|R o o f V e n t i l a t i o n
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 space. |
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
- 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
- The roof system itself will deteriorate prematurely.
- Cooling units will need to be serviced or replaced prematurely because of excessive use.