Calculating Your Ventilation Requirements

Step 1
Calculate attic square footage
How: Multiply length of attic (in feet) times width of attic (in feet)
30’ x 40’ = 1,200 square feet
Step 2
Calculate NFA (Net Free Area) needed for this attic by using the “1 in 300” rule
How: Divide attic square footage by 300
1,200 sq. ft. ÷ 300 = 4 square feet of NFA needed
Step 3
Convert square feet of NFA to square inches
How: Multiply square feet of NFA by 144
4 sq. ft. x 144 = 576 square inches of NFA needed
Step 4
Split the amount of NFA needed equally between the intake and the exhaust
How: Divide square inches of NFA needed by 2
576 sq. in. ÷ 2 = 288 square inches of NFA needed equally for exhaust & intake
Step 5
Calculate # of lineal feet of Exhaust Vent needed
288 sq. in. ÷ 16.9 = 17 lineal feet
Our option is that in the ratio of intake to exhaust, the intake should be slightly more than the exhaust.

Ice dam, on a smaller scale, is a problem of house and building maintenance in cold climates. An ice dam can occur when snow accumulates on the slanted roof of a house with inadequate insulation and ventilation in the attic. Warmth coming up through the roof melts the snow. Meltwater flows down the roof, under the blanket of snow, onto the eave and into the gutter, where colder conditions on the overhang cause it to freeze. Eventually, ice accumulates along the eave and in the gutter. Snow that melts later cannot drain properly through the ice on the eave and in the gutter. This can result in:
Leaking roof (height of leak depends on extent of ice dam).
Wet, ineffective insulation.
Stained or cracked plaster or drywall.
Rotting timber.
Stained, blistered or peeling paint.
Under extreme conditions, with heavy snow and severe cold, almost any house can have an ice dam, whereas a house that is poorly insulated and ventilated will have ice dams during normal winter weather. Giant icicles hanging from the eave are one indication of a poorly insulated, poorly ventilated attic.