Contemporary energy codes that emphasize insulation continuity and reduced thermal bridging, together with design trends, architectural preferences, and current fenestration product options, shape building enclosure design.

Fenestration within rough openings has become a critical, complex, and evolving aspect of that design. The fenestration alignment within the rough opening (in other words, how its vertical plane is positioned relative to the adjacent cladding, insulation, and air and water barriers) not only affects thermal performance, but also the air barrier continuity, the water penetration resistance of the overall wall assembly, and the more readily apparent aesthetics of the design.

Fenestration alignment, therefore, demands early multidisciplinary design scrutiny, but the process is not always straightforward (Figure 1). Often, multiple interdependent, iterative, and competing interests influence fenestration alignment in the exterior wall section. These include architectural design intent, structural load path, fenestration product type (i.e. window, storefront, or curtain wall),¹ and
the proprietary characteristics of the specific product.

In this article, the authors outline a structured, holistic decision-making framework for design professionals to use as a roadmap as they navigate today’s interdependencies in fenestration alignment.

Introduction

Fenestration systems have evolved over time through technical innovation and in response to project needs and design trends. High-performance features once reserved for custom, high-end fenestration systems like curtain walls are now trickling down into the commodity market of more cost-effective product types like storefront and windows. The industry’s desire for customization has led to fenestration products becoming increasingly proprietary, adding a new level of complexity to selecting an appropriate fenestration position in the exterior wall section.

This article primarily focuses on fenestration applications within single-story or multi-story punched openings in commercial buildings (Figure 2). A punched opening is defined as a discrete elevation of a glazing system that is entirely surrounded by another opaque building wall system.² These openings can be infilled with different fenestration types, including window, storefront, or curtain wall products.

Until the mid-20^th century, fenestration systems in punched openings (generally window products up until that time) were often simply supported along each edge in a mass masonry wall assembly, and the surrounding masonry fulfilled the primary building enclosure functions. These functions include serving as a substrate for structural attachment, airtightness, watertightness, thermal resistance, and the finish material of the opaque exterior wall.

In contemporary cavity wall construction, model energy codes are pushing the envelope forward (including sometimes in a literal sense) by requiring increasing amounts of insulation in the opaque exterior wall cavity, where it can be more easily configured continuously. From a hygrothermal and thermal perspective, these code evolutions are beneficial. They can increase the building’s energy efficiency, improve occupant comfort, and reduce the risk of condensation accumulation in undesirable areas of the exterior wall. But along with the benefits come design challenges at punched openings. Mainly, how does a building enclosure design account for the increased cavity insulation thickness relative to the position of the fenestration product (Figure 3) while maintaining the key features of fenestration perimeter design detailing?

For example, design professionals often ask themselves whether they should push the fenestration out towards the exterior to achieve thermal barrier continuity and control thermal bridging, or to achieve a more planar appearance. Does shifting the fenestration out lead to unintended consequences and complications for fenestration structural attachment and air/water barrier integration? Alternatively, a design professional may ask themselves whether to inset the fenestration within the backup wall, either to achieve a facade appearance in which the glazing plane is deeply offset within the opaque facade, or out of necessity because the opaque exterior wall system is precast concrete with insulation on the inside. And if so, how might they deal with the associated complications, such as thermal bridging and gaps that may require cavity closures with this approach?

In any case, starting by outlining the design considerations impacted by the fenestration alignment in the rough opening is a valuable exercise.

Design considerations that influence fenestration alignment

While there are many factors that influence the fenestration alignment in the exterior wall section, these considerations are frequently the primary drivers:

Architectural design intent

Architectural design intent dictates the cladding and fenestration materials, as well as the fenestration product type. Materials and products are selected to achieve the desired geometry, size, exterior frame exposure, glazing makeup, and, ultimately, the preferred fenestration and glazing depth relative to the surrounding cladding. The design intent also informs the material selection and profile of trim or cladding treatment around the exterior perimeter of the opening. Additionally, the design intent is responsive to the project budget and project stakeholder requirements, and thus product selection often trends to the least costly option that can achieve the desired architectural intent.

Energy code requirements

Local jurisdictions and the selected compliance path (e.g. prescriptive versus performance path) determine the thickness of exterior cavity continuous insulation (c.i.). The effects of thermal bridging are also considered on a project-specific basis and some building codes. For example, in Washington, D.C., thermal bridges are incorporated in building analysis under certain compliance paths. Voluntary or mandatory sustainable accreditations (such as Passive Building Certification) also affect a project’s energy performance requirements.

Structural load path

The fenestration’s gravity load (e.g. self-weight) and the lateral load applied to the fenestration (e.g. wind) must transfer to the building structure. Fenestration products can be attached to the building using a variety of methods, such as clips, strap anchors, sill angles, wood blocking, receptor systems, or flanges. Some fenestration products offer several attachment options. Note that flanged windows inherently limit fenestration alignment options, whereas other fenestration types may offer greater flexibility. In general, the fenestration anchorage configuration and materials will depend on the rough opening size, construction type, design loads, and fenestration type.

