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Design Criteria
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Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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Warm Roof Assembly Moisture Control
The designer shall specify the following three factors in designing moisture control for warm roof assemblies:

1. Ventilation: Adequate ventilation under the roof deck is required. The designer shall specify a ventilation system that will exhaust the moisture generated from inside the building before it can migrate into the roof assembly.
2. Insulation: Rigid insulation boards shall be installed over the roof deck of warm roof assemblies. The required R-value shall be specified so as to facilitate the control of heating and cooling loads while at the same time protecting the primary insulation from the degrading affects of accumulated moisture. The designer shall specify the insulation R-value required per the local building codes, the building occupancy, and the climate location. The designer shall also specify the required moisture control for keeping the insulation dry so that R-value is not impaired through accumulated moisture in the insulation.
3. Vapor Retarder: The designer shall specify a Fields vapor retarder for most warm roof assemblies, especially for roof assemblies installed in the northern United States, and for buildings that have high relative humidities.

Cold & Warm Low-Slope Roof Assembly Ventilation
Moisture generated from within the interior of a building shall be suppressed from passing into the roof assembly by a Fields vapor retarder, and the moisture shall be satisfactorily vented outside so that moisture does not accumulate and condense in the joist spaces and other cavaties under the roof system.

Cold Low-Slope Roof Assembly Ventilation: Cold low-slope roof assemblies have no high, attic type spaces. Therefore, these assemblies have no natural chimney effect created by rising, warm moist air as in steep slope roof attics. Installation of continuous soffit vents around the building's perimeter may not remove sufficient amounts of moisture from the joist spaces in cold low-slope roof assemblies. Depending upon the depth of the joists and the thickness of the insulation put in the joist space, it may be necessary to raise the roof deck or shim it above the top of the joists so as to facilitate air movement along the underside of the roof deck. The underside of the roof deck is where moisture is most likely to accumulate and remain, causing wood to decay and steel to rust.

Installation of raised area dividers and curbs with mechanical vents aid the ventilation of cold low-slope roof assemblies. Air shall move along the underside of the roof deck for adequate ventilation to be achieved. If air is restricted from free movement beneath the roof deck, each joist space shall be individually vented so as to allow for the movement of sufficient volumes of moisture-laden air within the joist spaces.

Although moist warm air will move through the fibrous insulation commonly found in cold low-slope roof assemblies, air and moisture movement is restricted and the frictional forces created by the closely aligned fibers in the insulation will impede adequate ventilation of the roof joist spaces. Also, some joist insulations are encapsulated in plastic film, which may restrict free movement of moisture-laden air, further complicating the ventilation of a cold low-slope roof assembly.

Designers shall evaluate the need for specifying a Fields vapor retarder in cold low-slope roof assemblies so as to plan for minimizing the quantity of interior moisture entering the roof joist spaces. Cold low-slope roof designs shall specify adequate free air movement immediately beneath the roof deck, allowing for the exhausting of warm moist air to the outside. Mechanisms that facilitate positive ventilation such as raised area dividers with ventilators, and continuous soffit vents are beneficial for the movement of air through the roof joist spaces.

Warm Low-Slope Roof Assembly: Ventilation Venting interior moisture with a heating, ventilation, and air conditioning (HVAC) system is the primary means of minimizing humid air from migrating into warm low-slope roof assemblies. The success of venting warm low-slope roof assemblies is dependent upon the type of insulation installed. Fibrous rigid insulation boards have the propensity to allow some movement of moisture-laden air, depending upon the method of attachment of the insulation boards. Fibrous insulation boards allow restricted movement of air through the insulation boards. Most other types of mechanically attached rigid insulation boards will not allow migration of moist air except at insulation board joints.

The moisture-laden air entering warm low-slope roof assemblies without an effective vapor retarder generally enters between the joints of the insulation boards, and accumulates on the bottom side of the cold, impermeable BUR membrane. The moisture can then move laterally dampening the top surface of the insulation boards. The extent of lateral moisture migration depends on the method of attachment of the BUR membrane. Completely adhered BUR membranes to the top surface of roof insulation boards prevents any appreciable lateral migration of moisture beneath the BUR membrane, while mechanically or partially attached BUR membranes allow lateral migration of moisture to the top surface of the roof insulation boards.

One way surface vents are prohibited from being installed on warm low-slope roof assemblies because of the large number of flashed penetrations necessary in the BUR membrane, and because vents into a warm low-slope roof assembly may allow entry of moisture into the roof assembly as weather changes occur. The U.S. Army Corps of engineers has demonstrated that commonly available one way roof vents are ineffective in drying wet insulation boards. Ventilation of warm low-slope roof assemblies with one way roof vents is impractical, marginally effective at best, and in some instances detrimental to the long-term performance of the roof assembly. Moisture control for warm low-slope roof assemblies is most effective with a well designed HVAC system, an effective vapor retarder, and the installation of moisture resistant components.

Direct Ventilation of Building Interiors
If building temperature control is unimportant, and if process heat generated within the building is such that it requires venting, then ventilation ducts straight through walls near the bottom of the roof deck, or ventilation ducts straight through the roof system shall be the most positive means of eliminating excessive quantities of moisture from inside the building. The installation of structural components resistant to moisture is also required.

However, if energy conservation is important, or if interior temperatures must be maintained within specified tolerances, heat exchangers may be installed in the HVAC exhaust system to warm make up air taken from outside.

Interior generated moisture during construction of the building shall be mechanically exhausted to the outside. Heating interior air in an attempt to dry out wet interior construction components may generate additional moisture and raise the interior relative humidity of the building above the design level. Unless construction generated moisture is effectively removed from a building's interior, the effects of the moisture may damage the roof assembly before the building is completed.

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Tacoma WA 98421

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