Present building code requires either a.) One square foot of venting for every 150 square feet of attic space. Or b.)  One square foot of venting for every 300 square feet of attic space if a minimum of 50% of the vents are in the upper portion of the attic.   Ideal application is to provide lower venting (Soffit venting is ideal, or if the style of your house doesn’t make this practical, roof venting installed near the bottom of the roofing is effective) and higher venting (near the roof ridgeline) to take advantage of the natural convection process, wherein hot air rises and sucks in cooler air into the lower vents.   

I would recommend you go with the second requirement of 1/300 ratio.  Although it is less venting, if properly designed it is by far the more effective type of venting.  I also routinely recommend passive venting styles-they not only cost less to install but I really like the cost of running these systems-nothing! Powered vents don’t seem to last very long and the noise can keep you awake all night if installed near bedrooms, especially as they get older and noisier. 

First figure the square footage of the attic.  This is usually fairly easy if you add the square footage of the home plus about 225 square feet for a single car garage or 450 square feet for a 2 car garage.  In addition you will need to add the square footage of soffits (eaves) or patio ceilings.   Once you have this total divide by 150 or 300 depending on the method of venting you choose.  This figure is the square footage of venting needed. 

Second, choose the type of venting you want to install.  The most common types are turtle/static vents or O’Hagin vents.  Turtle vents are about one foot square and look sorta like a turtle shell. O’Hagin vents are made to look like concrete tile, so can be hard to see if painted the color of the tile.

Vents are marked with the amount of venting they provide with the designation NFVA (Net Free Ventilation Area) or FVA. 

If you choose turtle vents, you have to pick metal or plastic.  No preference here, except that the plastic  are usually larger providing more venting area.  If metal each of these will provide about .27 of a square foot (40 sq. inches) of venting and if plastic about .42 of a  square foot (61 sq inches). These vents can be used with either asphalt shingle roofs or concrete tiles. 

The O’Hagin vents are very common in this area.  These metal vents are made to have the profile of a concrete tile so can be flat or shaped in the style of mission tiles.  They range from .59 to .68 of a square foot (86 to 98 sq inches). 

A check with several roofers in the area puts the cost of installing these vents at $35 to $50 each.  Turtle vents are available for about $8 at local warehouses.  O’Hagin vents are available at roofing supply companies. A do-it-yourselfer can install either type, but roof leaks can occur if you’re not careful to follow instructions. 

O’Hagin installation instructions are available at: O'Hagins's INC.

Turtle/static vent installation instructions can be found Here (PDF File).

If you have soffit vents (usually metal with little perforations) there is another potential issue for your attention.  In most local installations of blown in insulation, the insulation is tight against the roof decking effectively cutting off ventilation of the soffits.  Building code requires 1” of ventilation space between decking and insulation.  Good building methods require baffles to be installed prior to insulation installation.  If your insulation is installed tight you should rake it out to provide some ventilation.

While were at it, take a look at your insulation levels.  If insulation is low you are paying more for both heating and air conditioning than you should be.  If you have blown in fiberglass you should have about 14 inches to meet the code of R38.  If it’s cellulose look for about 10 inches.  “In a home with poor ceiling insulation, heat movement through ceilings may account for 30 percent or more of the total cooling cost. With a well-insulated ceiling, this source of heat may account for only 12 to 15 percent of the total cooling cost. Thus, high attic ventilation rates are most important for poorly insulated ceilings.” B. R. Stewart, Texas A&M University “Attic Ventilation for Homes”

Attic ventilation-pay once to get it right or continue to pay the electric utility and the air conditioning repairman-it’s your choice.