Air barrier continuity and water penetration resistance

Fenestration products must integrate with surrounding building enclosure systems to maintain air barrier continuity and avoid water penetration. How exactly this occurs depends on wall type (rainscreen cavity wall versus barrier wall), fenestration product type, and rough opening flashing detailing.

Fenestration product type

Aside from a design choice guided by architectural preference, the fenestration’s product type also directly influences alignment. Each product includes, by manufacturer design, a “wet-dry” line, meaning portions of the product outboard of the wet-dry line are intended to manage and drain incidental moisture that penetrates the outer plane of the system. The wet-dry line of the fenestration product must integrate with the wet-dry line of the adjacent opaque wall assembly with a primary weather seal. The position of the wet-dry line for window, storefront, and curtain wall products is conventionally different. Non-flanged window products and storefront products typically have the primary weather seal positioned at the outboard edge of the frame, while curtain wall products often have the primary weather seal positioned at the “shoulder” of the curtain wall mullion, which is inset from a wept rainscreen seal located near the outboard edge of the frame. Flanged window products typically achieve their primary weather seal to the surrounding rough opening flashing by bed-setting the jamb and head flanges in sealant and stripping-in the jamb and head flanges to the air/water barrier with a self-adhering flashing. The primary weather-seal location can also vary by fenestration product within a given type. Common primary weather seal and wet-dry line locations are illustrated in Figure 4. Although this is not the focus of this article, an interior air seal is also a key design feature (to restrict air infiltration/exfiltration and benefit water penetration resistance) for all fenestration types.

Interdependency and iterations

Throughout the facade and fenestration design process, mindful decision-making informed by the factors highlighted above can help develop coordinated rough opening perimeter details. Some of these decisions, like modifying the material or profile of perimeter trim to conceal the exterior cavity, are easier to adjust later in the design timeline compared to others, such as changing the fenestration product type or load path and attachment to the structure. Regardless of the magnitude and consequence of the change, altering one factor will generally impact others.

Adding to the challenge of this inherent interdependency is the notion of design iterations. The authors’ experience is that even if the ownership, design, and construction teams agree at the start of a design on the primary factors, it is reasonable to expect some level of decision-making iteration throughout the design phase that affects the fenestration product itself and the surrounding conditions. For example, someone working on a project that begins with a particular basis-of-design window product that is designed to cantilever off its supporting condition by a certain distance could learn during contractor pricing that the product exceeds the project budget. This results in substitute products being presented. Aside from the substitute product’s performance characteristics requiring review, the team would need to evaluate whether it can similarly cantilever at its sill or whether a supplemental structural support element, such as a steel shelf angle attached to the building structure, would become necessary. But if a steel angle is added to the design, how does this affect thermal bridging at the windowsill, associated interior condensation potential, and code compliance? This is an example of iteration converging with interdependency.

Sometimes, even the construction buyout and/or submittal phase can introduce changes (e.g. substitution of an alternate product based on contractor input or preferences) that, by their mere lateness relative to the design schedule, present a greater probability and magnitude of pitfalls than if they were introduced earlier.

Decision-making framework—A fenestration alignment road map

Fenestration perimeter design detailing that is not conceived systematically or that changes without revisiting and coordinating with other relevant design considerations can lead to problematic outcomes. No one wants to continually reroute back to the beginning of the decision-making process and duplicate effort, but failure to consider interdependent factors can result in leaky, inefficient, or unnecessarily costly details. So how can project teams leverage the list of design considerations introduced above in a manner that is not only comprehensive but also conducive to the iterative design process?

The authors suggest a road map that includes the following steps:

1. Establish surrounding exterior wall systems

Determine adjacent cladding types, exterior wall water management function (rainscreen cavity wall or barrier wall), exterior cavity insulation type and thickness, depth of backup wall construction, and depth of exterior cavity. This includes determining the energy conservation code compliance approach and whether the building enclosure design is affected by regulatory or voluntary sustainability programs. Determine if a hygrothermal analysis of the opaque exterior wall is necessary and evaluate the make-up of the wall from the standpoint of moisture accumulation potential. For cavity wall systems that include combustible components, identify how code-required standards such as NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components, influence the exterior wall design (e.g. air cavity depth constraints for tested assemblies). Size each exterior wall component. Essentially, establish the opaque exterior wall’s dimensional characteristics at the outset of this process.

