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ROOF DESIGnt-ONSIDERATIONS •DOE Energy Calculator A number of basic considerations factor into determining —Go to www.orni.gov/scVroofs+walls/facts/CoolCalcEnergy.htm the need and location of a vapor retarder: to find the Cool Roof Calculator. •Temperature and Relative Humidity —This tool measures the energy savings for low-slope roofs with —Vapor flows based upon a difference in vapor pressure between non-black roof surfaces. two locations,and flows from higher to lower pressure regions. —Normally,the higher the temperature,the higher the vapor •GAF CREST pressure. —GAF CREST(Cool Roof Energy and Sustainability Tool)is —In determining the need for a vapor retarder for most typical a unique tool designed to help contractors consultants and conditions,the exterior winter temperature and the interior building owners compare the energy cost savings of different winter relative humidity are the most critical factors. roofing systems options. —Temperature information is readily available from the National Weather Service. WATER VAPOR TRANSFER —Relative humidity information is typically available from the building HVAC design professional or the building operations Typical single-ply roof assemblies do not include vapor retarders manager.Relative humidity can also be field-measured. 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 •Vapor Retarder Location of the roof assembly during the balance of the year.Under normal —Vapor retarders are intended to be installed as close to the conditions,this type of cyclical water vapor flow does not cause warm side of the roof assembly as possible.Normally,this a significant deterioration of the roof insulation or reduction in places the vapor retarder directlyon the structural deck or insulation thermal performance. directly over a minimal la er of insulation.Note:The vapor retarder is not part of the GAF roofing system and is For projects where there is a significant difference in vapor pressure not covered by GAF warranties or guarantees. between building interior and exterior,the volume of water vapor flow is much greater,and control of water vapor transfer into —A sufficient amount of insulation must be installed over the and through a roof system becomes an important consideration. vapor retarder to place the dew point location above the level of Without adequate control provisions such as a vapor retarder, the vapor retarder(keeping it on the warm side). the roof insulation can become saturated with water,with a —Determining the need and location of the vapor retarder is the corresponding reduction in insulation thermal performance. responsibility of the design professional. Structural deck damage and/or condensation into the building interior may also occur. Vapor flow is referenced in various ways.The following are common 'Sealing At Perimeter And Penetrations terminologies: —Vapor retarders must be completely sealed at all perimeter and penetration locations. •Permeance —Sealing methods must be selected in accordance with the type —The time rate of vapor transmission thromh a flat material or of vapor retarder being installed. construction induced by vapor pressure difference between two —All'leakage into the roof system at perimeters and penetrations specific surfaces,under specified temperature and humidity will significantly reduce the effectiveness of the vapor retarder conditions. by allowingense and cause roof deterioration.moist air to penetrate into the roof assembly,where —Units of permeance are expressed as:(gr)/(h) (sq.ft.) (in.Hg). it can cond —The permeance,or perm rating,of a material is a performance evaluation specific to a sample of material,and not a specific 'Building Usage property of the material. —Building usage such as swimming pools,food processing,paper —In general,the perm rating of a vapor retarder should be<0.1 manufacturing,foundries,etc.,that result in increased internal perms(Class 1). temperatures and humidity conditions will likely require the use of a vapor retarder. •Relative Humidity —These generalizations are not intended to substitute for actual ty vapor Clow calculations based upon specific building and —Relative humidity is the ratio of the pressure of water vapor climatic conditions. present in air to the pressure of fully saturated water vapor at the same temperature. —Relative humidity is expressed as a percentage. The Case 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 •Dew Point Temperature membrane. —The temperature at which air becomes saturated with —A vapor retarder provides a good safeguard against vapor saturated vapor(100%relative humidity)and condensation migration in case a building's use changes from a"dry"use to begins to form. a"wet"use. —Dew Point Temperature is expressed as OF or°C. —A vapor retarder can reduce or eliminate condensation inside the roofing system. 17