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A correctly designed attic ventilation system is one of the most important features of a new roof. There has been much discussion on this subject because of variety of improvements in the construction of new homes and the many climates changes in the country we live.
What the pro’s say.....
Typically, the building codes require that a minimum of 1 ft.² of net free ventilation be provided for every 150 ft.² of attic floor space. This can be accomplished by the use of dormer vents, turbine type vents, ridge venting systems, gable end vents, eave block vents and static vents to mention a few. There is a reduction of the ventilation ratio from 1 and 150 to 1 and 300 with the combination use of a vapor barrier in the attic. After 8/1/09 this is no longer an exception. 1 in 150 is the rule.
Most local building codes are based on "model building codes". In recent years, the several residential model codes (ICBO, BOCA, and SBCCI) have been consolidated into the International Residential Code 2003 (IRC-2003). This code is referred to as a model code. Local building authorities make the final decisions as to their local needs but most local building departments adopt the IRC-2003 with little or no modification.
NRCA The Nation Roofing Contractor Association also recommends a balanced ventilation system of 1 square foot of net free ventilating area for every 150 square feet of attic be provided for steep sloped roofs.
So it is good practice to use the 1 and 150 rule.
Why Ventilate ?
Simply stated it is to prevent condensation from occurring on the undersides of roof sheathing and insulation during colder months and to evacuate hot air from the attic during hot days. This arguably extents the life of the roof coverings plus has additional benefits of reduced HVAC use.
In the winter condensation can occur from warm air of the living condition raising and meeting the colder air in the attic space. Again by cycling the air with the use of a balanced ventilation system, fresh air this will lessen the condensation.
It is my experience that in the warmer months an attic can reach upwards of 160 degrees. With the use of a breathable or balanced attic ventilation system, the high temperatures can be evacuated and introducing a supply of clean fresh air, that is lower in temperature, will reduce the temperature of the attic space.
Of course the country we live in has a variety of climates and weather conditions, the design may need to be adjusted. Thus the constant discussion goes on and the local rules may vary.
So what do I do?
Hire a professional licensed contractor who has the knowledge to design a balanced ventilation system. If your contractor doesn’t bring up the subject of your attic ventilation he probably will not have the proper amount of ventilation budgeted for the job. Most manufacturers of roofing products require attic ventilation or the warranty may be affected.
So in conclusion…. The benefits are great for a small cost during a roofing installation. The average residential house costs a few hundred dollars. With the reduced cost a running the HVAC system and damage that may occur cause by condensation, it is a wise choice.
Bob is the director of technical advice for Ask A Roofer.com.
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