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Understanding Attic Ventilation

In cold climates, vented attics or roofs are supposed to maintain a cold roof temperature to control ice dams caused by melting snow while venting moisture from the interior of the attic. The melted snow can be caused by heat loss from the conditioned space to the attic, usually due to air leakage and conductive losses.

In hot climates, vented attics or roofs drive out hot air from the attic to lessen the building’s cooling load.

Control of ice dams, moisture accumulation and heat gain can also be successfully addressed by unvented attic or roof design. Spray polyurethane foam (SPF) can be applied in sufficient thickness to satisfy local energy code requirements, directly to the underside of roof sheathing between rafters or joists of any slope in all (heating, mixed and cooling) climates. This configuration controls the entry of moisture laden air into the insulation and also eliminates dew-point occurring at the underside of the roof deck and anywhere in the insulation, in all (heating, mixed or cooling) climates. Due to the fully adhered properties, air and moisture are displaced out of the insulated space - including at rafters and sheathing. Moisture cannot enter the insulated space from any direction, eliminating the requirement for roof venting.

To learn more about vented vs. unvented attic space, click here to read  the interesting article "Understanding Attic Ventilation" by Joseph Lstiburek, PhD, P. Eng., Building Science Corporation.