased only a hundredfold, and ten times as much oxygen must enter per minute through each square millimeter of skin, ten times as much food through each square millimeter of intestine. When a limit is reached to their absorptive powers their surface has to be increased by some special device. For example, a part of the skin may be drawn out into tufts to make gills or pushed in to make lungs, thus increasing the oxygen-absorbing surface in proportion to the animal's bulk. A man, for example, has a hundred square yards of lung. Similarly, the gut, instead of being smooth and straight, becomes coiled and develops a velvety surface, and other organs increase in complication. The higher animals are not larger than the lower because they are more complicated. They are more complicated because they are larger....Comparative anatomy is largely the story of the struggle to increase surface area in proportion to volume."

The same natural laws apply to our buildings. Getting air and light into a small building, is relatively simple, easily accomplished by windows. Heating and cooling can also be accomplished passively, but beyond a certain size natural ventilation and daylighting don't work and heating and cooling become difficult and require increasing amounts of energy.

We invite you to join in this discussion and research.

Some possible topics to address:

How size and height affect:

Embodied energy

• Structure
• Construction methods
• Materials

Operating energy

• Passive strategies
• Active Systems

Building shape, form, skin

Energy load per occupant

Land use and transportation

• Density, sprawl
• Transportation patterns

Larry Strain is an EBNet board member and a senior partner with Siegel & Strain Architects, an award-winning architectural design, research and consulting firm committed to sustainable design with a particular expertise in green building materials. Larry's Re-Sourceful Specifications -- guideline specifications for environmentally sustainable materials and methods is an outgrowth of this research and is now incorporated into the GreenSpec Directory, published by Building Green, Inc.

Notes

1. Table 2, Page 38 The influence of construction materials on life-cycle energy use and carbon dioxide emissions of medium size commercial buildings. Thesis submitted to the School of Architecture, Victoria University of Wellington Nicolas Perez Fernandez, July 2008

2.  On Being the Right Size J.B.S. Haldane, 1928