May 14, 2003

Article

Here is a recent article published by CASMA in "Canadian Roofing Contractor and Design" - April / May 2003 edition. It was written by Andy Lodge, Technical Committee Chair, Canadian Asphalt Shingle Manufacturers' Association.

DOES ATTIC VENTILATION REALLY MAKE A DIFFERENCE?

For decades, people designing roofs and installing shingles have accepted the fact that ventilation is needed. The national and provincial Building Codes require ventilation of attic spaces. Shingle manufacturers, and manufacturer associations, such as the Canadian Asphalt Manufacturers' Association (CASMA) provide detail application instructions, limited warranty requirements and technical literature regarding the need for ventilation.

Recently, others have presented one or two theories stating that ventilation is not needed; that is has minimal effect on the temperature or longevity of the shingled roof system, and that the heat and moisture dissipation is not necessary, given good insulation and vapour barriers.

Computer models and theories are valuable scientific tools, but why is it that ventilation has been required and successfully used for so many years? Is it simply an " old wives " tale that ventilation does make a difference in the overall roof system performance?

Admittedly, there have not been many scientific studies performed on the benefits of ventilation. In fact, much of the importance of ventilation comes via empirical evidence provided by long time roofers and manufacturers. Many individuals know that roofs / attics without good ventilation simply do not last as long as roofs and attics with good, open and thorough air movement.

In 1999, a CASMA member company's Technical Department received a call from a homeowner seeking advice on ventilation requirements. Through the conversation, it was learned that the homeowner had a portion of his roof which was unventilated - where the shingles were in poor condition, and the remainder of the roof, which was ventilated, had shingles that were in good condition.

The roof was only 11 years old, and the home is located approximately 100 km north of Toronto, Ont. The homeowner wanted to make sure that the next roof would last much longer, and therefore wanted to comply with proven industry design wisdom, including ventilation requirements.

The homeowner agreed to make his roof available for a CASMA sponsored study. An independent roof analytical company was contracted to evaluate the roof ventilation and the shingle condition in both the non-ventilated (heated and unheated areas) and ventilated areas.

The roof was initially inspected in October 1999. It was confirmed that approximately one-third of the roof was installed on an unventilated "sandwich" roof deck in which the insulation is sandwiched between two plies of wood sheathing. The shingles in the area over the heated residence showed clear evidence of premature aging, including excessive loss of granules. The other portion of the attic over the house was conventional ventilated attic, using full soffit venting and upper roof vents. The shingles in this area were in very good condition and did not require replacement. It is important to note that both observations were made on the same (south) side of the dwelling. The shingles over the sunroom were also in good condition, however, none of these shingles were directly exposed to the south.

When the roof was scheduled for re-shingling - August 2000 - the consultant returned to obtain sample shingles for laboratory analysis. (NOTE: the entire roof was re-shingled, and a ventilation space was added to the non-ventilated portion). CASMA wanted to confirm that the difference in the observed shingle condition was not due to differences in shingle construction. (NOTE: shingle deterioration did not follow any discernible "bundle" patterns).

Shingles were sampled in duplicate from locations in the unventilated area, the ventilated area and from an unheated sunroom area located beneath the insulated deck. Mass analysis from all five samples showed normal manufacturing variation, revealing that all shingles from the initial installation were likely taken from the same production lot.

Flexibility, or pliability, is a property tested and required by many shingle product standards as an assessment of product quality. When tested for pliability at cold temperatures, all samples from all roof areas failed; this was not surprising given the 11-year old age of the roof. Shingles normally only pass the cold temperature flexibility tests at time of manufacture; loss of flexibility is simply one of the ways shingles naturally age.

However, when the samples were tested for flexibility at room temperature, all the samples from the unventilated roof over the heated part of the house failed (zero out of 10 passed on one shingle, and zero out of 10 passed on the second shingle), yet about half of the specimens from the ventilated house area samples passed (four out of 10 passed on one shingle, and five out of 10 passed on the second shingle). See Table One.

This test showed that the shingles applied to the unventilated, insulated deck changed significantly, due solely to the effects of ventilation. The shingle sample from the sunroom (insulated deck but an unheated area of the house) showed some flexibility (two out of 10 specimens passed). This was perhaps partly due to the fact that this area of the roof was over an unheated structure, and partly due to the fact that these shingles were not directly exposed to the more severe southern location.

Results from this study showed that lack of ventilation can significantly reduce shingle longevity. According to the consultant's report, the shingles in the unventilated area were effectively "baked" due to lack of airflow. No computer models were used; this was a real roof, with real asphalt shingles. CASMA is attempting to find more case studies of ventilated versus non-ventilated roofs to continue to further confirm what experienced industry personnel have known for decades - ventilation is an important factor in roof performance.

Besides reducing the temperature of the applied shingles, ventilation is also critical in reducing ice dams, and, by eliminating moisture from the attic cavity, condensation is decreased, which would limit mould growth.

For additional information on ventilating roofs, please refer to CASMA Technical Bulletins on this subject.

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