Critical considerations

Vapor retarders and air barriers must be used properly

  • Photo 1: This protection board deteriorated as a result of significant moisture accumulation. Photo courtesy of Simpson Gumpertz & Heger Inc., Waltham, Mass.
  • Photo 2: The moisture trapped in the concrete deck likely was from the original construction.Photo courtesy of Simpson Gumpertz & Heger Inc., Waltham, Mass.
  • Photo 3: Corrosion spots on the panels' top surface were caused by through-corrosion that began on the panels' underside.Photo courtesy of Simpson Gumpertz & Heger Inc., Waltham, Mass.
  • Photo 4: Moisture was found trapped within this roof system, which caused deterioration of the paper insulation facers.Photo courtesy of Simpson Gumpertz & Heger Inc., Waltham, Mass.
  • Photo 5: These insulation boards were not weighted properly while the adhesive cured. As a result, the boards did not attach appropriately to the substrate.Photo courtesy of Simpson Gumpertz & Heger Inc., Waltham, Mass.
  • Photo 6: This asphalt has a glossy surface in some areas, indicating it cooled before insulation made contact.Photo courtesy of Simpson Gumpertz & Heger Inc., Waltham, Mass.

Vapor retarders long have been a common part of low- and steep-slope roofs in many U.S. climates. Increasingly, air barriers also have become a routine requirement for all types of roof systems. Starting with Massachusetts' building code in 2001, several other state building codes now require an air barrier as part of any exterior assembly. This trend will continue nationally when building codes and LEED® requirements adopt the ASHRAE 90.1—2010, "Energy Standard for Buildings Except Low-Rise Residential Buildings," which requires air barriers.

However, roof systems continue to fail because of a lack of reliable vapor retarders and/or poorly detailed air barriers.

Some roof system designers, roofing contractors and manufacturers remain unclear about the differences between these two components—when they are needed, their intended functions and the requirements for effectively installing each. Often, the same material layer is used to provide both functions, adding to the confusion.

It's important to be aware of the differences between air barriers and vapor retarders and the problems that can occur when they are improperly designed or installed.

Four barriers

Exterior envelope assemblies typically require the following four components to separate interior building conditions from ambient exterior conditions:

  • A water barrier to keep rain and other precipitation from entering a building
  • A thermal barrier, or insulation, to help buildings maintain appropriate indoor temperatures for occupant comfort
  • A vapor retarder to keep moisture in the form of vapor from diffusing through exterior building assemblies to control condensation within the assembly
  • An air barrier to keep air from flowing through exterior building assemblies, which causes energy loss and can carry moisture that contributes to condensation within the assembly

The primary function of any roof system is to provide a water barrier. In addition, building codes typically require insulation to provide a thermal barrier. The other two barriers will be discussed in more detail.

Vapor retarders

Depending on the climate, the source of moisture vapor drive is either from inside a building or from ambient air conditions. However, a significant source of moisture in roof assemblies can be from construction or roofing materials.

Vapor pressure differentials drive vapor diffusion—warm, humid air has a higher vapor pressure than cooler, drier air. The difference in vapor pressure between a building's interior and exterior creates a vapor drive, wherein moisture within the air with higher vapor pressure attempts to migrate to the cooler, drier area. Vapor migration occurs at a microscopic level by diffusing through materials' pores.

Vapor migration through a roof system is controlled by a vapor retarder, usually a sheet membrane such as polyethylene or aluminum foil; a roof membrane also may act as a vapor retarder. The appropriate location of a vapor retarder within the roof assembly depends on the climate type, building occupancy and building's HVAC system. Vapor retarders traditionally are placed on the insulation's warm side to prevent vapor from migrating through the system to cold surfaces and condensing. Although several climates exist throughout the U.S., the following general guidelines commonly are used:

  • Heating climate: Areas in the northern U.S. primarily require heating of occupied spaces, and the vapor drive typically is from interior spaces to the exterior during winter. A vapor retarder commonly is placed on the insulation's interior side to prevent the warm interior air from accessing cool exterior surfaces.
  • Cooling climate: Areas in the southern U.S. primarily require cooling of occupied spaces, and the primary vapor drive is from the exterior spaces to the interior spaces during warm weather; therefore, the appropriate location for a vapor retarder is on the insulation's exterior side to prevent the warm and humid exterior air from accessing the cooler interior surfaces. However, because the roof membrane or system often provides sufficient vapor resistance, no dedicated vapor retarder is used.
  • Mixed climate: Areas in the central U.S. require significant heating during winter and cooling during summer, and the vapor drive changes directions seasonally. A vapor retarder sometimes is placed on the insulation's interior side, and the roof membrane provides vapor resistance for warm weather conditions.

