110-volt AC attic fans have been around for a long time. They are big and boisterous and require an electrician or knowledgeable homeowner to do a hard wire hook-up. These costs can easily exceed $600 - $700.

 

For the same price or less (if you do it yourself), the more elegant solar powered attic fan can do the job. Using a simple 10-watt photovoltaic module to directly power a 12-volt DC fan, these self-contained units can quietly and effectively move 800 cubic feet of air per minute. This is enough to keep the attic 30-50 degrees cooler. In some places it can mean the difference between needing or not needing air conditioning.

No matter how well insulated your ceiling is, excessive heat in the attic will find it’s way into your living space. Insulation simply slows it down. By mid-day, an army of millions of Btu’s have marched through your insulation and are assaulting your living space. Solar attic fans will reduce the air conditioning load in the living space below and make hot summers more endurable for those who don’t use air conditioning.

A single solar attic fan can cool about 1,500 square feet of attic area. Solar attic fans don’t use batteries or require an electric outlet. They have a small photovoltaic (PV) module that powers the fan when the sun is shining, cooling the attic during the part of the day when heat builds up. The fan should be installed more or less in the middle of the attic to serve the entire space. The fan will draw air from the eaves and from other vents. The idea is to draw air from all sources equally.

Attic fans are very simple to install. My crews do them in an hour or two. The biggest challenge to the do-it-yourselfer is psychological: “Do I dare cut a 14” hole in my roof and trust that it won’t leak?” If you have a tile roof, metal, or perhaps even a cedar shake roof, you may wish to defer to a professional. However, if you have a conventional composition shingle roof, it’s not as scary as you think. You can do it!

First you need a few tools, which most do-it-yourselfers will have. Determine the general location of the fan, near the peak, in the middle of the attic space to be cooled. Then, determine the exact location of the attic fan. Inside the attic, measure 12” to18” below the peak and make a mark centered between two rafters. Next, drill a hole from the attic onto the roof. Leave the drill bit in place so it can be easily found from the roof. On the roof side, locate your drill bit and tie a string around the bit that is 7” long. Use it to draw a circle that will be 14” in diamater. Double check the opening size of the fan model you’re installing. Cut around the circle using a sawzall or jigsaw. Be sure to catch the roof plug by grasping the bit rather than letting it fall in.

 

Next, as a trial test, attempt to push the unit into place making sure the top edge of the unit goes under at least 2 or 3 courses (horizontal rows) of roofing until the opening in the fan is directly over the hole in the roof. Inevitably you will hit nails or staples holding shingles in place as you attempt to place the unit. Do not force! Instead try to locate the obstacle by gently lifting shingles, and looking for the nail or staple in the way. If the obstacle is a nail, remove the nail with a flat pry bar. If a staple, drive a larger, flat head screwdriver under the staple and pry up. Repeat the process until all nails or staples in the way are pulled.

In order for the unit to slide all the way into place you will have to trim away about a 2” semi-circular arc from the first course of shingles directly over the top half of the fan unit. Make a mark on the roof to one side of your hole at the up-and-down mid-point to help determine the position of the fan when centered over the hole. You can go back into the attic to make sure.

 

Now that the unit fits directly over the hole you’re ready to fix and caulk it into place. Lift the bottom edge of the base and caulk all the way up and a few inches beyond the point where the unit goes under the shingles. Next, screw the base down. The screws should pass through the caulked perimeter. Caulk over the screw heads and you’re done.

Now, that wasn’t so nerve racking, was it? Notice for yourself how much cooler the attic is now with the fan working. As an extra precaution, you might want to put dabs of caulk under the loosened shingles where nails or staples were pulled to help hold them in place.

Cedar shake roofs are done in the same fashion, though it’s more difficult to get nails out and re-secure loosened shakes. If cedar shakes are broken, metal flashings must be installed under the cracked shake extending up to the course of roofing directly above the damaged shake.

Cascading water must be directed to the next lower course of roofing having full integrity. Metal flashings are like an “ace in the hole” for any mistake on the roof. If, while trying to pull out a nail, for instance, your pry bar pierces a shingle course above the 14” hole you’ve cut in the roof, all isn’t lost. Just slide a metal flashing under the damaged shingle, and make sure the top of the flashing extends a few inches into the next course of shingles above your mistake.

These metal flashings are available at most hardware stores in light and dark colors to match existing roofing. They come in 4” x 6” and larger pre-cut sizes. Make sure the metal is aluminum or plated for exterior long-term use.

All solar attic fans have optional thermostats. The manufacturers claim it is good to vent the roof year round, which means there’s no need for a thermostat. But, if you’re worried that on cold winter days you may be expelling warmer air from the attic and increasing heating load, then the thermostat is advisable. Thermostats snap in place in the wiring between the module and fan and dangle freely into the attic space.

There are a handful of solar attic fan manufacturers. Most have fixed photovoltaic (PV) modules to power the fan, which means that the unit has to be placed in the location most favoring the sun. One manufacturer offers a module which can be tilted. On a roof whose peak runs north and south, this unit can be placed near the peak on either side, with the module tilted up and oriented to the south. This is accomplished first by tilting the module, then spinning the base to face the module south before caulking and fixing to the roof. I’ve even placed these on north-sloped roofs with the PV module tilted to face south.

I’ve been asked about solar attic fans to cool upper level living areas finished to the rafters. Generally this is not an acceptable use, since in winter months the hole in the roof allows heat to escape even if the fan motor were disabled. If a well-insulated and sealed cover were used, however, it could work.

There are few solar energy technologies more simple, elegant and cost effective than solar attic fans. The significant benefit for relatively low cost makes it an excellent investment both in terms of energy savings and personal comfort.

           
“We’ve had our solar attic fan installed for about a year. For years, our family has used air conditioning to maintain the home at the same comfortable level. We’re on an equal pay program with the electric utility. Since the attic fan went in, our monthly payment has gone down US$10 per month. No other energy conservation measures were employed last year, so it appears that the attic fan deserves the credit. We love how it quietly and unobtrusively saves energy and money.” —Dr. Judith Ris, Vancouver, Washington