Building enclosure design of exterior soffits
Treating an exterior soffit as merely a surface on which to hang cladding materials can render the building enclosure vulnerable to performance problems.
Commercial building designs have long incorporated exterior soffits, better known outside of the building design/construction industry as the underside of any projecting building feature. Aside from soffits at the eave or rake edges of a steep-slope roof, the soffits that are the focus of this article are located on the underside of a steel or concrete-framed projecting building level or on the underside of other horizontal projections, such as balconies and canopies.
Traditionally, soffit designs provided an exterior ceiling material simply to conceal unsightly structural features. But when an exterior soffit connects the facades of consecutive building levels, it composes part of the building enclosure and accordingly requires performance-based design attention. With current code requirements, increasingly complex facade articulations, and the abundance of contemporary materials incorporated into architectural facade designs, this is not always a straightforward task.
The alternative—treating an exterior soffit as merely a surface on which to hang cladding materials—can render the building enclosure vulnerable to performance problems. Soffit design strategies intended to avoid water penetration, air infiltration, condensation, and premature material deterioration are highlighted below.
The authors’ approach to soffit design is a microcosm of their broader building enclosure design consulting approach. It involves systematically examining the soffit’s assembly of structural and architectural materials, and their integration with adjacent building enclosure systems, from the following standpoints.
For rainscreen building enclosures, the authors generally define “water management” as the ability of the building enclosure to facilitate the travel of water along the water-resistive barrier, and then intentionally discharge it from the exterior cavity. For the inverted field of the soffit, enabling the water discharge is relatively straightforward provided the soffit cladding materials have some free area for water to escape the cavity (e.g., cladding panels with perforations).
For a soffit below the structural floor slab of an enclosed space, the free area for water discharge does not need to be significant, since by the nature of its protected configuration, this type of soffit generally does not experience bulk water exposure from above. However, the upward surface of soffit cladding materials can be exposed to water from the adjacent vertical cavity where it can become trapped and have deleterious consequences.
Diverting water traveling down the vertical cavity to the exterior with a continuous through-wall flashing system is therefore an important feature of soffit design. A prudent location of discharge for this water is on the downward facing surface of the soffit cladding at the outside corner of the soffit, where it is relatively inconspicuous from an architectural standpoint.
When the facade and soffit cladding material is metal composite material (MCM), the small and discrete “condensation weep” holes that are a feature of some MCM soffit panels typically do not provide sufficient free area for cavity drainage when the vertical panels have open joints. The authors prefer to detail a continuous slot (e.g., MCM panel joint), with an insect protection screen, adjacent to the through-wall flashing’s hemmed edge for this purpose (Figure 2).
The above considerations are important when the vertical cavity adjacent to the soffit is tasked with managing incidental water (i.e., limited to minimal water that penetrates closed facade cladding joints). However, “important” becomes “vital” when the vertical cavity must manage a significant water volume. For example, some designers route balcony water drainage off the edge of the balcony and between the slab edge and the slab’s facade cladding. The authors typically recommend avoiding the risks of this drainage strategy by including internal plumbing drainage for balconies that include facade and soffit cladding below them.
When the facade and soffit cladding material is permeable, such as cement plaster, including the appropriate drainage accessories within the cladding system itself is also critical. One example is the drip screed at the outside corner of a cement plaster soffit (Figure 3). The drip screed is often omitted from the design or construction processes and the authors have witnessed the consequential premature deterioration to the soffit cladding.
Material Selection and Durability
Soffit cladding material choices are as diverse as the cladding options for walls. Cement plaster soffit systems are heavy and labor intensive to install but have been used effectively in soffit applications for many years. Metal panel soffit systems, including plate panel and MCM systems, can provide modern appearance and straightforward detailing of attachments, but can be more costly than other options. Natural dimension stone cladding materials are typically heavy and require carefully engineered attachments and structural backup systems, but designers can consider composite thin stone options with a lightweight structural backing at soffit locations to reduce the dead load of the system.
Contact with bulk water and condensation can deteriorate some materials faster than others, so it is prudent for the soffit design to acknowledge that, particularly when the soffit is below an exterior space such as a balcony, the cladding should be sufficiently durable to not rapidly deteriorate if the upward facing surface is wetted. Gypsum-based sheathing products are a common value engineering choice as a soffit cladding backing material given their economical cost, relatively light weight, and finish options that include simulating a traditional cement plaster soffit. However, even when marketed for exterior use, the products with paper facers are vulnerable to premature deterioration from moisture and should be designed and constructed with particular care.