For certain climates and roof systems, analysis of moisture transmission using computer models is appropriate to ensure appropriate vapor retarder selection and placement. Some building codes include prescriptive requirements for vapor retarder placement; for certain project conditions, the best type and/or correct placement of a vapor retarder may conflict with prescriptive code requirements. The designer must recognize the code requirements but also understand how the overall system will perform in a given climate.

Vapor retarders also may be required to control moisture migration from within construction materials. The most common issue is with new construction using cast-in-place concrete decks where moisture within new concrete can become trapped within a roof system. Permanent steel form decks commonly are used for cast-in-place concrete decks, and even if these decks are vented, the moisture within the poured concrete does not dry out during the short term.

In warmer climates, an interior vapor retarder is not needed to control vapor drive from inside the building, but vapor from the concrete deck can become trapped between the steel form deck and roof membrane, which typically is impermeable. This trapped moisture can attack adhesive bonds within the roof assembly, causing delamination and roof system failure.

Air barriers

Air barriers are intended to limit airflow through the roof assembly in both directions (exfiltration and infiltration). In addition to energy loss, air flowing through the assembly can carry water vapor within it, which can contribute to condensation within the assembly. The amount of water vapor depends on the air's relative humidity. Unlike vapor diffusion, airflow occurs primarily at a macroscopic level through gaps, holes, joints or other openings that create a path through or within a roof assembly. To this end, the air barrier must be a complete, continuous layer to control airflow.

The air barrier's location within the roof assembly generally is not critical and is not governed by climate as with vapor retarders. The more important considerations for air barriers' locations are the attachment method; ability to resist air pressure caused by wind, stack effect or mechanical pressurization of a building without tearing or displacing; and ability to reliably achieve continuity and seal all penetrations. An effective air barrier must be continuous at all intersections and interfaces between the roof and exterior wall systems. All joints and seams between building materials making up the air barrier must be sealed.

In some circumstances, an air barrier can function as the same layer as the vapor retarder. In warm climates where the vapor retarder should be installed on the insulation's exterior side, the roof membrane may provide the functions of the water, vapor and air barriers. In this case, the roof membrane must be integrated with the exterior wall air barrier to provide continuity. Steep-slope roofing materials, such as metal or slate, often are not airtight; in cold climates, the interior vapor retarder often is required to function as the air barrier.

Case studies

My company has conducted several case studies regarding vapor retarders and air barriers. Examples follow.

Low-slope single-ply roof in mid-Atlantic region

This building's roof system consisted of a normal weight concrete deck placed on metal forms, tapered polyisocyanurate insulation, fiberglass-faced gypsum protection board and an adhered single-ply TPO membrane. The insulation was adhered using polyurethane foam adhesive applied in a ribbon fashion. Within this system, only the roof membrane and concrete deck could be considered an appropriate air barrier. No dedicated vapor retarder was included below the insulation.

After about four years, the membrane and insulation had debonded almost completely from the substrate. The protection board had deteriorated as a result of significant moisture accumulation (see Photo 1). An investigation showed airflow from the building's interior into the roof system contributed significantly to the failure. At the roof edge, the precast concrete wall panels extended beyond the roof to form a parapet, and firesafing was installed in the joint between the slab and precast panel. A canted sheet-metal piece was used to support the insulation and roof system across this joint. Roof openings showed air was flowing from the building's interior and through joints in the sheet metal, indicating positive pressure relative to the exterior. During cold weather, this airflow caused condensation within the roof system.

In addition, further roof openings and samples testing showed the concrete roof deck was wet (see Photo 2). The moisture trapped in the concrete deck likely was from the original construction. New concrete dries slowly, and without sufficient cure time, moisture within the concrete can become trapped within the roof system, particularly when a steel form deck is used. Roof membranes are relatively impermeable and can function as vapor retarders. Without a vapor retarder on the concrete deck, moisture within the concrete deck was driven to the roof assembly's exterior side during cold weather, condensing at the underside of the membrane (a vapor retarder) and causing widespread deterioration of the gypsum protection board and loss of adhesion of the sheathing facer, which provided roof membrane attachment.

