Roof Vents and Ventilators Information

Centrifugal Fan Powered Exhauster via Loren CookRoof vents and ventilators are used to ventilate the attic or crawlspace of a building, circulating air for cooling, and moisture mitigation. More elaborate HVAC systems can use roof vents to ventilate the whole building. Simple roof vents are screens installed in the soffits or eaves of the house or a square or triangular louvered gable vent on each end of the house’s top. Larger, more enhanced vents are mounted on top of the roof. These larger vents must be carefully installed as to avoid water leakage or destruction by high winds. The vents must have a cap or hat on the top to prevent rain from coming straight down the stack. 

Roof vents are very important for the life of the roof, since they mitigate moisture buildup under the roof. Moisture is the main cause for premature roof failure due to the mold and mildew build up that moisture promotes. These vents ventilate and cool the building by exhausting the super-hot air that builds up in the attic or crawlspace under the roof. In the winter, by mitigating attic moisture, ventilators prevent ice dams from forming on the roof. 

Ventilators may be passive or powered. Passive ventilators rely on the chimney effect and often have an air powered turbine, driven by the rising hot air inside or wind flow outside. Powered ventilators have an electric fan inside and can move more air for a given pipe diameter, at the cost of electrical energy. Some powered vents have built in solar panels to power their fans. The drawback of  solar power only ventilators are that they won’t work at night, in the shade, or work very well on cloudy or winter days when solar radiation is at its lowest.

Specifications

To select the proper vent for the desired application the airflow needed in the building must be calculated. There are established air flow building standards that can be used to calculate the flow needed. Effective attic ventilation requires air to vent out at or near the peak of the roof via the ventilators and replacement air to be drawn in under the eaves of the house, by soffit or gable vents. Any through-the-roof cut is a potential source of leaks, so the ventilator must be installed correctly, and then periodically inspected for water leakage. 

Vents usually come with screens to deny bugs and other wildlife entrance. Stock should be regularly inspected to signs of animal damage or for security risks. In areas prone to high rain and storms, such as Florida hurricanes, roof vents have to be made to resist side-blowing rain and high winds. In these areas, hoods should be installed over the vents if a large tropical storm is expected. 

Installation

Passive Turbine Vent via GAFLarger roof ventilator units, usually powered, are often mounted on a roof curb. A roof curb is a rectangular extension piece that goes through the roof into the building and connects with the rectangular exhaust duct. The ventilator is mounted on top of the curb. The curb raises the ventilator a foot or two above the roof. This helps it vent more through a longer chimney length and allows more wind to hit the vent. The curb also helps keep the ventilator’s electrical components drier by keeping them away from a wet rooftop. 

The most common ventilator is the slant back ventilator. It fits well on a typical pitched roof and provides enough flow for a residential house attic or a restroom ventilator. Slant back ventilators also are naturally resistant to rain and are unobtrusive and inexpensive. Most beneficially, slant back ventilators are easy to install in comparison to other types. 

Resources

Roof Vents or Attic Vents - Networx

A Crash Course in Roof Venting - Building Science Corporation  

Image credits:

Loren Cook | GAF



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