inside ases | chair’s corner
Keeping Your Cool
Comfort without air conditioning is a folk art.
By JOHN REYNOLDS,
Hot weather is upon us, with higher electricity bills
for air conditioning. Can we reduce electricity
use, avoiding the stiff-neck discomfort of constant
forced cold air? Can our bodies and buildings follow natural
thermal cycles that respond to the climate?
John Reynolds, FAIA,
is chair of the American
Solar Energy Society
Board. Contact him at
Shade is a first line of defense. We’ve all experienced
tree-shaded streets that are so much cooler than sunny
ones. The same applies to our buildings. When planting
trees for summer shade, think about admitting winter sun
(avoid evergreens to the south), blocking winter wind
(use evergreens to the windward side) and celebrating
the seasons. Some deciduous trees provide colorful bark
in winter, blossoms in spring, shade and food in summer,
color and food in the fall. Trees provide seasonal, large-scale shade for the whole building envelope.
Windows are most vulnerable to solar gain. To shade
them, consider deciduous vines. With a much smaller root
ball, vines can shade high-rise facades as well as apartment
balconies. Fashioned by their support structure, they serve
as living awnings. They can celebrate the seasons and provide a local source of food. My pole green bean vines grow
very fast and cast dense shade for my south windows. And
fresh-picked green beans are a delight.
Windows are most vulnerable to solar gain. To shade
them, consider deciduous
Architectural shades for windows work for businesses
and homes. Not that long ago, awnings over retail and
office building windows were quite common. They can
be cranked open or closed as necessary. They block sun
when temperatures are too hot, admit it when too cool
and protect from glare and rain. Facades are enriched with
three-dimensional projections and with the fourth dimension of time, as shadows slowly move with the sun. What
elegant richness, in contrast to slick, flat, glass facades.
Overhangs above south-facing windows are popular,
due to their peculiar advantage at this orientation: In summer months (spring to fall equinox), if the overhang shades
to the bottom of the south window at noon, it will also
shade to the bottom of the window at all other hours. In
winter months, the shadow of the overhang at noon is at
its deepest; in the morning and afternoon, the window gets
more sun than at noon. So the south overhang shades in the
summer and interferes less with sun in the winter. Designers of these fixed overhangs face a dilemma: The spring
equinox is much cooler than the fall equinox. If you design
for complete shade in hot August, you also block the sun
in cool April. I favor fixed overhangs with deciduous vines;
the overhang shades from May through July, and vines do
the rest. The vine leafs out when the weather is warm and
retains its leaves until late fall’s cool weather.
External shades of all types are subject to wind and
weather, and in winter, they can threaten those below
with icicles. It is so much easier to shade inside the window. But compared to an externally shaded window of the
same size, completely unshaded windows will gain twice
as much summer solar heat if facing south and about 3. 7
times as much heat if facing east or west. Unshaded horizontal skylights gain a whopping 7 times as much summer solar heat. Adding either drapes or Venetian blinds
inside an unshaded window will cut heat gains, but the
sun has already entered. The exterior shade is clearly best
at excluding solar heat.
The heat sinks available to cool a building are air (the
drier the better), earth (especially when moist) and the
cold night sky. The common air conditioner (an air-to-air
heat pump) uses outdoor air as its heat sink. The passive
cooling equivalent is the open window, but that is of little
use when the air outside is hotter than we desire inside.
In climates where summer night air is quite cool, buildings can be flushed with night air, and thermally massive
indoor surfaces can store the “coolth” for the next day’s
comfort. In humid climates, the air is not an effective heat
sink for passive systems.
The earth’s heat sink is well used by geoexchange
systems, also known as ground-air heat pumps. The low-energy equivalent is the earth tube, an underground passage where outdoor air temperature gradually approaches
the earth temperature. The deeper this passage is below
ground, the greater the difference between the temperature of the hottest summer air and that of the earth. These
systems almost always use a fan to propel the air.
The cold summer night sky may seem the least likely
heat sink, because roofs are insulated more thoroughly
than any other building surface. There are a few low-energy approaches, such as the roof pond and the night roof
spray system. The roof pond, advocated by Harold Hay,
slides insulated roof panels open by night to expose bags
of water to the cold night sky. These bags are in direct contact with a metal ceiling below. The lower the humidity,
the better the system performance, as high humidity can
produce condensation on the ceiling. The roof night spray
system stores water in an insulated tank; by day, the water
circulates through cooling panels in the building, and by
night, it is sprayed on the roof, cooled by exposure to that
cold sky, then re-collected and stored.
Insulate, shade and find the appropriate heat sink:
passive cooling’s big three. ST