Attic Roof Ventilation

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 difference 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 the 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.

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.

There is also the problem of mildew which is both damaging and can cause health problems.

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.