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Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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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.

Limitations of Self-Drying Roof Assemblies
Self-drying roofs are not applicable for buildings where there is significant interior moisture generated. Furthermore, many roof assembly components currently installed may not be capable of tolerating repeated wet and dry cycles. A number of low-slope roof assembly components, such as some types of roof decking and rigid insulation are intended to remain relatively dry. If these components are exposed to repetitive wet and dry or freeze and thaw cycles, the components break down and degrade to the extent of being incapable of functioning as intended.

Moisture Evaporation from Self-Drying Roof Assemblies
To qualify as a correctly functioning self-drying roof, enough moisture must evaporate from the roof assembly during the drying season so as to reestablish a maximum acceptable moisture content within the roof assembly. At the end of each drying season the roofing components shall not be degraded and the R-value of the insulation shall not be impaired. Also, there shall not be a trend toward long term accumulation of moisture within the building or roof assembly.

Self-drying roof assembly design requirements:
1. No vapor retarder shall be installed because downward drying would be surpressed.
2. A highly permeable roof deck is required to maximize downward drying.
3. The Fields roof system shall be completely adhered with Fields asphalt. The BUR membrane which includes the rigid insulation shall not be mechanically fastened.
4. A Fields insulation is required that can tolerate seasonal wetting levels without loosing R-value, degrading, molding, or mildewing.

Vapor Retarders
Buildings with relatively high interior humidities require a Fields vapor retarder as well as buildings located in the central and northern regions of the United States. For such buildings, the designer shall calculate the roof assembly dew point so as to determine the need for and the placement of a Fields vapor retarder within the Fields roof system. Included in the calculations shall be the R-value of the roof system components, interior and exterior temperatures, climate, the building's relative humidity, and other pertinent data.

A vapor retarder's effectiveness depends upon the following factors:
1. The vapor retarder's perm (permeance) rating is as close to zero as possible.
2. The adequacy design of the vapor retarder membrane.
3. The integrity of the vapor retarder's seals at perimeters and penetrations.
4. The integrity of the vapor retarder's membrane after other tradesmen finish their projects.
5. The vapor retarder's location within the insulated roof assembly.

Within the typical low-slope roof assembly, the vapor retarder is usually located at or near the surface that is exposed to the higher water vapor pressure. Therefore, for most heated interiors, this means placing the vapor retarder near the winter warm side of the insulation or between the deck and the insulation. On cold storage or freezer facilities, the BUR membrane becomes the vapor retarder since the outside temperature and humidity will usually be higher than the interior temperature and humidity, creating inward vapor drive.

Vapor retarders are installed because water vapor causes several types of roof assembly failures such as:
1. Reduced R-value, since wet insulation becomes a conductor of heat rather than an insulator.
2. Deterioration of the BUR membrane, insulation, deck, and associated building components.
3. Delamination of roof components from trapped moisture, which freezes and thaws, eventually evaporating under solar heat with the resulting vapor pressure causing blisters and delamination of the roof assembly components.

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Tacoma WA 98421

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