Moisture created by condensation in the roofing assembly can be detrimental, potentially causing mold growth, rotting of wood decks and rusting of metal decks, and the reduction of insulation R-value.
Condensation occurs when warm, moist air comes in contact with a cool, dry surface. The temperature at which this happens is called the dew point. The movement of warm, moist air toward cooler, drier air, is called vapor drive. Outward vapor drive occurs when warm, moist air inside a building moves toward cooler, drier outside air. Inward vapor drive occurs when hot and humid exterior air moves toward cooler, air-conditioned interior air.
Moisture in Roof Assemblies
The amount of moisture and potential for vapor drive in a roof assembly is dependent on several factors:
- Interior & Exterior Temperatures: The greater the temperature difference between the interior and exterior of a building, the greater the potential for vapor drive.
- Interior & Exterior Humidity Levels: Buildings with high humidity or prolonged humidity generation (e.g., swimming pools, food processing facilities/bakeries, laundromats, etc.) have a high potential for vapor drive.
- Insulation: The insulation between the roof membrane and the deck is critical for keeping the dew point above the membrane and out of the roof assembly. Inadequate insulation can allow warm, moist interior air to reach the underside of the cooler membrane and condense. Insulation that is only one layer thick can allow air to travel up through the gaps between insulation boards and condense on the underside of the roofing membrane. Two staggered layers of insulation can reduce the flow of air that can get to the underside of the membrane.
- Vapor Retarders: Air movement is critical in the transfer of moisture. The use of a properly installed vapor retarder in a roof assembly (such as a polyethylene plastic sheet) reduces the amount of air/moisture that can travel through it. Determining if a vapor retarder or air barrier is needed and where it should be installed is the responsibility of a roofing professional.
Proper Roof Design is Crucial
Every building is different, and each has its own design requirement for reducing moisture in the roofing assembly. Professional roof system design is always recommended for reducing the potential for condensation and moisture buildup in any roof assembly.