The Importance of Roof Ventilation
By George Dwyer
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 driers unless these items are properly ventilated
through the roof. In some cases the condensation can be bad enough to be
mistaken for a roof leak.
Here are some problems associated with an improperly ventilated attic space.
Sumps between rafters (deck deflection) can happen because after awhile
(sometimes several years, sometimes only a couple years), a plywood roof deck
can warp or deteriorate and become spongy and dangerous to walk on. This occurs
because one side of plywood decking needs be able to "breathe" by being exposed
to circulating air. The adhesives used in the plywood can deteriorate or Dry Rot
can occur because of condensation.
Water vapor will condense first on anything metal inside the attic; this will
eventually cause the metal to rust. Heads can rust off nails, metal plumbing
straps or straps holding HVAC ducting can rust in two causing the ducting to
crash down on top of the ceiling joists or through a suspended ceiling. This
problem is more common in humid climates.
In colder climates – generally where the average January temperature is 32° F
(0° C) or colder – high inside humidity (40% or greater) combined with low
outside temperatures can cause frost to form on the bottom of the roof deck. See
Dry Rot in the glossary.
Insulation can trap moisture which will reduce the R-value of the insulation
and create a nice environment for the propagation of certain molds, spores, and
fungi which will also cause problems. See Dry Rot in the glossary.
There is also the problem of mildew which is both damaging and can cause
health problems. The roof system itself will deteriorate prematurely.
Cooling units will need to be serviced or replaced prematurely because of
excessive use. Ice Dams - ice dams are the result of melting snow continually
refreezing at the roof perimeter and then backing up under the shingles and
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.
There are many types of attic vents available today. There are static vents,
power vents, ridge vents, turbine vents, soffit / cornice vents, gable vents,
starter vents, and cupola vents. These all come in a wide variety of sizes,
styles and shapes. Some will ventilate better than others depending on the roof
configuration, attic size, climate, etc.
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 air into the attic; and exhaust vents, which are located near or on the
ridge line of the roof and allow 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 back from it.
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