Attic Ventilation – Important Primer
Attic Ventilation - - Its Importance and How to do it
Attic ventilation is probably one of the most neglected areas of home construction and remodeling. All too often not enough thought is given to providing adequate ventilation to these spaces.
My guess is that virtually everyone who is reading this Bulletin has entered a closed up car which has been sitting in the sun. The temperature and the air in the car can be stifling. Because the air in the car cannot escape, the energy of the sun heats it up. As the sun continues to 'bake' the car, the air molecules get more and more excited. The temperature begins to rise.
Contrary to this, a car left in the sun with the windows down will not be as uncomfortable. The only difference between the two situations is air movement. The car with the windows down allows air to circulate through the car. This circulation or movement creates a cooling effect simply by allowing the heated air to escape from the car.
Attics and Cars
When you think of it, your attic is really not that much different than a car. If your builder did not provide for adequate air movement, your attic is nothing more than a confined space.
A confined space in an attic can cause serious problems both in the summertime and in the winter. The air which is trapped in the confined attic space almost always contains water vapor. This water vapor can and will damage your home.
Condensation & Steam
If you allow air to circulate, it achieves a balance or equilibrium with the air it is mixing with. The moisture content of the air will be balanced as well as the temperature. These are important considerations in attic spaces.
As air is heated, its ability to hold moisture in the vapor state is increased. In fact, as the air becomes heated, water vapor is actually attracted to it. If this happens inside your attic, the plywood face which is exposed begins to absorb this moisture. It is not unlike being in a large pressure cooker.
As we discussed earlier, this moisture can cause problems with the plywood.
On the contrary, in colder weather the confined air in the attic can also cause problems. The roof framing members and plywood of a roof, when not heated by the sun, usually are the same temperature as the outside air. Air constantly leaks from the inside of our houses into the attic. This air almost always has a higher moisture content than the air inside the attic during winter months. If this air is allowed to collect in the attic space, it possibly will condense on the framing members or the plywood sheathing. This moisture can begin to rot the wood.
The Solution - Air Movement
You can eliminate these problems by allowing air to move rapidly and freely through your attic. Remember the car with the windows down? Attic ventilation technology has advanced rapidly in the past 10 years. New products are available which make attic ventilation simple and easy. They are inexpensive and readily available.
The key to effective attic ventilation is air movement. Remember, the trick is to get the air in your attic to mix with the air outside as quickly as possible. This can be achieved with flow-thru technology.
Older homes often have triangular gable vents. Any air which exits the attic from these vents must be replaced. These vents are usually only effective if the wind is blowing directly into the vents. That usually doesn't happen very often.
Virtually, every house has a soffit. This soffit creates an overhang at the edge of a roof. Because of the manner in which roofs are framed, these soffit areas are perfect entry points for air to enter attic spaces.
In new construction, continuous ventilation strips are often installed in these soffits. This provides an ample and excellent source for air to begin its upward flow into the attic area.
To make sure that the air flow is not blocked by attic insulation, baffles should be installed. The space between the roof and the outside walls narrows as the roof passes over the outside bearing walls. Insulation can block these areas if not carefully installed. Baffles prevent this from happening.
The baffles provide an unobstructed channel from the soffit area to the attic.
Continuous Ridge Ventilation
After the soffit has been vented, the air must have a place to leave the attic. This is best achieved by installing a continuous ridge vent along the highest points of the roof. This type of venting can also be installed along roof hips. It is important to install as much of this ridge and hip (where applicable) ventilation as possible. This ventilation material combined with the soffit ventilation strips provides an effective flow-thru ventilation system.
The Vacuum Effect
Wind which flows along an object creates a vacuum effect due to a lowering of air pressure. This vacuum can be beneficial when you use continuous ridge type ventilation.
As wind blows across your roof from any direction, it can create a vacuum effect along the ridge vents. Air is sucked from your attic space as long as it can be readily replaced along the soffits. This is one of the reasons soffit vents must be installed. The vacuum effect happens even in the slightest breeze.
Continuous ridge vents are available in many styles. The original vents were aluminum strips which sat on top of the ridge. These were unsightly and they sometimes leaked.
Newer designs allow these vents to be covered with the standard cap shingles that traditionally appeared at the ridge or along hips. These vent strips are made from either hard plastic or dense coarse plastic interwoven fibers.
When applied properly they provide a sleek look to a roof. All you see is a thin shadow line where the cap shingles are raised above the regular shingles.
Pot Vents & Power Vents
You can also use standard pot vents and power ventilating units to move air from your attic. These items work best when used in conjunction with continuous ridge vents.
Pot vents are simply aluminum covers that cover holes which have been cut in your roof. They come in various sizes and designs.
Power ventilators have electric powered fans attached to a pot vent. Often these are thermostatically controlled to turn on when the attic temperature reaches a certain temperature. They turn off automatically when the attic has cooled to a preset temperature on the thermostat.