low energy building design
For Aggressive
Every south-facing window
is a basic passive solar
space-heating system.
The challenge is minimizing
heat loss while maximizing
solar energy collected
throughout the day.
Houses in Carrboro, N.C., demonstrate the key
passive solar design elements: all houses on an
east-west axis, many south windows, and few if
any east and west windows. We also see the common design mistake demonstrated by the yellow
building, which will be excessively shaded in the
winter afternoon by both its projecting wing and
the projecting blue building.
By Norbert M. LechNer
Norbert M. Lechner ( lechnnm@auburn.edu) is an
architect, professor emeritus in the College of Architecture, Design, and Construction at Auburn University, LEED-accredited professional and ASES Fellow.
He is an expert in energy-responsive architectural
design with an emphasis on solar-responsive design.
Lechner’s book, Heating, Cooling, Lighting: Design
Methods for Architects, is used by more than a third
of all architecture schools in the United States and in
architecture schools worldwide. He is also a sought-after speaker, giving keynote lectures and workshops
at universities and conferences around the world.
As we discussed in the November/ December issue, low-hanging solar- responsive design strategies reduce a building’s energy consumption and typically cost less than the high-hang-ing solar-collecting technologies — or nothing
at all. (See “Choose the Low-Hanging Fruit” by
Lechner.) In this issue we focus on passive solar
heating techniques.
Most passive solar space-heating systems
are of three major types: south windows, also
called direct gain; Trombe walls, where south-facing glass covers a mass wall (see the Zion
National Park Visitor Center, facing page); and
sunspaces. A Trombe wall system delivers no
light, only heat into a space. Although sunspaces
are the least efficient and most expensive, they
are popular. Besides their basic construction
cost, they are expensive to insulate and shade.
The most sustainable sunspaces are those that
have no east, west or overhead glazing. In effect,
they are south-facing building spaces that are
not mechanically heated or cooled. Their south
façade would be all glass and may be more than
one story high.
There is no truth in the idea that solar energy
is too expensive to collect. It all depends on the
system. Remember, basic passive solar heating
is free if implemented during design, and the
enhanced systems usually pay for themselves in
a few years. Only the sunspace is hard to justify
on financial grounds.
Any building that requires space heating
should use passive solar design, not only because
it is environmentally favorable but also because
it can cost as little as nothing. Every south-facing
window is a net gainer of energy in the winter
because it collects more energy during the day
than it loses over a 24-hour period. Thus, every
south-facing window is a basic passive solar heat-
ing system.
The Earth-sun relationship is such that vertical south windows receive the most sun in the
winter and the least in the summer — just what is
needed (see figure 1, facing page). On the other
hand, horizontal glazing (horizontal or near-horizontal openings such as skylights) and east
and west glazing get the most sun in the summer
and the least in the winter — just the opposite
of what we need. Consequently, the most cost-effective strategy, which is actually free during
the design phase, is to move some of the east and
west windows to the south façade. Keep in mind
that the more south windows in a building, the
more heat is collected during the winter. Because
passive solar heating will fail if the windows
are not exposed enough to the winter sun, the
designer can use a site-evaluation tool to ensure
sufficient solar exposure. Both commercial and
