Roof and Attic Ventilation
Page 1 of 5
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 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
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.
- 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.