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Any time of the year, weather can be your home's worst enemy. In summer, excessive attic heat and moisture can be incredibly tough on your roof, causing premature deterioration, like wood rot and shingle damage. As temperatures climb outside, hot air gets trapped inside your attic, causing your air-conditioning to work overtime and your utility bills to soar. By the time winter rolls around, the insulation in your home can be ruined, or its effectiveness greatly reduced, when exposed to moisture due to poor attic ventilation. Even worse, the continued melting and refreezing of ice and snow from an improperly vented roof can dramatically accelerate roof failure.

How weather effects ventilation
Two natural forces – wind pressure and thermal effect – cause air to move into and out of an attic. When wind strikes the end, corner, or roof of a house it will be forced in another direction (usually upward), because wind can not pass through a solid structure. When this happens, a vacuum effect is created near the ends of the structure. As this happens, this newly directed wind enters the attic via vents within the areas and it exits through the other pressure zones around the ends of the structure, acting almost as a natural fan in an attic. The pressure caused by these misguided winds will greatly depend on the speed of the wind itself.

Wind changes direction constantly, sometimes up to 30 degrees within a matter of seconds.  Pressure in certain spots causes the vents to either from intake vents to exhaust vents or exhaust vents to intake vents.  When this occurs, effective ventilation is minimal.

A home's exterior can also effect the ventilation of an attic.  Wind produces a cooling effect on the outside of the home, which in turn causes the interior to cool a couple of degrees as well. 

Thermal effect on ventilation
The second major natural force affecting attic ventilation is the height-temperature difference, or thermal effect.  This force results from the difference in temperature of the air in the attic and that outside, combined with the difference in elevation between the lowest, and the highest vent opening.  Also, winter temperature differences are small in a well insulated house.  Under these circumstances, this force is very limited.  In summer, if the vent openings are in the soffit and at the ridge, the air moves adjacent to the hot surface of the roof sheath.

The combined effect of both forces
Wind pressure and thermal effect operate separately or in combination.  When both forces are equal, the total air flow will be approximately ten percent greater than for either force operating independently. This percentage decreases rapidly when either force increases over the other.

Types of Ventilation

Roof Louvers are mounted at the ridges of roofs.  Some have a baffle design and some may have a turbine design, with both vents putting out about the same energy of winds in the attic. The turbine design is more efficient in circulating the air and pushing it out of the attic.  These vents, although effective, don't do much for circulation if they are the only ventilation systems put into the home. 

Gable End Louvers are inexpensive and inconspicuous and are mounted in the top "point" of the gable.  When the wind hits perpendicular to the ridge, a portion of each louver serves as an intake and the other portions of the louver serve as the exhaust.  As a result, air flow is confined to a small area adjacent to each louver.  When wind hits parallel to the ridge, the rate of air flow through the louver is approximately 70% of the wind velocity.  They might look good, but the air flow is not uniform through the entire cross section of the attic. 

Rectangular Louvers are sometimes mounted in the gable ends of the attic to serve as ventilation intake and exhaust, similar to gable end louvers.  These vents also make a wonderful design element for your home.

Continuous strip soffit vents, or uniformly close-spaced soffit vents provide effective CFM (cubic feet per minute) regardless of wind direction.  They are installed in a horizontal plane so that wind is parallel to the vent face regardless of wind directions, with the ventilating area installed on opposite sides of the building, providing a balance between intake and exhaust areas.  When wind hits parallel to the ridge, air enters the soffit vents on each side toward the downwind end, and leaves at each side toward the windward end, providing a counter flow effect.  When the wind is perpendicular to the ridge, air moves through the attic in the same direction as the wind on the exterior of the building. 

Several tests established the rate of air flow through soffit vents and the relation of this to air movement in the attic.  It was found that, at low wind velocities, approximately 70% of the total volume of air moves in the area within four inches of the floor.  At higher wind velocities, 20% of the total volume remains in this area near the floor.  With a diagonal or longitudinal wind, there is a supplemental air flow longitudinal to the attic, moving with the wind along the attic ceiling, and counter wise near the floor.  This secondary air movement causes dead air above the floor and below the ridge.  It is in the area that frost can form on the roof sheathing during winter.  Because most of the air flow is near the floor, it might appear that the soffit vent would be effective for summer cooling of that area.  The problem is that it does not reduce the temperature of the roof sheath.  Thus a greater quantity of heat is radiated to the attic floor without changing the temperature of the attic air. 

