by nithya_caleb | December 16, 2018 12:00 am
by Scott Armstrong, BSSO, CET, LEED AP
Building shapes have become increasingly complex and cladding systems are often soaring high above the roof level or beyond adjacent façades. Whether used as a design statement or to conceal mechanical or rooftop components, parapet extensions, wing walls, soffits, and other appurtenances create many challenges for design and construction teams. Successful parapet and soffit design means knowing how to stop.
Understanding service conditions
Building enclosures are intended to control air, water, and thermal transfer between dissimilar environments. As we push for taller, larger, more complex, higher performing buildings, the stress on enclosures increases. For example, tall building enclosures are often subject to combined pressures from HVAC supply, wind, and stack effect. These combined pressures can result in long-term positive pressure in some areas and modulating pressure in others.
Designing and constructing is only one aspect of the built environment. A building enclosure must perform its functions in a durable manner and be maintainable. In some cases, the industry expects an enclosure to be functional for more than 50 years with little or no intervention.
It is no secret most enclosure failures occur at transitions and any enclosure specialist will readily identify areas most prone to failure (Read “Improving Tie-in Details for Better Envelope Performance” by Andrea Yee and published in the July 2018 issue of Construction Canada). This article focuses on two extremes of vertical enclosures: parapets and soffits.
Evaluating deterioration and failure
What do parapet and soffit failures look like? Sometimes it is obvious, such as moisture on exterior cladding materials or significant cladding deterioration, but more often, failures present less obvious symptoms. These could be occupant discomfort, space temperature fluctuations, poor building pressurization, or perceived water infiltration. Since many mechanical systems are designed to accommodate a wide range of heating and cooling demands, many of these symptoms can be masked by adjusting dampers, increasing air flow to some areas, or adding supplemental conditioning. Unfortunately, these strategies are short term and energy intensive, and do not address underlying defects contributing to enclosure failure.
Parapet case study
This author’s firm was recently consulted on a newly-constructed mid-rise commercial office tower featuring an approximately 3-m (10-ft) curtain wall parapet extension above the roof. During tenant fit-up, moisture damage was occurring at interior gypsum board window heads and on curtain wall vertical mullions. The damage was initially attributed to condensation on the interior of the back pan due to high interior moisture levels and cold exterior temperatures. The back pan was insulated on the interior by a batt-filled steel stud assembly. This meant back pan temperatures were below the dewpoint range and, as expected, interior air was leaking into the batt-filled cavity and condensing. This article will not debate the ineffectiveness of batt-filled steel stud walls on the interior of curtain wall systems; rather it will focus on remedial strategies attempted and outcomes.
The first attempt to remedy this interior condensation problem, by a different consultant, included adding heat tracing cables to the interior of the back pan to try and maintain the back pan temperature above the dewpoint. The author’s firm was not able to determine whether the heat tracing was able to increase the back pan temperature above the dewpoint; however, repeated damage to refinished interiors confirmed the heat tracing was ineffective at mitigating condensation. Clearly, a deeper investigation was needed to identify other contributing factors and to also develop an effective and low-energy solution.
The author’s firm directed a contractor to provide several interior test openings to assess the back pan, slab edge, and parapet extension detailing. This was coupled with an exterior visual review and infrared thermographic review from the roof-side of the parapet. The investigation revealed air leakage into the parapet and exfiltration at the top of the building as significant contributing factors in the recurring moisture damage. It was acknowledged an effective remedial strategy would need to control air leakage primarily while being minimally invasive and cost effective.
The design and construction documentation provided the necessary data to develop an air barrier plan specifically for the parapet and associated detailing. The temperature drop from interior space to the parapet void space is precipitous, so installing an air seal transition at the roof slab underside to stop interior air flow into the parapet appeared to be the most suitable approach. Since the vertical curtain wall mullions (effectively hollow vertical tubes) extend from the interior to the parapet void space, it was also necessary to provide an air seal transition inside the mullion. Otherwise, interior air could bypass the slab edge air seal and cause condensation.
Fortunately for this client, the trouble was quickly identified and a solution could be implemented prior to occupancy, during the fit-up stage. The air sealing strategy was deployed at the top floor around the entire building perimeter. Once air sealing was complete, no further reports of condensation or moisture damage were reported and the tenant occupied their space on schedule. While this matter was resolved quickly with little disruption, these issues could have been identified on the design or shop drawings and resolved prior to the tenant fit-up stage, saving time and money. Some appropriate design strategies will be presented later in this article.
Soffit case study
Soffits can be deceptively with several complex design and construction challenges, such as:
Soffits are also constructed by many trades over a protracted project schedule with, too often, few integrated design and delivery strategies.
A few years ago, the author was involved with an investigation of a deteriorated soffit and requested to develop remedial strategies with the aim to improve long-term durability and address occupant comfort concerns. While people generally think of soffits being located close to grade, often on the underside of an overhanging second or third storey, the soffit in this case study is located on the seventh floor of a newly constructed building. Regardless, the defects and conditions presented by this case are very typical and occur more frequently than one would expect.
Upon arriving at site, it was noted a large portion of the soffit had failed and fallen to a lower roof. Accessing the area by an elevated work platform allowed an up-close review of the failed components and of adjacent, intact assemblies. Immediately, a discontinuous air barrier permitting warm, moist, interior air to leak into the soffit space was identified. Once in the soffit space, the air condensed and led to moisture loading of the soffit sheathing. The coating on the soffit sheathing and inadequate venting of the soffit void prevented drying of the sheathing and, with material softening, contributed to failure of the panel retention system.
