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Self-Drying Roof Assemblies
A self-drying roof is any roof assembly that is
installed without a vapor retarder. However, there may be other forms
of secondary protection for the roof assembly from moisture originating
inside the building or migrating upward from damp soil conditions. These
forms of protection may be well designed HVAC systems, or a combination
of adequate HVAC and dehumidification equipment in conjunction with an
exterior drainage system for buildings located in damp soil conditions.
These types of self-drying roofs have no primary form of protection from interior moisture and no form of designed condensation control for the roof other than the natural, seasonal, downward drying. Low-slope cold roofs that have vented joist spaces may take advantage of the drying effects of natural passive ventilation.
Self-Drying Roof Assembly Performance
Most successful self-drying roof assemblies have
been warm roof assemblies where the BUR membrane has been completely adhered
into asphalt. Another factor that contributes to the successful performance
of fully adhered self-drying roof systems is that many of these systems
have included low permeable rigid roof deck insulation that has also been
adhered into asphalt. Mechanically attached self-drying roof systems have
a greater potential for air leakage or the transport of moisture vapor
from the interior of the building into the roof assembly, and therefore,
a greater potential for condensation to form within the roof assembly.
It has been proven that air can move laterally under a membrane that is
not completely adhered. During times of wind and certain building pressure
conditions, a pumping action can occur that actually suctions air into
the roof Assembly. During these conditions, warm moist interior air can
be transported into the roof assembly, where cold surfaces can cause the
moisture to condense. Beads of moisture have been found on the underside
of a number of mechanically attached self-drying roof systems.
Solar Warming of Self-Drying
Roof Assemblies
In order for self-drying roof assemblies to function
correctly, there shall be adequate solar warming of the BUR membrane that
promotes downward drying. Heat energy absorbed from the sun causes the
evaporation of winter accumulated moisture from within the roof system,
and drives it downward through a vapor permeable roof deck into the building's
interior. The building's ventilation system must be such that moisture
evaporating from the self-drying roof assembly is vented, preventing excessive
build-up of interior moisture. The moisture evaporating from the roof
assembly shall be vented to the outside or absorbed into the interior
air where it is eventually vented outside the building. Downward drying
of winter accumulated moisture is required in order to prevent damage
from moisture accumulation in a building.