Mechanical Attic Ventilation - When Going Natural Isn't Enough!
Part 2 of 2 (Return to Part 1 - Natural Attic Ventilation)
Mechanical attic venting...
If you have noticed any of the symptoms of inadequate ventilation as mentioned earlier and cannot increase ventilation through conventional means, you may be a candidate for one of the various mechanical attic ventilators. They all increase ventilation through the use of fans or turbines which forcefully remove air from the attic. As you will see, they work in concert with other vents to lower attic temperatures in the summer and also remove moisture in the winter.
Gable Mounted Ventilators...
Gable mounted ventilators are the simplest type to install, and can really increase the air flow through the attic. They are mounted inside of an existing gable vent, and in many cases require no carpentry at all to install. A typical unit can ventilate over 2000 square feet of attic space. You will need electrical skills, however, since these are powered units.
There is a second type of gable mounted ventilator that is mounted through the wall, with its own self-opening shutter system to prevent the entrance of bees, bats or other undesirables into your attic when not in use. These units require more advanced carpentry skills for installation but the reward is increased ventilation. Unlike the gable vent-mounted units, they do not have to blow through the gable vent's protective screening. As mentioned in the article on whole house exhaust fans, insect screening can decrease the total air flow through a vent by up to 25%!
Roof Mounted Ventilators...
Roof mounted ventilators come in two flavors... powered and unpowered. Unpowered ventilators, also called turbine attic ventilators, rotate with the slightest breeze and cause a powerful updraft in the attic to effectively pump out the heated air. Varying in size from 12" up to 24" in diameter, the largest of these turbines can move as much air (with a little assist from Mother Nature's breath) as the most powerful electric ventilators!
Avoid steel ventilators... as Neil Young once sang "Rust never sleeps!" Instead insist on rust-proof aluminum or plastic ventilators. The SupaVent unit shown right is made from ABS plastic and is virtually indestructible! Supavent Turbine Ventilator graphic courtesy of Edmonds Industries Australia.
Powered roof mounted ventilators are the most effective ventilators available able to ventilate over 2500 square feet of attic floor space. They share the same two advantages of the turbine ventilators... the high mounting position allows them to most effectively suck out the heated air, and you have greater flexibility in choosing the mounting position... especially useful if your attic is "complicated" due to architectural design!
If your roof faces the right way (south-facing roofs are the best) you might be able to benefit from the energy saving features of a solar powered attic exhaust fan. Though not as powerful as the largest electric ventilators, solar units offer the advantages of zero energy cost during use. They are the most effective when needed the most since their fans increase in power and speed as the intensity of the sunlight increases. And even if you are "electrically challenged", there's no problem 'cause there's no wiring! Pictured right is the Fan-Attic Solar Ventilator.
Whole house exhaust fans as attic ventilators??
If you already have a whole house exhaust fan, you already have some of the benefits of attic ventilation fans. Each time you use your exhaust fan the attic air is quickly cooled. Initially there will be "hot spots" since the exhausted air will be directed towards the existing vents, but the sheer volume of the exhausted air will soon cool the entire attic.
Of course, most of us will not use our whole house fans during the day since it would be silly to draw heated air into your home just to cool the attic! If your existing attic ventilation system is doing its job, a whole house exhaust fan will only add to the benefits of a cooler attic in the evenings.
See the article on whole house exhaust fans for more information!
Watch out for unintended consequences!
Murphy's law rears its head in the strangest places... your attic, for instance. Who would think that beautiful powered ventilator might do absolutely nothing to cool your attic? If you have lots of ventilation already, adding a powered ventilator may be a waste of time and money.
This is especially true if you have a soffit-ridge vent system, the most efficient natural ventilation system. Face it... ventilators are stupid machines! They will draw air from wherever they can with the least effort. This means that a ventilator will draw air from ridge vents, gable vents or soffit vents. Fine. But if the fan draws much of its air from a nearby gable or ridge vent, there is no benefit to anyone except the electric company! So placement of the ventilator is essential... as well as careful consideration of whether or not it is a wise investment in the first place.
If you have installed an unpowered roof turbine vent, you have a year-round ventilation increase (unless the two feet of snow on the roof freezes it solid). Unfortunately, powered systems use heat sensors to control their operation. They only run when the attic reaches a certain temperature... typically ranging from as low as 70 degrees F to as high as 140 degrees F. This is great for the summer, but what about the winter? As you have learned, the moisture that accumulates in a poorly ventilated attic can be as destructive as the summer heat. Unfortunately, a thermostat is not a viable solution for wintertime ventilation. A suggestion... install a timer that will automatically turn on the ventilator for a certain number of hours each day to give you a true four-season ventilation boost!
Safety considerations with powered ventilators...
It is a wise idea to have a master cutoff switch for your attic ventilator in your living space. This allows you do perform maintenance safely as well as being able to stop the air flow if you need to go into the attic. No need to pull cooler house air into the attic, right?
All attic ventilators should be installed with a fusible link, a type of fuse that melts in the presence of high heat, disconnecting the power. It is mounted near the fan in the attic, automatically turning off the fan in the event of fire... a valuable safety feature!
Do-It Yourself installations...
Both powered and unpowered ventilators come with detailed installation instructions, and should be within the abilities of even the novice do-it-yourselfer. Getting electrical power safely to the ventilator (if needed) requires electrical knowledge concerning proper wiring practice and local code requirements.