Ventilation and Moisture Control for Residential Roofing

The proper ventilation of attic areas is a very important design consideration. If implemented correctly, proper ventilation methods can help ensure the maximum service life of roof assembly materials, and can improve heating and cooling efficiency. Minimum ventilation requirements are specified by insulation manufacturers for energy efficiency and by building code officials for code compliance. In addition, ventilation is required by shingle manufacturers to ensure the performance of the roof. Overlooking this consideration may result in these moisture related problems:

  • Premature failure of the roofing including blistering
  • Buckling of the roofing shingles due to deck movement
  • Rotting of wood members
  • Moisture accumulation in the deck and/or building insulation
  • Ice dam formation in cold weather

In cold climates, internal building moisture is often a primary cause of roofing problems. Tighter construction techniques and heavier insulation applications help seal the side walls and create a more effective retarder against cold air penetration. Occupancy-generated water vapor will eventually reach the cold underside of the roof deck and condense. This may cause wood to rot, plaster to crack and paint to peel. Proper attic ventilation allows water vapor to escape before it condenses at the roof deck. Proper ventilation also helps to reduce the occurrence of many problems such as expansion/contraction of decking and ice damming in cold, snowy climates.

During the summer months, very high roof deck temperatures are caused by the sun's radiant heat. Eventually, the heat from the deck permeates the attic space, and finally reaches the living space. This, of course, decreases cooling efficiency. Additionally, recent research has reinforced the theory that prolonged exposure to extreme heat accelerates the aging of asphalt roofing products. By properly ventilating the underside of the roof deck, heat buildup and its related problems can be reduced.

The calculation of ventilation requirements is dependent on three primary factors; the size of the attic, the placement of the vents and the "rating" of the vents. There are two types of vents, intake vents and exhaust vents. It is a combination of these types that provides free-flow ventilation, the most efficient way to handle problems of unwanted heat and moisture in enclosed areas.

In most cases, a minimum free-flow ventilation area equal to one square foot per 150 square foot of attic floor area must be designed and properly installed to provide proper ventilation. where a properly designed and installed eave and ridge ventilation system is employed, a free-flow ventilation area equal to at least 1 square foot per 300 square feet of attic floor area is often sufficient. Combination eave and ridge venting is generally recognized as a superior venting technique.