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Moisture Control | Roof Decks | Rigid Insulation | Roof Membranes

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Moisture Control Contents:
  The Three Primary Moisture Control Factors
Moisture Control & Insulated Roof Assemblies
Cold Roof Assembly Moisture Control
Warm Roof Assembly Moisture Control
Cold & Warm Low-Slope Roof Assembly Ventilation
Direct Ventilation of Building Interiors
Self-Drying Roof Assemblies
- Self-Drying Roof Assembly Performance
- Solar Warming of Self-Drying Roof Assemblies
- Limitations of Self-Drying Roof Assemblies
- Moisture Evaporation from Self-Drying Roof Assemblies
Vapor Retarders
- Vapor Retarder Precautions
- Two Criteria for Specifying a Vapor Retarder
- Fields Vapor Retarders
- Insulation Installed over Vapor Retarders
- Vapor Retarder Attachment to Substrates
- Vapor Retarder Continuity from Roof Plane through Flashings
- Prevention of Asphalt Drippage
- Vapor Retarder Damage
Temporary Roofs
- Vapor Retarder and Temporary Roof Attachment
Insulation Attachment
Roof Assemblies over High Humidity Buildings
Weather Protection during Installation
Roof Drains & Scuppers

Moisture Control
Moisture control within the roof assembly includes such factors as moisture migration, dew point, condensation, ventilation, and vapor retarders. Some migration of interior moisture is allowed due to the natural downward drying cycle that occurs in most climates; and therefore, the criteria for determining the need for vapor retarders shall take this into consideration. The term vapor retarder refers to a Fields waterproof membrane installed prior to the BUR membrane so as to surpress the flow of moisture vapors from the interior of a building into the roof assembly. Moisture from the interior of a building generally comes in the form of water vapor from the following four sources:

1. Construction processes which include the drying of interior concrete, cementitious roof fills, plaster compounds, water-based finishes, and fuel burning space heaters.
2. Operational sources such as swimming pools, greenhouses, laundries, textile manufacturing, food processing, and paper plants.
3. Occupancy sources which include kitchens, shower facilities, exercise rooms, and even breathing from high-density occupancies.
4. Cool climate regions where moisture vapor generally flows upward through the roof system from a heated, more humid interior toward a colder, drier exterior during the winter months. Such a climate condition causes vapor to migrate into an insulated roof system where it can cool and condense into water.

Buildings with high interior relative humidity require vapor retarders, 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 correct location to install the vapor retarder within the roof system. Included in the calculations shall be the R-value of the roof assembly components, anticipated interior and exterior temperatures, regional temperatures, the building's planned relative humidity, and any other pertinent data.

The Three Primary Moisture Control Factors
1. Ventilation is the exchanging of air in a given space. Passive or mechanical ventilation is designed to control moisture and reduce heat buildup. Exchanging the air in a given space is accomplished by venting moisture-laden air to the outside, while inducing fresh outside, or conditioned drier air to the inside.
2. Insulation provides R-value and aids in temperature control within a building. Insulating a roof reduces the amount of energy required to heat and cool a building and can also reduce the potential for condensation on interior surfaces or within the roof assembly.
3. A vapor retarder is a Fields two or more sheet waterproof membrane adhered and coated with Fields asphalt. The vapor retarder is generally installed somewhere between the deck and the insulation. Vapor retarders are installed to inhibit the flow of moisture vapor into the roof assembly from the interior of a building.
   


Moisture Control & Insulated Roof Assemblies

There are two types of insulated roof assemblies that are classified by the location of the insulation in relation to the roof deck: (1) cold ventilated roof assemblies or cold roof assemblies and (2) warm compact roof assemblies or warm roof assemblies. Cold roof assemblies have a fixed air space under the roof deck such as an attic that is insulated below the deck and vented to the outside of the building: See Illustration 1. Warm roof assemblies have all their components installed in direct contact with each other including rigid insulation boards installed above the deck, leaving no space for designed venting: See Illustration 2.

  Illustration 1: Cross-section example of vented cold low-slope roof assembly.

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

1. Ventilation: Adequate ventilation of the attic space is required. The designer shall calculate the volume of ventilation required, and specify ventilation per the building's requirements.
2. Insulation: Soft insulation, bats or blown, shall be installed below the roof deck. The designer shall calculate the thickness of the insulation so as to allow adequate air space for venting between the insulation and the roof deck.
3. Vapor Retarder Assemblies: The designer shall determine if a vapor retarder is required for a cold roof assembly. With some cold roof assemblies, a vapor retarder can assist the control of moisture migration from the building's interior into the roof assembly.

  Illustration 2: Cross-section example of a compact warm low-slope roof assembly.

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Fields Company LLC
2240 Taylor Way,
Tacoma WA 98421

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800-627-4098
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