Durability considerations should not end with the soffit finish material. The soffit support system (e.g., furring, fasteners, hangers, etc.) must also be engineered to perform for the expected service life and exposure conditions of the soffit; simply turning vertical wall cladding support system details along the soffit may not be appropriate. Additionally, adhesive-based anchoring systems for soffits should be very carefully vetted before specifying as some adhesives are not designed for permanent duration loading in tension. Typically, mechanical attachments, designed with redundancy in mind, are more appropriate for overhead applications.
Exterior Soffits Case Study 1
A new high rise apartment development features multiple towers over a common podium covered by a large amenity terrace. The tower facades include frequent bay projections that add interest to the building facades and afford residents with stunning views of the thoughtfully landscaped amenity terrace below as well as the adjacent urban waterfront. The bay projections extend down the full height of the facades and stop just above the amenity terrace forming small covered exterior alcoves.
Among other luxury features of the project development, the residential unit interior finishes were intended to be high-end. In that spirit, the exterior bay projections and their soffits (which form ceilings for the amenity alcoves below) were intended to appear as interior ceiling finishes, so gypsum wall board (GWB) sheathing with an exterior-grade skim coat plaster was specified and installed.
The drainage design for the open-joint rainscreen cladding of the bay projections above was to route any cavity water draining down the façade between the lowest slab edge and its metal panel cover. The concealed slab edge construction included an aluminum brake metal though-wall flashing with transverse joints lapped in gun-grade sealant.
Shortly after tenants occupied the residential units, complaints regarding water-stained amenity ceilings began. Diagnostic water testing revealed that the through-wall flashing system was systemically ineffective, both because the transverse joints permitted water penetration and because a cold-formed metal framing (CFMF) accessory of the soffit finish sub-framing system protruded between the terminal point of the flashing and the weep slots in the metal panel cladding.
Both the water that penetrated the through-wall flashing and that struck the protruding CFMF ended up on the upward facing surface of the GWB soffit sheathing, where it quickly stained and deteriorated the GWB (Figure 4). These conditions were exacerbated given that the open-jointed rainscreen cladding system on the bays above provided ample cavity water to the flashing and wind-driven rain events funneled air and water along the soffit surface including into the continuous vent space along the edge of the gypsum finishes.
A remedial program required that all through-wall flashings were replaced (along with associated edge portions of the façade air/water barrier above), the metal panel slab edge cover was removed and reinstalled, the perimeter vent system modified to minimize wind-driven water penetration, and significant removal/replacement of the GWB occurred.
Continuous air barriers are required to enclose conditioned spaces by contemporary building codes in most U.S. climate zones. Defining and simplifying the path of air barrier system continuity across the soffit and selecting the materials and details that compose the air barrier system should be key steps in the early design phases.
One common decision point that relates to the path of the air barrier arises when design teams establish the elevation of the soffit cladding such that a significant cavity space (i.e., more than a few inches) is formed between the cladding and the structural floor slab above. Here, the authors often observe one of the principal soffit design pitfalls – design drawings that fail to illustrate the path of the air barrier system altogether.
Designers who do bear in mind the need for air barrier continuity will either route the air barrier system up to and along the underside of the floor slab, or will generally follow the soffit cladding plane, which is offset and parallel to the floor slab. The former approach is typically preferred (in conjunction with coordinated thermal barrier and ventilation design; more on these concepts in later sections) since it:
- Minimizes the risk of uncontrolled interior moisture-laden air condensing within the soffit area where the moisture cannot be easily managed and can subsequently prematurely deteriorate soffit components;
- Simplifies the mechanical system design by eliminating the need for dedicated heating/cooling of the space above the soffit; and
- Simplifies constructability, since a sheathing material to support the air barrier is not required along the path of the soffit cladding.
When selecting the air barrier materials, designers have choices to consider and coordinate with the surrounding construction. If the structural floor slab of the projecting building level above the soffit is cast-in-place concrete on removable forms, one option is to use the concrete itself as a component of the air barrier system and terminate the air barrier material from the exterior wall below the soffit onto the underside of the floor slab.
Alternatively, a self-adhered membrane, adhered continuously to the entirety of the underside of the concrete slab, can function as the air barrier. The authors find that, since the force of gravity may eventually overcome the adhesive bond of the membrane to the underside of the concrete slab, the former option is typically preferable, especially when combined with a termination bar to secure the air barrier membrane from the wall below the soffit to the underside of the slab.
In some cases, the materials selected for the projecting structural floor slab can make the air barrier termination detail described above complicated to construct and prone to poor performance. For example, terminating a self-adhered membrane air barrier onto the underside of a concrete slab placed over corrugated steel deck is significantly challenged by the flutes of the steel deck. In this case, the steel deck can function as the air barrier in the field of the soffit area, but special details that specify how to fill the flute spaces (e.g., spray foam insulation) are required.
Note: This article does not include discussion on the design of vapor retarders as that subject matter for soffits is similar to exterior walls, in which vapor retarders have been well-documented.