A couple of other potential contributing factors to this soffit failure were identified. They are as follows.
To resolve these issues, several measures were recommended. These included:
None of these items would have significant impact on the design concept or the construction budget had they been included. However, the absence of these measures resulted in gross soffit failure and a considerable repair bill.
Detailing for success
These examples highlight the need for attention at critical junctures, such as parapets and soffits. Detailing these assemblies requires an understanding of structural building movement, thermal performance requirements, air control, material limitations, construction technologies, and trade co-ordination. Transitions in these areas are frequently overlooked and misunderstood.
Parapet design strategies
Everyone loves rules of thumb so here is a suggestion for a new one: the one meter rule. In other words, parapets extending more than 1 m (3 ft) above the roof require extra care to control air leakage, mitigate thermal losses, and reduce condensation risk. Parapets less than 1 m can be adequately insulated and air-sealed by conventional means. Of course, there are always exceptions to rules but, for the purposes of this article, here are simple strategies for tall parapets.
Air leakage control in parapets is important since moist, interior air leaking into parapets can lead to concealed (or not) moisture accumulation, especially in colder climates. In fact, perceived roof “leaks” can be the result of parapet air leakage and back pan condensation. Installing an effective air seal at the roof deck level provides compartmentalization to the parapet and helps mitigate the condensation risk.
Additionally, air seals are sometimes needed within a cladding system. As discussed earlier, vertical curtain wall mullions often act as shafts carrying interior air into parapet voids. In the simplest instance, these air seals can comprise a sheet metal and sealant closure on the building interior and foam-filling of vertical curtain wall mullions. More complex or taller parapets may present other opportunities, such as starter tracks just above the roof level to accommodate simpler tie-ins. Solid opaque walls can incorporate an air seal comprising a continuous through-wall membrane at the deck level that transitions to the wall AB/VR control layer.
Thermal control at parapets can present unique challenges. Parapet walls are sometimes designed to accommodate loading from building maintenance equipment such as boatswain’s chairs or swing stages and, as such, are often reinforced with elements anchored directly to the building structure. A prime example is a hollow structural steel (HSS) parapet upstand. These may look great from an air sealing perspective, with continuous membranes, fully-welded plates, and solid materials, but they present significant thermal bridges. This undermines thermal performance and increases the condensation risk. Structural thermal breaks (STBs) may help; however, these may increase cost and affect construction scheduling.
It is important to understand how parapets transition from one to another around the building perimeter as well. Recent architectural trends, such as slot windows, present challenges since they require transitions from unitized (curtain wall) to site-built (opaque wall) systems, each with very different air, vapour, and thermal control layers and materials.
What about a low parapet that becomes a high parapet? Or a low parapet extending behind a shear wall, effectively limiting heat flow to the void space and increasing condensation risk? Each project is unique and likely requires bespoke solutions.
Soffit design strategies
While the one meter rule can apply to soffits, too, many other conditions make soffit design a unique challenge. In most cases, the preferred design is to have a warm, interior zone directly below the occupied floor with suspended control layers. This zone is filled with services such as HVAC supply, plumbing, electrical, and cabling. Maintaining a warm zone below the occupied floor provides optimal thermal comfort to occupants since the floor will generally be maintained at interior space temperatures.
The control layers (typically membrane and insulation) separate this warm, interior zone from a vented—or exterior—soffit space. This vented soffit space permits installation of infrastructure, such as lighting, signage, suspended access supports, and security services, behind architecturally appealing soffit cladding systems. This vented soffit space also permits maintenance of infrastructure elements without compromising the carefully detailed and constructed control layers. Of course, the soffit assembly itself is only one component in the enclosure assembly. Success relies on effective transitions.
Transition design and construction requires knowledge of construction sequencing, a general understanding of structural movement in buildings, and detailed insight into different enclosure systems and how they manage air, water, and thermal. As discussed in the soffit case study, soffit control layers typically comprise “thick” site-built assemblies, such as steel studs, sheathing, AB/VR, insulation, and cladding, whereas many cladding systems comprise “thin” shop-fabricated assemblies, such as curtain walls or precast. The increasing specialization of all sectors, including design and construction, means workers familiar with one system are rarely familiar with another.
Since curtain wall systems are installed prior to soffit systems, it is often left to the soffit worker to complete the tie-in between the two systems. Since curtain wall systems are unique to the manufacturer, one strategy does not apply in all cases. It is incumbent on the design professional and construction quality assurance (QA) team to ensure these transitions receive proper oversight.
One method to improve the simplicity of air barrier transitions between soffits and curtain walls is to remove the curtain wall neck in the transition zone. This permits installation of a continuous, solid transition material on the exterior side of the vertical mullion against which insulation can be installed for a reasonable thermal barrier. The closure must be designed to accommodate differential movement as noted earlier and must be able to resist expected pressures.
Designing and delivering increasingly complex and high-performing buildings requires an understanding of environmental conditions, building materials, construction technologies and delivery models, and physics. Failure to appreciate common demands on modern building enclosures in areas like parapets and soffits can lead to early and costly failures.
Scott Armstrong, BSSO, CET, LEED AP, is senior façade specialist, building sciences, WSP. Armstrong has 20 years of experience in high-performance buildings, building enclosures, façades, existing building repair and renewal, roofing and green roofs, and integrated design. He is past secretary and board member of the Canada Green Building Council (CaGBC) Greater Toronto Chapter. Armstrong can be reached via e-mail at Scott.R.Armstrong@wsp.com.
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