At the Roof Edge: Water, Air, Thermal, and Vapor Control

"Out on the edge you see all the kinds of things you can't see from the center." - Kurt Vonnegut
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Sponsored by GAF
By Benjamin Meyer, AIA, LEED AP

Air Control

Air control diagram for flush roof edge (left), platform frames parapet (center), and balloon framed parapet (right).

Goal: Most buildings require a continuous air barrier. If you think of a building as a solid 3D shape, like a cube, then the air barrier must be continuously detailed across all six sides of the building enclosure to be effective.

Principles: To achieve continuity, the air control layer requires much more than selecting a material or specifying a lab-rated assembly. Air control discontinuities in parapets can lead to water ingress, impact occupant comfort, waste energy from loss of conditioned air, cause damage from significant condensation moisture, and transmit airborne contaminants through the building enclosure. The amount of moisture transported through the building enclosure via an air leakage pathway at normal interior-to-exterior pressure differences is many times greater than the amount of water vapor that can pass through a permeable material due to vapor diffusion alone. When it comes to the air control layer, parapets are among the most challenging areas to get right.

Roof membranes are generally very good at blocking airflow, but unless they are designed to be part of the continuous air barrier system, and tied into the other five sides, the building will still leak air.1

For low-slope roof systems, it can be beneficial to design the primary air control layer as the roof deck or to the topside of the roof deck. An example of this would be air sealing the penetrations to a concrete roof deck or installing a dedicated membrane to the roof deck, prior to installing insulation. Clearly identifying and communicating the air control layer in the roof system simplifies detailing at penetrations and transitioning at the parapet wall or flush edge.

Air control elements highlighted in red, across parapet cavity example.

Installing an air barrier after the parapet wall is in place is difficult to get right. It requires significant coordination among trades to install the air control layer up and around the parapet wall, transition to the coping cap flashing, and terminate to the wall system air control on the other side of the wall. One alternative is to connect the air control layer from the roof side of the wall to the exterior wall by insulating within the wall cavity with a closed-cell spray foam. While this may be the “fussiest” option with regards to blocking, trade coordination and use of specialty trades, in some cases, such as balloon framed light-gauge stud walls, it may be the best (or only) option.

The case of a flush edge is fairly straightforward; maintain continuity of the air control layer either over or under the roof edge blocking and terminate over the wall air barrier system. The use of engineered transition membranes (pre-manufactured materials used to tie two (often dissimilar) air barrier materials together) at the roof-to-wall interface can accommodate the installation of incompatible materials. A transition membrane may also help resolve any scope-of-work issues between roofing contractors and wall air-barrier contractors as installation of an engineered transition membrane should be scoped early.

Air barrier “strip-in” example with platform framed parapet.

When the parapet wall is built on top of the roof deck, as in a platform framed parapet, it gets a bit trickier. The best option for continuity is to “strip-in” the air barrier to the roof deck before framing the parapet wall above the roof deck. Though the strip-in method is preferred as a way of keeping conditioned air out of the parapet, it requires significant trade coordination and is not often implemented in the field. To accomplish it successfully, the stripped-in portion of the air barrier should be installed with excess material on either side of the roof edge. Then frame the parapet wall on top of the roof deck, and connect the excess stripped-in membrane to the air control materials on the wall and at the roof deck.

Thermal Control

Thermal control diagram for flush roof edge (left), platform frames parapet (center), and balloon framed parapet (right).

Goal: Maintaining continuity of the insulation layer, especially the continuous exterior insulation, across the parapet is important to achieve the intended energy performance and to prevent moisture condensation on cold surfaces.

Principles: In current IECC and ASHRAE 90.1 national model commercial energy codes, the basic prescriptive requirements for both walls and roof systems include the use of continuous insulation in many climate zones and construction types. Continuous insulation is far more effective than cavity insulation, which is tucked into the voids between framing members. In parapets, the framing members are exposed to exterior conditions on both sides of the wall, rendering cavity insulation highly ineffective. Across the flush roof edge and parapets, maintaining continuity of the “continuous insulation” can be tricky. Even with continuous insulation designed in the roof and wall systems, a common thermal discontinuity emerges where the roof system meets the backside of the parapet wall. These discontinuities are important because they represent thermal bridges in the thermal control layer.

For the flush edge condition, the thermal discontinuity primarily results from the intersection of roof edge blocking for terminating the roof system and wall cladding at the transition. The compactness of this detail makes it difficult to simply add insulation. The roof edge blocking should be a wood-based material, which has a much lower thermal conductivity than steel. Using solid wood blocking (versus open steel components) also greatly reduces the potential for air movement at the transition from roof to wall that can reduce the overall thermal resistance at this location. Roof framing members over the wall below should be covered by the continuous insulation from the wall system below. That is, don’t stop the continuous insulation short of roof framing edge conditions!