In addition, the polyurethane adhesive's integrity deteriorated as a result of moisture within the roof system. The roof system samples also revealed an inadequate amount of urethane adhesive was used and the insulation boards were not sufficiently weighted to provide intimate contact with the adhesive before the adhesive cured, contributing to the roof system attachment failure.

Based on some initial moisture analysis, concrete decks could take years to dry to a level that is acceptable for installing roofing materials without a vapor retarder. Therefore, installing a vapor retarder on the concrete deck would be the most reliable way to prevent moisture in the deck from entering the roof system. It also would be critical to have the vapor retarder function as an air barrier and be integrated with the precast concrete wall system to avoid air exfiltration at the roof edge condition.

Indoor swimming pool in northern Midwest

This building consists of brick masonry walls and a combination of a steep-slope standing-seam metal roof system and low-slope EPDM roof system. The swimming pool created a condition of high interior temperature and humidity levels (80 F, 60 percent relative humidity). The metal roof system consisted of prepainted steel panels, asphalt—saturated felt underlayment, plywood, polyisocyanurate insulation, polyethylene vapor retarder and acoustical (perforated) metal deck.

Corrosion spots on the painted steel roof panels, among other reported leakage problems, prompted an investigation. It was discovered the corrosion spots observed on the panels' top surface were caused by through-corrosion that began on the panels' underside (see Photo 3). The underside of the panels and attachment clips holding the panels down were severely corroded. Furthermore, the plywood sheathing had deteriorated heavily, with large sections that had turned to debris with no structural capacity.

The corrosion and deterioration were caused by air flowing from inside the building through the roof system to the exterior Although a vapor retarder was provided, none of the roof components were designed or installed to provide an adequate barrier to prevent airflow; laps in the polyethylene and felt layers were not sealed; joints in the insulation and plywood were not sealed; and the standing-seam metal roof system was not airtight.

Without an effective air barrier, the interior air was able to flow out of the building because of positive pressure induced by the mechanical system and condense on the cold plywood and metal surfaces above the insulation layers. Imbalances in the mechanical system could have been corrected to help control this problem. But if the vapor retarder had been designed and installed to make an airtight interior component with sealed laps and transitions to other systems, the interior air would not have been able to reach the cooler roof assembly components that allowed the condensation to occur.

Low-slope roof in mid-Atlantic region

This building's low-slope roof system consisted of single-ply TPO, paper-faced polyisocyanurate insulation tapered to drains and a precast concrete deck with a cast-in-place concrete topping. The insulation was adhered with polyurethane adhesive, and the roof membrane was adhered to the insulation with typical contact adhesive.

An investigation was performed to determine the cause of debonding the insulation and membrane. Moisture was found trapped within the roof system, which caused deterioration of the paper insulation facers (see Photo 4); as a result of the facer deterioration, the boards debonded from each other and the membrane debonded from the insulation. Microbial growth from moisture accumulation was found in the paper insulation facers.

In addition, the insulation adhesive was installed haphazardly, and the insulation boards were not weighted properly while the adhesive cured so the boards did not attach appropriately to the substrate (see Photo 5).

It was determined the moisture likely originated within the concrete topping. The roof system did not include a vapor retarder, and moisture in the topping was allowed to condense in the roof system's upper portions during cold weather. The insufficient application of insulation adhesive contributed to the extent and severity of debonding insulation.

Steep-slope roof in New England

This building includes brick masonry walls, a steep-slope copper standing-seam roof system and a low-slope TPO membrane roof system. Evidence of moisture problems first were observed by extensive white staining and moisture accumulation on the brick veneer near the roof edge conditions.

The copper roof was applied over rosin paper, felt underlayment, plywood, metal furring with mineral wool insulation, self-adhering rubberized asphalt sheet vapor retarder, gypsum sheathing and a metal deck. The masonry wall system consisted of brick veneer, a drainage cavity, rigid insulation and fluid-applied waterproofing applied to a concrete block backup wall. The roof membrane was applied over gypsum sheathing, polyisocyanurate insulation, a vapor retarder, gypsum sheathing and a metal deck.