Determining Your Ventilation Requirements

When determining soffit ventilation options, your choices are many. Each has its own net-free area. Net Free Area Defined: The net free vent area is the actual, unobstructed area where air can freely flow from outside, to inside, to outside. For every 750 cfm of airflow, it is recommended that there be a minimum of 1 sq. ft. of attic venting, i.e. roof vents, gable vents, soffit vents, ridge vent. This allows the air flow of the fan's exhaust air into the attic and out of the home freely.

To calculate your net free area, add up your total number of vents in feet. If your vent(s) are screened take approximately 1/2 away from your total. For example:

You have four 1 sq. ft vents, and they are screened. You would have a 4 sq ft gross of venting, however since the screens will block air flow, you would need to subtract approximately 2 sq. ft of venting from your gross, giving you a 2 sq. ft net free area.

The importance of a properly balanced ventilation system is hard to exaggerate. Experts say inadequate intake ventilation causes 95% of all ventilation problems and will typically void the shingle warranty.

So what is balanced ventilation? Balanced ventilation can be defined as equal amounts of air per foot coming in through the continuous soffit vents and going out at the ridge vent. More air coming in at the soffit will allow the ridge vent to do a better job of removing hot air at the peak. If the ridge vent has more air flow than the soffit, the ridge vent could back draft and pull hot air, debris, rain, etc into the attic rather than expel as designed. More air flow will create more air exchanges which will serve to lower attic temperatures, akin to lowering car windows or opening the front and back door of a house, creating a strong pull-through of ventilation.

So how does this stack up in the real world? An example of a simple gable design home with a 40' ridge: ridge vent net free area = 40ft x 18sqin/'nfa = 720 sq.in of net free area; therefore, you need at least 720 sq.in of net free area coming in at the soffit. Soffit vent net free area = 40ft(2) x 9sqin/' NFA = 720 sq .in. of net free area, so the system is balanced.

What's wrong with this picture? This assumes an ideal world where no insulation blocks the air of the soffit vent and you can run the soffit vent the full length of all available horizontal soffit areas. Builders typically break up the front of the home with design features, leaving maybe half of the front or less available to ventilate

In this situation, as long as half of the front could be ventilated, you can still balance with a 12.7sqin/' NFA continuous vent; the calculation would be (40 + 20) x 12.7 = 762 sq.in of net free area. Having more air flow at the soffit than ridge creates no problems. These net free area calculations are based on .030 vent material thickness as standard. Be careful with products providing airflow resistance inherent in product designs. We recommend running ridge and continuous soffit vents from end to end in all available horizontal areas for best aesthetics and ventilation .

Finding a Solution

Roof louvers combined with soffit vents provide both high and low ventilation. However, it is practically impossible to install enough high vent area for a balanced system.  The result is the combination provides about the same amount of ventilation per square inch of vent area as soffit vents alone.  The air movement is confined to a few areas adjacent to the roof louvers and close to the attic floor.

Gable end louvers combined with soffit vents doesn't alter the air flow patterns characteristic of each type of vent used alone.  Most of the air movement is adjacent to the attic floor, thus the effectiveness of per square foot of vent area for summer cooling is similar to that of either type of vent alone.  It is possible to install a substantially larger amount of vent area and secure more cooling.  For winter moisture control, the effective CFM per square foot of vent area is usually less for the combination than for either type of vent used alone.  When the wind is perpendicular to the ridge, the combination increases the air flow by as much as 50%, compared to soffit or gable louvers used by themselves.

Ridge vents combined with soffit vents are the recommended solution. This combination provides a more rapid and complete change of attic air than any other combination of vents or louvers.

