Venting Size for an Attic

Vent Sizing

• The two main factors governing the size of your vents is the size of your attic and whether your insulation has a vapor barrier installed. In attics without a vapor barrier, your attic will need one square foot of venting for every 150 square feet of floor space in the attic. If your attic insulation has a vapor barrier, the ratio of venting to attic space increases to one square foot of vent for every 300 square feet of space.

Vent Placement

• The main vents for your attic are usually placed in the soffit of the roof. The soffit is the flat panel on the underside of the roof where it extends past the exterior walls of your home. If your home uses a vaulted or cathedral style ceiling, you will need to install additional vents along the ridge line of the roof to ensure adequate ventilation can flow throughout the attic.

Rafter Vents

• Rafter vents are used to ensure that your insulation does not block the soffit vents. Rafter vents are installed on the underside of your roof deck where it slopes down above the soffit vents. These vents allow you to install insulation up to the edge of your ceiling without blocking your vents. You should install a rafter vent for each soffit vent and fully cover the spaces above your exterior walls with insulation. This combination of venting and insulation will prevent heat from leaking into your roof, improving your energy efficiency and preventing ice dams during the winter.

Ventillation Fans

• Most homes use natural ventilation to cool the attic, however you can also install temperature sensing fans that will automatically ventilate your attic when it reaches a preset temperature. These fans are effective but often use more energy than they save. If your attic already uses fans, you can install solar powered fans that will decrease your energy costs. Solar powered fans also decrease the amount of moisture entering your attic, but they can't run at night or when it is raining.
  In most homes two fans will provide adequate ventilation for the attic.