Roof Ventilation
The best type of ventilation occurs
when cooler air can enter the attic through soffit or fascia intake
vents. These vents can be seen from the ground and are located behind
the gutters. As the cooler air warms and rises, the warm air is vented
out through exhaust vents mounted on or near the top of the roof.
Without these vents, heat and moisture
will build up in the attic area and combine. This combination causes the
sheathing to rot, shingles to age prematurely, and the insulation to
lose its effectiveness.
Attic ventilation is an important part
of roofing. Proper attic ventilation extends the life of a roof and
reduces problems because it minimizes the temperature differential
between the attic and the air outside. Proper ventilation will remove
moisture and heat from the attic. Trapped heat and moisture can raise
energy costs, cause ice dams, and damage roof system components as well
as structural and personal items located inside the attic where
temperatures can easily reach 150° F (65° C). Condensation that forms
inside attics can be caused by the use of washing machines, dish
washers, bath tubs, showers, and tumble dryers unless these items are
properly ventilated. In many instances the condensation is bad enough to
be mistaken for a roof leak.
One problem associated with an
improperly ventilated attic space is:
- Ice Dams - ice dams are the result
of melting snow continually refreezing at the roof perimeter, and then
backing up under the shingles and underlayment causing leaks. Proper
ventilation used in conjunction with heavy insulation and an air
barrier can create a Cold Roof Assembly which will help eliminate ice
dams.
To properly ventilate an attic, two
types of vents are needed. Intake vents, which are located at the
downslope edge of the roof (a.k.a. eaves) and allow fresh cooler air
into the attic; and exhaust vents, which are located near or on the
ridge line of the roof and allow warmer air to leave the attic. The use
of an exhaust vent in conjunction with an intake vent uses the natural
forces of wind pressure and thermal effect, collectively known as the
Stack Effect, to ventilate the attic space. HINT: Make sure your attic
insulation doesn't block the intake vents. If necessary, use baffles to
keep the insulation away from the intake vents.
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 instance, 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.
There is always concern regarding the
best type of ventilation. Both intake ventilation and exhaust
ventilation installed at an approximate one to one (1:1) ratio are
required for maximum air circulation. One square foot of ventilation per
300 square feet of attic floor space is a general guideline and is a
code requirement in some areas.
There are several common
misconceptions about attic ventilation. Many people believe if they have
only power vents or turbine vents working near the ridgeline, that their
attic is properly ventilated. In order for an exhaust vents to properly
function, they have to have intake vents working with them. Air has to
enter somewhere. If there are no intake vents air will enter through
some of the other exhaust vents. The result is circulation of only the
air immediately surrounding the vents or in between the vents.
Another common misconception is that
"more is better". Many people believe they can improve the ventilation
of their attic by installing vents throughout the roof surface. This can
cause a Ventilation Short Circuit. For instance, if a roof has a full
soffit and ridge vent system, and additional vents are installed halfway
up the slope, the ventilation is now hampered because air is now exiting
out the vents in the middle of the roof before it reaches the ridge
leaving the attic partially un-vented. Depending on wind pressure, air
can also be taken in at the intermediate vents reducing the
intake at the eaves.
There is also the problem
of weather infiltration. Wind blowing across a roof surface creates
negative air pressure. Nature will automatically try to compensate for
it by moving air from a location of higher pressure, such as inside the
attic. When the air is removed from the attic in this manner, it has to
be replaced. If the proper intake ventilation isn't used, air will be
brought into the attic through the exhaust vents and will, at times,
bring moisture with it.
