Most buildings require a continuous air barrier, and the air barrier must be continuously detailed across all six sides of the building enclosure to be effective. Air control discontinuities in parapets can lead to water ingress, impact occupant comfort, waste energy from loss of conditioned air, cause damage from condensation moisture, and transmit airborne contaminants through the building enclosure. 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 (read more about roof membranes here). For low-slope roof systems, it can be beneficial to use the roof deck as the primary air control layer or include an air control layer on the topside of the roof deck.
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. 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. 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.
Moisture Management Of Parapet Walls
Not all wall, roof, and parapet scenarios require a vapor control layer. In fact, adding a vapor barrier to a parapet design without understanding the hygrothermal impact can lead to unintended moisture problems, such as preventing an assembly from drying from incidental moisture. Consulting with a building enclosure professional can help clarify the moisture-related impacts of adding a vapor barrier to an assembly.
The spacing of weeps 406, 813, 1219 mm (16, 32, or 48 in.) on center (oc) is one example of a common practice without scientific support given moisture management and modular spacing patterns have little correlation. While there may be rules calling for certain spacing (i.e. 2006 International Building Code [IBC] 2104.1.8?Weep Holes), that does not mean there is supporting research.1 Some things are just done long enough they become standard practice.
With the old weep technology and its spacing, water indeed got out of the cavities and cores of masonry walls. However, it was not necessarily all the water, always through the weeps, or a fast process. Moisture management in masonry walls is about getting the water away from, off of, and out of the construction detail as quickly as possible. The length of time moisture remains is in direct proportion to the amount absorbed into the materials.
The process of determining moisture management zones begins at any part of the exterior envelope. In most cases, since moisture moves from a high point of entry to a low point in the exterior building envelope, starting at the top makes sense.3
The coping on the parapet wall is the roof of the parapet and must be waterproofed (Figure 9). One of numerous exterior building envelope details with many responsibilities, coping stones are frequently positioned out of sight. The intersection of the roof and bottom back side of the parapet is another moisture management detail with numerous roles. The roof flashing and the parapet wall counter flashing must be designed to be both waterproof and movement-absorbing; they must be able to accommodate expansion and contraction of the roof assembly.
There is a misconception patterns on the exterior of the building envelope veneers (e.g. stucco, wood, brick, or stone) are simply decorative. In truth, their primary function is protection. They direct moisture away from sensitive details, such as windows and doors. In the past, the construction industry understood this multipurpose concept and had the sense to make them both functional and aesthetically appealing. The current trend seems to concentrate solely on the aesthetic aspect. The unintended consequence of this singular focus is the creation of surface patterns (or details) that actually cause moisture management problems (Figure 11).
Window flashingZone 3 is the group of six windows on the second and first floors on the right and left sides of the exterior building envelope (Figure 12). In many cases windows or numbers of windows should be grouped into a single risk zone because their moisture management details are so interconnected and interdependent.
Louvers and windowsZone 4 is the pair of louvers and windows on each side of the entryway (Figure 13). Obviously, the two types of openings are different, but the moisture management detail is virtually the same. Further, their proximity to one another joins them into one, unified moisture management risk zone.
Front stoop steps and stoop platformZone 8 is the front stoop steps and platform. The seventh and eighth risk zones are the perfect example of the interdependence of moisture management systems. In the case of the stoop platform and steps, the slope-to-drain of the surfaces and their ability to resist moisture penetration is absolutely critical.
A detail that will allow for replacement of the stoop platform and steps without major impact on the veneer wall system is the appropriate design (Figure 16). This is an example of how a comprehensive understanding of moisture management risk zones influence the original building design and its detailing to allow for future maintenance, repair, and replacement of the exterior building envelope components with the least amount of interruption to adjoining details.
Bottom of wallZone 9 is the set of two garden level windows on each side of the front entryway stoop (Figure 17). Window openings at this elevation on an exterior building envelope have several unique moisture management concerns, including their proximity to grade level and accumulating moisture, along with the potential for splashes.
ConclusionUnderstanding weeps and identifying unique moisture management risk zones on and in the exterior building envelope are critical for creating and maintaining a sustainable building. However, while these moisture management risk zones can be identified as separate and unique for the purpose of designing and detailing, they are not and cannot be disconnected from each other when it comes to moisture management.
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.1For 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.
d. After the parapet wall is in place, the remainder of the continuous air barrier can be applied to both the roof and wall systems. It is notable that if the air control layer is also intended to act as the WRB in the wall and parapet system (as shown in Figure D), the application should start from the lowest point and work upward; this allows the subsequent layers to be lapped in shingle fashion. After the air barrier is applied to the walls (light red), the pre-applied strip of material can be lapped and secured over it (dark red in the center of the wall). The air barrier can then be applied to the roof deck (light red), up the backside and over the top of the parapet wall, and then terminate downward on the outside of the wall, lapping over in shingle fashion (dark red at the top of the wall). Lapped edges hidden behind the layers of air barrier materials are shown with dashed lines.
e. The roof insulation and high-density coverboard can now be installed to the roof deck. These could be mechanically-fastened or adhered roof systems, but the use of mechanical fasteners through the entire roof insulation can have a significant effect on the thermal performance of the building.7 Continuous insulation is also applied to the backside of the parapet wall to maintain continuity. The parapet blocking for the coping cap can now be installed. In this case, it also includes a layer of tapered insulation to provide slope back to the roof area and extend the continuous insulation to the top side of the parapet. Wood blocking is included as required to accomplish fastening to meet ANSI/SPRI ES-1 uplift requirements. After the parapet blocking is in place, a piece of counter flashing (shown in blue) is required to lap over the WRB prior to the installation of the walls continuous exterior insulation; flashing will later lap over the coping cap, but without this piece pre-installed, there would be a discontinuity in the water control layer. The bottom edge of the flashing, under the wall exterior insulation, is shown as a dashed line. 2ff7e9595c
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