In the old days, attic ventilation was not important.
Houses had very little insulation and were not very air tight. Now, homes are well insulated and tightly sealed. Attic ventilation is needed for a comfortable home. Building codes now usually require 1 square foot of ventilation for every 300 square feet of attic area, divided equally between intake and exhaust vents. This is simple arithmetic, no guesswork is involved
Without attic ventilation, summertime attic temperatures can exceed 160 F. Even at night the attic stays hot. The excessive heat reduces the life of your roof and eventually penetrates to the living space of your home. A well ventilated attic will make your home noticeably cooler and reduce the load on your air conditioning.
Attic ventilation removes moisture from your attic. Moisture in your attic can cause mold and mildew. Attic ventilation can also help prevent ice dams.
Water vapor is produced from bathing, cooking and household appliances. This moist air rises through the attic until it contacts the cold winter air, where it can condense into water droplets on the underside of the roof. It then can drip onto the insulation, reducing its efficiency. The water can cause mildew and mold, rot the wood, damage and spot ceilings and paint, and cause damage to the roofing. Also, ventilation helps prevent ice dams, which is especially important in Colorado. Ice and snow on a roof will melt and run down the roof to the cooler eaves, where it can freeze and create a dam, trapping moisture on the roof. Ventilation helps to prevent this by keeping the roof deck temperature consistent.
How much attic ventilation do I have?
|Type of vent||# of vents||multiply by||si nfa
|=||Total si nfa|
|RV51 or box vent||________||multiply by||51||=||_________|
|Large dome or unused
attic exhaust fan
|turbine - 14"||________||multiply by||110||=||_________|
|ridgevent||____ linear feet||multiply by||18 si/ft||=||_________|
|Gable vent 12" x 12"||________||multiply by||60||=||_________|
|Gable vent 12" x 18"||________||multiply by||90||=||_________|
|Soffit vent 4" x 16"||________||multiply by||26||=||_________|
|Soffit vent 6" x 16"||________||multiply by||45||=||_________|
|Soffit vent 8" x 16"||________||multiply by||65||=||_________|
|Total vent area||=||_________|
Vents are rated in square inches (si) of net free area (nfa).
Net free area is the amount of actual free space, allowing for the screens and louvers.
To convert square feet to square inches, multiply the square feet by 144 (number of square inches in one square foot).
To download this as an Excel spreadsheet (with photos) CLICK HERE.