When determining the ventilation needs of your home, a professional roofer will calculate the required “net free” ventilation space needed for your roof, and select ventilation equipment that will meet these needs. A vent’s effectiveness is measured by its Net Free Vent Area. The Net Free Vent Area is the portion of the opening in the vent that actually ventilates.
For example, a vent can have an opening that measures 12 inches by 12 inches. This would appear as if it would yield 144 square inches of ventilation area. Because of louvers, an insect screen, or some other type of blockage, the actual ventilation area could be as little as 40 percent of that, yielding about 58 square inches of ventilation area. This 58 square inches is what’s known as the Net Free Vent Area, and is the amount that should be used when calculating how much venting you need.
Rafter Ventilation for Existing Buildings
The purpose of rafter ventilation is to reduce or eliminate heat build-up, and reduce or eliminate water vapor that can condense in the ventilation spaces.
Achieving rafter ventilation can be difficult or impossible for existing buildings, and can involve significant costs. However, every attempt should be made to design and install an adequate ventilation system.
If you suspect you may have poor rafter ventilation, contact Minnesota Building Contractors today. Call 507-206-9607 or submit a free roofing evaluation request.