Attic ventilation is not a hit and miss affair. The technology evolved through a systematic understanding of the interaction between buildings, and the elements in this case thermal transfer. However roof venting is not a firm requirement in the building codes. Hence it is a design and construction preference at the option of the architect or homeowner.
Attic Ventilation in Winter and Summer
Attic ventilation in Calgary maintains a cold temperature in the roof space in winter, to control ice damming when snow melts above. It expels warm air rising from the conditioned living space into the roof space and out through soffit vents.
This maintains a consistent temperature across the entire roof and eaves, so melting snow can drain consistently and not form ice dams over eaves. Attic venting also has the secondary purpose of expelling humidity that might otherwise accumulate under the roof membrane.
Attic venting also helps keep Calgary homes cooler on hot summer days, by releasing the solar-heated air and lessening the cost of cooling the living space below. These advantages do not however necessitate the system because there are other technologies available.
Attic Ventilation and Roof Design
The above principles work best in a simple, single-pitch roof where the air can flow freely and vent consistently. However, this is not the case with complex roof geometry involving multiple valleys, hips, dormers, and skylights.
In such cases, forced-air ducting may prove the answer to attic ventilation, provided the duct work is properly assembled without leaks. However, if the delivery system leaks significantly the benefits can weaken to the extent that ice dams still form.
The Effect of Extreme Weather on Vented Roofs
Wind driven rain can find its way through roof vents and cause soffit collapse and water damage. Therefore un-vented roofs are indicated for hurricane zones. Finally, climate change is increasing the risk of runaway forest fires. If you live in the woods, perhaps you should not vent your roof.
The Alternative: An Air Barrier at Ceiling Level
Ice damming and premature snow melting may also be averted by installing an impermeable thermal barrier at ceiling level. This obviates the need for attic ventilation in winter. because warm air cannot rise from the living space into the attic. Valiant Exteriors provides this information as a service to its readers, although it does not construe as construction advice.
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Vented Soffit in Need of Repair: Chris Bohn BY CC 2.0