In most cases, thermal insulation should be continuous and follow the path of the air barrier to limit condensation risk in the soffit cavity. Previously, we described the preferred route of the air barrier as excluding the soffit cavity from the conditioned building. But if designers choose to route the air barrier and thermal barrier along the exterior perimeter of the soffit cavity, the prescriptive insulation requirements of the building code may not singlehandedly manage condensation risk since the soffit space is likely to experience different in-service mechanical conditions (e.g., temperature and relative humidity) than typical building interior space. This is because a soffit space circulated with interior air (i.e., lacking dedicated mechanical systems for the soffit cavity) might be remote enough from the HVAC diffusers such that the soffit cavity does not receive the air exchanges needed to maintain temperature and relative humidity conditions to a degree that the prescriptive insulation thickness limits condensation.
In this case, adding mechanical systems or dedicated diffusers to the soffit cavity are typically the best approach, but they come with significant cost and design coordination (e.g., with structural framing) considerations. Basic computer thermal modeling can be a useful analysis tool to predict thermal performance and condensation risk of exterior enclosures, particularly exterior soffit cavities. However, advanced computer analysis is sometimes required to incorporate the nuanced effects of HVAC systems and other heat and air flow sources on the soffit cavity.
The soffit thermal barrier and mechanical system design should also consider the use of the space above the slab. For spaces with regular occupancy, avoiding occupant comfort issues associated with a cold floor should be addressed in the design if the soffit cavity is unheated or includes insufficient heating to render the slab comfortable for occupants. This concept should also be considered, but is less consequential, when the space above the slab is scheduled for transient use by occupants.
Locating the air barrier too far from conditioned spaces with mechanically circulated air can limit the beneficial interior heat and increase chances of condensation in enclosed spaces. Soffits with stagnate air in moderate to extreme climates can be sources of condensation if there is not a path for conditioned air circulation to all parts of the soffit. Structural framing can often create obstacles for air circulation and must be coordinated with the structural engineer. If the soffit is enclosed by the air barrier, at a minimum, unimpeded pathways for air flow should be provided between the interior space and the space enclosed by the soffit, but as described above, a dedicated mechanical system or diffuser for the soffit cavity may be required.
If the soffit is not enclosed by the air barrier, the cladding must accommodate adequate exterior ventilation to dissipate heat and moisture in the space between the soffit finishes and floor slab above. The authors are not aware of codes, standards, or research to inform the free area required to provide adequate soffit ventilation. However, the requirements for steep-slope roof attic spaces provide a reasonable basis for ventilation design of similar spaces. Traditional soffits that freely allowed air exchanges with the exterior typically had minimal condensation challenges, but resulted in drafty adjacent interior spaces. With the advent of air barriers and various cladding options which can sometimes perform as unintended air barriers, designers must carefully consider how and where to ventilate soffits.
Exterior Soffits Case Study 2
An elementary school renovation and modernization project in the Mid-Atlantic incorporated new construction (including curtain wall and MCM exterior wall cladding) with existing century-old mass masonry construction. The project delivery method was design-build. The new construction featured unique geometry, including an irregularly-shaped floor plate, to accentuate the contrasting architectural styles. The design also included a recessed first floor which resulted in an exterior soffit below a portion of the second-floor slab.
The curtain wall system extended down approximately 2 ft. below the second-floor slab edge, forming the exterior vertical perimeter of the soffit cavity. The design architect had considered two options for the soffit cavity: exclude the soffit cavity (and vent it to the exterior) from the conditioned space by extending the air and thermal barriers from the lower exterior wall straight up to the underside of the slab and sealing the slab edge to the back side of the curtain wall, or enclose the bottom of the soffit cavity with the air and thermal barriers and circulate interior conditioned air into the space.
The design that was reflected in the bridging documents appeared to show the former option, but the curtain wall subcontractor advocated for the latter option by highlighting that constructing an airtight seal between the back of the curtain wall and the edge of the second-floor slab would be challenging. The authors supported the design-build team in evaluating the merits and drawbacks of both options within the context of the curtain wall procurement process.
During this evaluation, since construction was already underway, we noted that at several locations a deep steel beam aligned above the first-floor exterior wall would have blocked virtually all airflow from the interior ceiling plenum into the soffit cavity. At that point in time, modifying the beams to allow air circulation into the soffit cavity or adding mechanical equipment to the soffit cavity would have been impractical, and proceeding with interior/conditioned soffit cavity approach would likely have resulted in condensation issues. In the end, the design-build team decided to proceed with the original design concept, which ultimately included creating an air-tight seal between the curtain wall and second floor slab using brake metal and silicone sealant, establishing a thermal break between the curtain wall mullions above and below the slab, and venting the soffit to the exterior (Figure 5).