Proper roofing system selection and design requires the consideration of many factors such as:
Construction Type
Warm roof construction will improve efficiency over the traditional cold roof design. A warm roof has the insulation on the outside of the structure which means the roof decking is kept warm. Because the deck does not have two opposing temperatures, dew point condensation is minimized or eliminated inside the structure.
Energy Efficiency
Thermal resistance is a heat property and a measure of a temperature difference by which an object or material resists a heat flow (heat per time unit or thermal resistance). Thermal resistance is the reciprocal of thermal conductance. Roof insulation installed above the roof deck is a practical means of achieving the necessary energy efficiencies. In addition, the use of white-colored reflective membranes can reduce the heat load on air conditioning equipment, as well as providing a moderating effect on the temperature in proximity to the building
Fire Resistance
Resistance by the roofing system to fire usually is required by code. Typically, a UL Class A or B rating is required, depending on the location and/or use of the building. Refer to current UL Directory to verify roof assembly requirements for specific fire ratings.
Wind Performance
Roofing systems should be capable of resisting the forces generated by the maximum anticipated wind speed for a specific location of building. Positive internal pressure is often a serious problem with roof membrane attachment. Conditions where the positive internal pressure is constant may cause the roof system to billow up and will reduce the overall uplift resistance of the roofing system. This effect is most pronounced in mechanically attached systems and can cause attachment concerns with all roof system types.
Reflectivity
A measure of solar energy that is reflected away from a roof surface. Dark materials absorb more heat from the sun and can be up to 70°F hotter than a reflective white surface given the same outside temperature and conditions.
Emittance
A measure of the infrared radiation emitted from a roof surface. Unlike reflectance, infrared emissivity may not be affected by dirt or discoloration of the surface of a material.
Water Vapor Transfer
Typical roof assemblies do not include vapor retarders as a standard assembly component. For these applications, there is a natural transfer of water vapor into the roof assembly during a portion of the year, followed by a natural transfer of water vapor out of the roof assembly during the balance of the year. Under normal conditions, this type of cyclical water vapor flow does not cause a significant deterioration of the roof insulation or reduction in insulation thermal performance. However, for projects where there is a significant difference in vapor pressure between building interior and exterior, the volume of water vapor flow is much greater, and control of water vapor transfer into and through a roof system becomes an important consideration. Without adequate control provisions, the roof insulation can become saturated with water, with a corresponding reduction in insulation thermal performance. Structural deck damage and/or condensation into the building interior may also occur.
Relative Humidity
Relative humidity is the ratio of the pressure of water vapor present in air to the pressure of fully saturated water vapor at the same temperature.
Dew Point Temperature
The temperature at which air becomes saturated vapor and condensation begins to form. A number of basic considerations factor into the need and location of a vapor retarder. Determining the need and location of the vapor retarder is the responsibility of the Design Professional.
Vapor Retarder
Vapor retarders are intended to be installed as close to the warm side of the roof assembly as possible. Normally, this places the vapor retarder directly on the structural deck or directly over a minimal layer of insulation or fire barrier. A sufficient amount of insulation must be installed over the vapor retarder to raise the dew point location above the level of the vapor retarder. Air leakage at perimeter and penetrations will significantly reduce the effectiveness of the vapor retarder by allowing moist air to penetrate into the roof assembly where it can condense and cause roof deterioration.
Factors for the use of a Vapor Retarder
- A vapor retarder can protect the long term thermal resistance of insulation sandwiched between the vapor retarder and the membrane.
- A vapor retarder provides a good safeguard against vapor migration.
Factors against the use of a Vapor Retarder
- A roof membrane leak may go undetected due to the vapor barrier and will trap moisture that can eventually destroy the roofing system.
- Lack of thermal resistance (not enough insulation) between roof membrane and the vapor retarder will center due point inside the roof assembly.