The ridge vent provides a continuous opening along the entire ridge line of the roof.  It is mainly available in aluminum, in 10-foot lineal sections, or in 4-foot self-contained units.  Ridge vent must always be installed in combination with soffit vents.  If a ridge vent were supposed to be installed alone, then part of it would serve as an inlet as a result of pressure differences at various points along the ridge.  This would cause weather infiltration and would confine air flow to the top of the attic near the ridge.  Because it is continuous along the ridge, the ridge vent provides uniform air flow along the entire roof sheath surface.  This can only be accomplished with an ample net free area of inlet located in the lower portion of the attic. 

Continuous soffit vent can provide this intake area, assuming uniform airflow that corresponds with the ridge vent.  Since the ridge vent is located at the highest point of the attic, and the soffit vents at the lowest, the system simulates air flow through thermal effect to a greater degree than any other combination of vents.  If there is little or no wind, appreciable attic ventilation can still be realized.  No other system makes this effective use of natural forces. 

Alcoa Delivers Solutions
Don't let weather destroy your roof's beauty. An Alcoa Roof Ventilation System promotes continuous, even air flow that reduces condensation and balances inside/outside temperatures throughout the year. It keeps attics drier in the winter and cooler in the summer - reducing interior temperatures as much as 50 degrees! And, no matter what the weather's doing outside, Alcoa Roof Ventilation Systems will keep you and your family more comfortable inside all year long.

Regardless of the style and type of your ventilation needs, Alcoa delivers the products to deliver optimal performance from Alcoa Vent-A-Ridge to various gable vent styles to multiple soffit alternatives ranging from aluminum to vinyl.

Alcoa Vent-A-Ridge
Alcoa's Vent-A-Ridge Roof Ventilation System adds a nearly invisible line of ventilation along the roof's peak. Unlike unsightly gravity and turbine vents, Alcoa's connector-less roof ventilation systems blend with the existing roof line, adding low profile venting without taking away from the beauty of your home.

Alcoa's unique ridge ventilation system is installed along the roof's peak, and a continuous louvered metal strip is added to the overhang on each side of the home. The ridge and soffit vents work in tandem, eliminating any need for costly, power-consuming attic vent fan units and providing total, season-long attic venting.

Air that is continuously drawn in through the soffit bathes the underside of your roof and then exhausts at the ridge cap. Wind passing over the home and rising hot air in the attic create a natural convection action that automatically drives the entire process without using any electricity.

When combined with a vented soffit system, the Alcoa Vent-A-Ridge effectively removes attic heat and moisture, adding years to your roof while substantially reducing your heating and cooling bills.

Vent-A-Ridge Advantages

• Same peak-performance
• Constant, even air flow from soffit to peak
• Wide selection of colors and optional mill finish

Soffit Ventilation Options
When selecting soffits to satisfy your ventilation needs, there are two elements you need to consider. First, the amount of ventilation you need from your soffit. This element is critical because the style of the perforation hole determines the amount of Net Free Area available in the soffit panel. Two hole styles are available including a perforated and a lanced. Review the chart below to view the difference you will notice that the perforated hole style offers the most Net Free Area per square foot. You can also choose a hidden vent soffit for a more pleasing aesthetic without giving up any ventilation capacity.

Styles:

• Perforated
  

• Lanced
  
  
• Hidden Vent
  
  
• Center Vent
  

The second choice you have when selecting your soffit is the material from which it is constructed. Two alternatives exist including aluminum and vinyl. Each alternative provides its own features and benefits including.

Alcoa Aluminum Soffit Systems

Alcoa Vinyl Soffit Systems

Alcoa Soffit Systems (see product catalog for further details)

Gable Vent Options
Alcoa provides gable vent solutions in various sizes and shapes. Including:

• Round
• Octagon
• Half-round
• Round Top
• Square
• Rectangular

For further details regarding sizes and net free area please consult your Alcoa Home Exteriors' product guide.

Avoid These Common Pitfalls

• Don't block soffit vents with insulation.
• Don't exhaust warm humid air from dryers, range hoods, bathrooms, or furnaces into the attic area.
• Don't draw furnace combustion air from the attic area unless soffit ventilation is increased proportionate to unit requirements.
• Do separate garage area from attic area when garage ceiling is not enclosed.
• Do close tri-level roof attic areas at junction points and ventilate each area separately.
• Don't expect efficient, trouble-free attic ventilation without adequate soffit ventilation.