Ventilation is rarely the cause of premature deterioration of shingles. Look at the class action suits against shingle manufacturers; shingle quality seems to be deteriorating…possibly due to high oil costs…asphalt being replaced with fillers??? Certainteed Shangle and Horizon brands barely lasted 10-12 years even with the light coloured granules which are known to extend shingle life due to reflection of sunlight!!
Building scientists Dr. Joe Lstiburek and Dr. Bill Rose who have studied roofing systems, venting and insulation systems rate ventilation about FIFTH on the list of causes of shingle failure/aging.
(1) shingle quality
(2) shingle colour
(3) shingle orientation (facing south, etc.)
(4) geographic location
(5) attic venting
“Myth Two is that attics need to have lots of ventilation. Again, venting requirements are not based on rigorous scientific research.
TenWolde explains that attic venting originally arose as a moisture-control strategy for cold climates. Other purported benefits, such as longevity of the shingles, arose later. It is widely believed that increased attic venting will prolong the life of roofing shingles by cooling them. But research shows that venting has very little, if any, effect on shingle temperature. The most important issue in shingle temperature appears to be the color of the shingles. Light-colored shingles reflect sunlight and don’t get as hot as dark shingles.
One possible real benefit of attic venting in climates with large snowfalls is to reduce snow melt on the roof to avoid the formation of ice dams. But according to TenWolde, a more effective——and energy-efficient——way to control snow melt in almost all climates in the United States would be to use air barriers and insulation to prevent heat from entering the attic.”
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From the Canadian Roofing Contractors Association (CRCA) with my bold and some comments in blue (more may follow later):
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VOLUME 53 FEBRUARY 2003
VENTILATION
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"Heat build-up
The Canadian Asphalt Shingle Manufacturers Association has issued a bulletin titled “Proper Ventilation for Asphalt Shingle Covered Roofs”. In it they state that both heat and moisture build-up in attics is the primary cause of many roof problems including blistering, distortion and curling of the shingles. It has been theorized that poor ventilation of attics can cause excessive heat build-up and high deck temperatures. Since heat is the major contributor to the aging of materials, this heat build-up is said to contribute to the deterioration of many roof-covering materials. **Although a recent study **(and studies by others) by Simpson, Gumpertz & Heger Inc., demonstrated that geographical location, building orientation, and roof colour have far greater influence on roof surface temperatures than the ventilation below, inadequate ventilation is often(still)cited as the reason for roof performance problems. (people don’t seem to learn!) Material manufacturers may not honour warranties where it can be shown that there is insufficient ventilation of the space below the roof."
Bill Rose, ASHRAE, Illinois Building Research Council:
"Airtight ceilings are a more reliable way to ensure a dry attic than venting, but in practice most houses fall into a middle ground where venting balances moisture input.
Joe Lstiburek, Building Science Corporation:
“Vented attic/roof designs have the advantage of a long, proven historical track-record. However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs.”
Anton TenWolde & William B. Rose, members, ASHRAE:
We recommend venting of attics in cold and mixed climates. However, if there are strong reasons why effective attic vents are undesirable, unvented attics can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below, or vice versa. The necessity and effectiveness of vents in cathedral ceilings in cold and mixed climates is still a contested issue. Unvented cathedral ceilings can perform satisfactorily in cold and mixed climates if the cavity is properly insulated, measures are taken to control indoor humidity and minimize air leakage into the roof cavity, and a vapor retarder is installed in the ceiling.