IN
SEARCH OF "FREE AREA":
Free area is that area of vent that is not restricted by louvers
or screen wire. It is usually related to the square footage of the
attic area: 1/50 would mean one square foot of free vent area for
each 150 square feet of attic floor. The amount of free area is
determined by the existence, or lack of, a vapor barrier, location
of the vents, and the climatic conditions since where the home is
located determines its need for condensation and heat control. The
rules recommended by the HUD (FHA) Minimum Property Standards (MPS)
are those most often followed. They may be inadequate. However,
they are quoted in older publications. Homes built only 10-12 years
ago often have too little ventilation by today's standards. Below
are the present generally accepted rules for determining the net
free area for attics:
Attics
without vapor barriers-
One square foot of free area is needed for each 150 square foot
of attic space (1/50). If half of the vents are in the eaves and
half are located at least 3 feet above the attic floor, you can
reduce the required free area to 1/300.
Attics
with 6 mil. polyethylene-
This attic calls for a free area of 1/300. This assumes that half
of the vents are in the eaves and half are located at least 3 feet
above the attic floor.
Vaulted
(Cathedral) ceiling-
You will need a continuous air space above the insulation, with
continuous eave vents and continuous ridge vents, or you must have
individual vents in the eave as well as near the ridge for each
rafter space.
Climate-
In areas having a high heat gain during the summer, attics with
or without a vapor barrier should have a total free area of 1/150;
half of the vents should be in the eaves and half near the ridge
of the roof.
Vent
size-
When figuring the size of the vents to be used, remember that the
figures above are for the net free area, which refers to the area
or the vent that is not obstructed by louvers or screen wire. Use
the factors in the chart on page 2 to determine the required gross
area of the vent openings. For example, if you are using gable and
vents with louvers and 1/8-inch mesh screening, you will need 2-1/4
square foot of vent before you have 1 square foot of net-free area.
Powered
attic fans-
Powered roof or gable attic fans are rated in cubic feet per minute
(cfm) at either "static air pressure" or "free air delivery." To
determine the minimum cfm you home requires, multiply the attic
floor area by 0.7 Add 15 percent for a dark roof. If the fan is
rated in "free air delivery," discount the cfm by 25 percent to
arrive at the correct figure.
For
example-
if you attic measures 1500 square foot, you require a fan delivering
1,050 cfm (or 1,208 cfm under a dark roof). For best results, you
must also provide adequate under-eave venting; 1 square foot of
attic floor space is recommended (1/150).
