Your house creates a microclimate of its own, and on a summer day the
extremes of this climate can be quite surprising. Around on the sunny side,
air temperatures soar, while on the shady side the temperatures may remain
The big flat roof turned perpendicular to the sun is an efficient solar panel,
harvesting the sun's rays and turning them into thermal energy — heat.
Roofs pointing south and west are particularly productive. Many American
homes capture and store this unwanted heat in their attics, where it is
not unusual for temperatures in that dark, still space to exceed 160 Fahrenheit.
Because of convection — cold air falls, displacing hot air — the ceiling
temperature just below the attic will lag behind, but the whole ceiling
is apt to act like a radiator, transferring the heat into the living space.
If your house is air conditioned, this extra heat will make the refrigeration
system work harder, and you will find the cost of this service on your
utility bill. If you rely on nature's air conditioning, you may be uncomfortably
Conventional insulation above the ceiling helps in the winter, when
heat is meant to be kept inside, but is less helpful in the summer, because
conduction — heat traveling through a solid like the ceiling — works well
in any direction, unlike convection, which works best when moving cold
down and heat up. Eventually, the attic's heat penetrates the insulation
and warms the living space.
The simplest, least costly remedy for this problem is an attic fan.
Properly installed, this fan will push the hot air out of the attic, so
that cooler air can come in and replace it. If this "make-up air"
is pulled from underneath the eaves on the shady side, for example, it
is quite possible to lower the attic temperature below the sunny-side air
temperature despite the successful harvest of inbound solar energy by the
We try to avoid active solutions like fans and air conditioning because
they consume energy, but in the case of the overheated attic we are presented
with an opportunity to convert the problem into its own solution: by placing
a photovoltaic module in the sunniest spot on the roof and using its output
to drive the attic fan! Our objections to active systems usually center on the
complicated control equipment, probes, relays, thermostats, and logic that
tell the business end what to do and when, but the beauty of the PV-direct
attic fan system is that the energy source is perfectly linked to the need:
the attic overheats most when the sun shines most brightly. By harnessing
the sun to spin the fan, we use the problem to generate its own solution.
Sticker shock is a real danger when planning a system like this; too
often, we forget that the operating cost of the equipment is often much
higher that the original equipment cost. Photovoltaic modules, called PVs,
are made of the same stuff as transistors and computer chips, a highly
refined and treated form of silicon. Think of it as a piece of blue metal,
which it is, and the price may seem staggering; think of it as a nearly
perpetual source of electricity, and it may seem nothing less than miraculous.
When considering what kind of attic fan system to purchase, do not forget
to account for the cost of electricity — house-current fans tend to use
more energy than DC (direct current) fans — as well as the cost of extending
a house-current circuit into the attic and connecting a thermostat or other
controller to make sure the fan only runs when it should. When we do the
reckoning, we conclude that the simplicity of the self-contained system,
a PV module connected directly to the fan, is a bargain over the life of
Using a low voltage DC fan adds two other features which are impossible
to evaluate from a monetary perspective but should be considered. Low voltage
electricity is inherently much safer than the AC (alternating current)
we generally use in our houses, and so you can safely do this simple wiring
job yourself if you'd like. Controls for low voltage DC fans and motors
are also simpler and more reliable, and so if you would like to add a thermostat
or a switch to control the direction the fan spins, you can do so cheaply
and with no fear of causing a fire or damaging your new equipment. An attic
fan is a perfect project for the do-it-yourselfer who wants to learn a
little about renewable energy.
Passive ways of controlling extremes inside the house — insulation,
ventilation — should have been built in to your house at the start, but
often are not. Adding them later may not be as successful.
The problem of attic heat build-up does not yield easily to conventional methods
of insulation. Unless you are willing to use active means like air conditioning,
you must keep the heat entirely outside the envelope. In Florida, experiments
have shown that a bright-white-painted roof cuts attic temperatures by
25% over a conventional white pebble roof. A radiant barrier, made of construction-grade
aluminum foil hung just below the rafters, can turn some of the radiant
heat around, but only after the heat is inside the perimeter, and by then
the struggle is already partly lost.
If you live in a climate with cool nights, the strategy of the whole
house fan may also make sense. By closing the house tightly during the
heat of the day while controlling heat income aggressively using overhangs,
deciduous plantings, reflective window covering or shutters, and, as a
last resort, air conditioning, the interior can be kept liveable. During
the cooler evenings, the whole house fan comes on to replace the air heated
during the day with fresh, cool air from outside. If your house contains
thermal mass — masonry floors or walls, masses of water or other materials
with high specific heat — these masses are recharged with the cooler evening
air and act as batteries for storing the cool until it is needed the next
day. If interior temperatures get out of hand during the day, these thermal
masses store daytime heat instead and can make the nighttime unbearable,
so proper heat management becomes very important. Classic old buildings,
constructed before this century's love affair with the kilowatt began,
often employ shading, thermal mass, and ventilation to maintain a comfortable
interior climate through the hottest days. Modern buildings, which one
architectural critic has called "all glass and no windows," require
heroic additions of expensive refrigeration to maintain a less healthy
One final note: it has been suggested by advocates of healthy houses
and workplaces that we may have defined our "comfort zone" too
narrowly for our own good. "Air conditioning flu" is a real hazard
for those of us who must go in and out between superheated urban reality
and supercooled shops, offices and homes. Even the best air conditioning
systems drastically reduce the humidity in the air, which puts added stress
on our respiratory systems. As energy costs continue to spiral ever upward
and fossil fuels become scarcer, it makes sense for us to broaden our notions
of comfort. We survive quite nicely at 85 degrees if the air is moving
enough for our personal evaporative cooling systems to operate.
But summer is AGES away, you might say. Well, maybe, but it sure is nicer to
work in the attic when the sun is less ferocious. The old pro sez,
do it NOW.