Investigating the problem revealed significant air exfiltration out of the building at roof openings, indicating positive air pressure within the building. The rubberized asphalt sheet within the copper roof system provided an air barrier for the steep-slope roofs but was not sufficiently integrated with the fluid-applied vapor retarder used in the adjacent brick veneer; the insufficient lapping of these materials allowed air to exfiltrate out of the building, causing condensation within the exterior wall cavity and staining on the masonry. At roof edge detail conditions, portions of the rubberized asphalt sheet were missing, leaving voids where interior air could flow into the wall system.

In addition, at transitions between the steep- and low-slope roofs, the rubberized asphalt sheet was not integrated with the roof membrane, allowing airflow into the roof system and causing moisture accumulation and deterioration of the sheathing beneath the roof membrane. The roof system's vapor retarder did not function as an effective air barrier, allowing air to exfiltrate into the roof system.

Because of the high humidity and positive pressure associated with the indoor pool environment, special attention to air barriers' integration between the different roof and wall systems was needed during development and construction to provide an envelope system with an integrated air barrier to control airflow. During construction, different trades may have been responsible for different components, requiring the general contractor to coordinate the transitions to achieve continuity of the air barriers.

Swimming pool in southern Atlantic region

The roof system above this swimming pool consisted of a TPO membrane adhered to polyisocyanurate insulation. Using hot-applied asphalt, the insulation was adhered to a precast concrete deck, which did not include a poured concrete topping layer. Reinforcement strips were used to prevent asphalt from dripping through joints in the precast concrete deck, but the asphalt mopping was not continuous and was thin in areas.

After five years of service, the roof system was debonded almost completely from the insulation. Test cuts in the roof showed the insulation facer failed from moisture accumulation; in most test cuts, part of the deteriorated facer remained adhered to the membrane and part remained adhered to the insulation board. When the insulation was removed at the perimeter, interior air could be felt flowing out of the building where the precast concrete roof deck was gapped from the adjacent concrete wall.

It also was discovered the insulation boards were not fully adhered into the hot asphalt—the asphalt's surface was glossy in some areas, indicating it had cooled before the insulation made contact (see Photo 6). Although this defect did not cause the moisture problem, it contributed to the roof system attachment failure.

Moisture migration analysis of the roof system showed that, even with the asphalt applied to the deck as a vapor retarder, the system would allow significant moisture accumulation within the roof system during winter because of the TPO membrane's low permeance. The analysis model predicted greater than 90 percent relative humidity in the insulation layers during winter, indicating significant risk of condensation and moisture accumulation. Although the building is located in a warm climate, winter temperatures are low enough and the indoor pool environment is humid enough to create moisture vapor drive from inside to outside during winter.

In addition, the roof system lacked a continuous air barrier to control airflow from interior spaces into the roof system. The building was positively pressurized, forcing the inside air through voids in the roof system where it could contribute to condensation within the system.

A continuous air barrier and effective vapor retarder would have been needed to control the moisture problems in this roof system. Applied to the structural deck, a single layer could have provided both functions and should have been integrated with the roof perimeter detailing conditions to provide a continuous barrier. Moisture migration analysis during the design process could have identified the inadequate vapor retarder design.


As these examples show, vapor retarders and air barriers can be critical to a roof system installation's success. A lack of vapor retarders over poured concrete decks and properly integrated air barriers clearly can lead to premature failures. Although some of the case studies include indoor pools with severe indoor environments that exacerbated moisture problems, they still demonstrate how roofs can fail through improper design and/or construction. In all examples, the manufacturers' warranties did not cover the causes of failure; many manufacturers defer to the designer for proper vapor retarder and air barrier design.

Regardless of the cause, premature roof system failures can be costly to all parties involved in a project. Some of the failures presented were caused by poor design choices, but improper installation issues also contributed to the failures; therefore, responsibility for the problems was divided. Clearly, proper installation by the contractor is a basic requirement.

In addition, though roofing contractors typically are not responsible for roof system design, those who understand the risks involved with improper design may be better able to protect themselves against claims. It is in a contractor's interest to review and understand the roof system design, including the four barriers, and also understand how the roof system is intended to be integrated with the adjacent construction.

Jeff Ceruti, P.E., is senior principal at Simpson Gumpertz & Heger Inc., Waltham, Mass.


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