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The Case of the Overheated Attic

by Michael Potts

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 quite comfortable. 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 warm.

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 roof.

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 the system.

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 working environment.

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.

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updated 1 January 2005 : 16:37 Caspar (Pacific) time
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