A vapor retarder layer sometimes is specified as an additional component in low-slope roof systems to minimize the potential for condensation to occur. When used, a vapor retarder layer can reduce water vapor diffusion from a building’s interior into roof system components. For a vapor retarder layer to function as it should, it must be placed in a proper location within a low-slope roof system’s cross-section. The following are some guidelines for proper vapor retarder layer placement.
For new construction situations, the designer of a building’s mechanical system typically is best-suited for determining whether a vapor retarder layer is necessary. When sizing and designing a building’s HVAC equipment, the mechanical system designer needs to consider outside conditions specific to the building’s geographic location, climate conditions and desired interior design conditions. These same considerations form the basis for determining whether a vapor retarder layer is necessary for the building’s exterior envelope, which includes the roof system. If a vapor retarder layer is deemed necessary for a building’s exterior wall assembly, the building’s designer also should consider a vapor retarder layer as a roof system component.
Because a building’s mechanical system designer typically has little to no knowledge of the building’s specific roof system design, the roof system designer, whether that be a building designer or roof consultant, usually determines where any vapor retarder layer will be positioned within a low-slope roof system’s cross-section.
Vaper retarder layer placement
For most buildings, the need for a vapor retarder layer is most pronounced during winter design conditions, which are characterized by relatively warm interior conditions compared with cool or cold exterior conditions. In these conditions, the predominate direction of vapor drive is from a building’s interior to its exterior. Warm interior air will cool as it passes through a roof system’s cross-section toward the outside. The rate at which this air cools depends on the materials it passes through and these materials’ R-values.
To prevent condensation, it is the roof designer’s goal to make sure the temperature at the vapor retarder layer is warmer than the dew-point temperature.
The dew-point temperature, sometimes referred to as dew point, is the temperature at which air becomes fully saturated with water vapor and when condensation begins to form. This is the temperature at which air has a relative humidity of 100%.
It is important to realize dew-point temperature is not a singular, static value. Instead, it is constantly changing based on changing temperatures and relative humidities. For this reason, NRCA advises dew-point temperature calculations be based on a building’s design conditions, such as those used for sizing and designing a building’s HVAC equipment.
To ensure the temperature at the vapor retarder layer remains higher than the calculated design dew-point temperature, insulation of a sufficient R-value must be designed and installed above the vapor retarder layer to maintain a temperature warm enough to prevent condensation.
When low-slope insulation is above the roof deck in membrane roof system designs, NRCA recommends the vapor retarder layer be positioned directly above the roof deck or as closely above the roof deck as is feasible. For example, over a metal roof deck, designs that use a thin, low R-value thermal barrier layer of insulation mechanically attached to the roof deck with a vapor retarder layer applied directly to the thin insulation layer usually are appropriate.
Also, special consideration needs to be paid to the design of any roof drains, roof curbs and other roof penetrations, as well as parapets and other edge conditions. At these areas, the vapor retarder layer needs to be properly detailed and terminated, and these areas also must be adequately insulated to prevent condensation.
For buildings located in year-round, warm climates with interior cooling, the direction of vapor flow will occur predominantly from the building’s exterior to its interior. In these situations, the vapor retarder layer should be designed on the exterior-side of above-deck insulation. In most instances, the roof membrane will function as the vapor retarder layer.
Additional information about vapor retarder layer design is provided in Chapter 2 of the Condensation and Air Leakage Control Section of The NRCA Roofing Manual: Architectural Metal Flashing and Condensation and Air Leakage Control—2018. Specific procedures and examples are provided for determining the design dew-point temperature, vapor retarder layer placement, amount of R-value necessary above a vapor retarder layer and the temperature gradient across a roof system’s cross-section.