Attic Ventilation

How Ventilation Helps Solve Attic Moisture Problems and Ice Dams

Although the problems of heat and moisture accumulation in the attic have different causes, they share a common solution: a high-efficiency ventilation system. In warmer months, a ventilation system exhausts hot air from an attic; in the colder months, it exchanges warm, moist air with cooler, drier air.

Winter creates a special attic ventilation problem in areas where snowfall and cold temperatures are common occurrences. The problem begins with the formation of ice dams which prevent melt water from running off the roof.

Ice dams can form when:

  • Warm air accumulates in the attic. Normally, the pocket of warm air in the upper portion of an attic won’t result in problems – unless the following conditions occur as well:
  • Lower areas of the roof remain cold. Especially near the eave, where temperatures may not be much higher than the ambient outdoor air. If the outdoor temperature is well below freezing, conditions are favorable for the formation of an ice dam.
  • A heavy snow cover accumulates on the roof. Snow provides the necessary moisture and it also acts as a layer of insulation, preventing heat loss through the sheathing. As a result, attic temperatures are typically warmer than they are on days when the roof is free of snow.

Under these conditions, ice dams form quickly. Heat high in the attic causes snow to melt near the roof peak. The water from the melting snow flows toward the eaves area, where the colder roof temperatures allow it to freeze. If conditions persist over several days, this refreezing of snow melt can form an ice dam.

The weight of a falling ice dam can damage gutters, fascia or shrubbery down below. Roof damage occurs when the water pooling against the dam begins to back up under shingles. The shingles get damaged or destroyed. Far more serious, however, is the damage caused at the plateline area. Insulation can be soaked, reducing its effectiveness. Plus, water can infiltrate into both exterior and interior walls, leading to structural damage. At the very least, mold spores and mildew can form, creating unpleasant odors and poor indoor air quality.

No amount of insulation, if used alone, can eliminate the formation of ice dams. An efficient attic ventilation system must be part of any solution.

A properly designed ventilation system creates a “cold roof” – a condition where the roof temperature is equalized from top to bottom. An equalized roof temperature helps eliminate the conditions that lead to the formation of ice dams.

Ventilation alone is not a complete solution either. Ventilation must be used with a waterproofing shingle underlayment and insulation. (It is difficult to say precisely how much insulation will be required. Many factors, from house design to its orientation to the weather, enter into the equation. A good rule of thumb, however, is to provide at least 13 to 16 inches of insulation. That’s the equivalent to an R-value of 40.

A Defense Against Ice Dams

To reduce the possibility of ice dams, use a three step approach:

  1. Install adequate attic ventilation. The most effective way to equalize temperatures is to create a “cold roof”. One of the most efficient and cost effective systems uses “Static Vents” (Maximum or Venmar Tower Vents)and evenly positioned “intake vents”(Soffits, Intake grilles or VMAX vents) to distribute airflow from eave to peak.
  2. Install adequate attic insulation. Attic insulation serves two purposes:
      • It reduces heat loss, which is a key factor contributing to creation of ice dams.
      • Adequate attic insulation diminishes the energy impact of having cold air flowing through the attic.

    Be sure there is adequate insulation around electrical fixtures, and wiring and plumbing chases. These areas often contribute to significant heat loss. Check existing insulation for water damage and for areas compressed by foot traffic or stored items. Finally, make certain existing insulation meets today’s R-value requirements.

  3. Install waterproofing shingle underlayment (commonly referred to as Ice and Water Shield) where required. Ice and water shield can minimize or eliminate water infiltration into the building structure.  Ice and water shield must be installed along the eaves and up the roof at least three feet beyond the interior wall line. Seeing how most houses have at least two feet of roof overhang, most roofing contractors will opt to give you six feet of ice and water shield coverage. (2 feet of overhang + 3 feet past the interior wall = 5 feet of ice and water shield coverage is needed for your home. Rolls of ice and water shield come in 3 foot wide rolls, most roofers will opt to do 2 rows totalling 6 feet total Ice and water shield coverage). Valleys should be lined with 36” wide piece of ice and water shield.