An example of thermal control continuity parapet.

For platform framed and balloon framed parapets, tactics for maintaining the thermal control layer may be specific to the wall framing material that extends past the roof. For walls composed of concrete, insulated precast, mass masonry or steel framing, the best approach may be to go up and over the wall with continuous insulation. In this case, continuous insulation is applied to the roof side of the parapet wall under the coping blocking at the top of the wall and connected to the continuous insulation on the exterior wall. If the parapet walls are tall cavity walls, this may not be ideal. Although insulated, the two-sided exposure and limited conditioning of the air in the cavity space within the parapet could still lead to condensation moisture on cold surfaces.

Image adapted from U.S. Environmental Protection Agency—Moisture Control Guidance for Building Design, Construction and Maintenance.

Looking up inside a balloon framed parapet. Condensation at the top of parapet from interior conditions.

Another strategy that is better-suited for wood-framed and very tall steel-framed walls is to effectively, but not literally, extend the roof thermal control layer through the backside of the parapet cavity wall and connect on the other side to the exterior wall continuous insulation. This is similar to the strategy described in the Air Control section above, using closed-cell spray foam to connect the control layer from the roof side of the wall to the exterior wall within the wall cavity. As stated previously, this may still be the “fussiest” option. It is, however, well suited for wood-framed walls where thermal bridging is less pronounced than steel framing, and with tall steel-framed cavities where even continuously insulated, air-controlled parapets can result in condensation due to their exposure and isolation from the regular interior space conditioning. It is important to note that when insulating across the parapet wall cavity, air-permeable insulation like fiber batts is not effective. If interior air can bypass or travel through the insulation, it can still lead to condensation and moisture problems in the parapet above the air-permeable insulation.

Vapor Control

Vapor control diagram for flush roof edge (left), platform framed parapet (center), and balloon framed parapet (right).

Goal: The primary function of a dedicated vapor control layer is to prevent condensation that results from vapor diffusion. Vapor diffusion occurs when water molecules in the air (vapor) pass through a solid material due to a pressure differential (high to low) on either side of the material.

Principles: Vapor diffusion through a solid material, even a vapor-permeable one, is a slow process. There are specific scenarios where enough vapor is able to diffuse through a solid material (not carried along by air leakage) to result in significant moisture accumulation over time. (Think of all the moisture that can potentially accumulate in a roof system as a concrete roof slab cures.) When it comes to vapor control, it is also possible to cause moisture problems by adding a vapor impermeable material to an assembly, intentionally or unintentionally. All materials—from insulation to membranes, air barriers, sheet metal, sheathing boards, paint, adhesives and so on—have some level of vapor retarding properties. Be sure to consult with a building enclosure professional to understand what materials may act as a vapor retarder in the roof, wall and parapet assemblies.

Not all wall, roof and parapet scenarios require a vapor control layer. In fact, adding a vapor barrier to a design without consulting with a building enclosure professional can lead to unintended moisture problems, such as preventing an assembly from drying from incidental moisture. Many times when vapor control is discussed, the conversation quickly slips into “air control” strategies to manage condensation-related issues—as air movement can transport up to many times more moisture than vapor diffusion alone. Vapor-retarding materials (and vapor-open materials) often also act as air barriers and can be incorporated into the continuous air barrier design. As designing and installing continuous air barriers becomes required in most buildings, the confusion regarding air barriers and vapor retarders still exists.2

Initial concrete roof deck moisture example.

For parapets, flush edges, and roof systems in general, one of the more challenging vapor control scenarios involves newly poured or “green” concrete roof decks. Significant initial moisture within the concrete will diffuse into the rest of the system or interior (due to its high vapor pressure) over a potentially long time. If the concrete is placed on a steel composite deck and can’t dry downward through the steel, then moisture in the concrete will drive to the exterior (upward) through the roof system, wetting the roof system along the way. A common strategy is to install a Class I or lower vapor retarder on the top surface of the concrete deck to prevent the moisture from rising. However, a self-adhered vapor retarding material may not always stick to high-moisture concrete. If a vapor barrier is to be installed above the composite concrete deck, a vented steel composite deck may be somewhat helpful as a means to provide a path for downward drying of the concrete, but this is not a definitive solution. Alternatively, an above-deck vapor retarder that allows horizontal movement of moisture with perimeter venting (think insulating lightweight concrete roof design) may also be beneficial.

 

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Originally published in May 2021

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