2. Establish performance expectations and overall project budget compatibility

Identify appropriate fenestration product type and basis-of-design product based on project-specific loads, available support conditions, dimensions, appearance and sight lines, and desired airtightness, watertightness, thermal, and other performance requirements (e.g. acoustic, condensation resistance). Engage the manufacturer’s technical support in this process if the project procurement method allows. At this stage, it is typically helpful to review the differences in performance characteristics between product categories.³ If window products are a category under consideration, review designations such as window class and grade with the project ownership team.⁴ This will inform the ownership team of the rationale behind the selected fenestration type, facilitate their engagement in budget-related decisions at the appropriate time, and emphasize the implications of interchanging products. When the project delivery method includes early contractor involvement, the construction team can helpfully provide product pricing comparisons to verify budget compatibility at a reasonably early stage in the design (e.g. early design development).

3. Establish a preliminary fenestration alignment based on architectural design intent

Using the basis-of-design fenestration type and product, position the plane of the glazing in accordance with the architectural design team’s preference.

4. Establish the general geometry and structural load path

The fenestration product type selected in the earlier step provides the backdrop for refining critical design decisions regarding the fenestration’s frame depth/profile and structural load path (including the attachment scheme). Available attachment strategies are influenced by the fenestration product type, manufacturer’s offerings, and surrounding wall construction. Note that some fenestration products have attachment options (e.g. strap versus bracket attachment) that affect the dimensions required gravity support. This step identifies:

(a) whether the fenestration’s structural attachment and gravity load support dictate a certain position for the frame and vertical plane of the glazing

(b) whether the backup wall of the rough opening itself can serve as the fenestration’s gravity load support or whether the sill of the rough opening needs to be extended with a supplemental structural shelf.

This step should also include an evaluation of whether the fenestration’s structural load path must accommodate differential movement. For example, if the head of the rough opening coincides with the underside of a structural slab that experiences live load deflection, the fenestration must accommodate that deflection within the fenestration system itself or through the rough opening using receptors or other means. Note that flanged windows do not lend themselves to such differential-movement applications.

5. Establish the wet-dry line

The wet-dry line is specific to each product (and not all products in the same product type have the wet-dry line at the same location) and dictates the primary weather seal locations, fenestration system drainage paths, and weep locations. Once the wet-dry line is confirmed, draw accurate, enlarged perimeter section details showing the primary weather seal location, rainscreen seal location (if applicable), and interior air seal location. Remember that the wet-dry line and position of primary seals on the fenestration frame are established by the manufacturer and validated through laboratory testing during product development.⁵ Deviating from the established locations of these features changes the performance of the system.

6. Configure rough opening perimeter flashings and cavity closure systems

The primary weather seal of the fenestration must engage the building’s air/water barrier. In a rainscreen cavity wall, this can occur either directly to the air/water barrier’s flashing accessories that turn into the rough opening, or, if the fenestration is aligned so that its primary seal is outboard of the vertical plane of the sheathing and air/water barrier, the primary seal can engage membrane or liquid flashing accessories that encapsulate an element (e.g. steel shelf angle at the sill or heavy gauge sheet metal “cavity closures” at the jambs and/or head). These elements, often metal, effectively bring the rough opening flashing out to meet the position of the primary seal. In a precast concrete barrier wall, the cavity closure concept may be applied on the interior side of the precast panel’s rough opening. In both cases, in the authors’ experience, relying on the cavity closure metal itself to serve as the airtight and watertight connection between the fenestration and the air/water barrier is fraught with performance pitfalls (given the splices that are involved) relative to fully encapsulating the closure element in liquid or membrane flashing. Also note that if differential movement can occur between the head and jamb conditions of the rough opening (see Step 4), then the head-jamb interface of the cavity closure system must also be designed to accommodate movement.

7. Develop exterior trim systems

Architectural trim may be required to close a wall cavity surrounding fenestration. Some fenestration manufacturers offer integral trim pieces attached to the fenestration product (e.g. via mechanical snap-in connections). These trim pieces, especially large ones that project substantially from the fenestration to cover a deep cavity, should have their wind resistance scrutinized to avoid the risk that they dislodge from the building during a design wind event. Calculations for trim are often delegated to the fenestration trade contractor. As with the fenestration’s structural attachment and perimeter flashing extensions, exterior trim must be designed for differential movement, if applicable. These elements are typically architectural only and should not be relied upon in lieu of a dedicated air/water barrier, though they can help deflect bulk water from reaching the exterior cavity.

8. Backcheck the preliminary fenestration alignment

Assess whether the positioning established in Step 3 allows the fenestration to satisfy Steps 4 through 7, and if not, adjust fenestration positioning.

Practical examples

Since it is impossible to anticipate every project-specific scenario, this section does not endeavor to illustrate all possible fenestration alignments or perimeter detailing outcomes; however, the generic practical examples of common design configurations shown below can be used as points of reference to complement the decision-making framework in the previous section.

For clarity, not all detail components are shown in Figures 5 through 7. These graphics focus on design features related to fenestration type, alignment, primary and interior air seals, structural load path, and perimeter flashings and cavity closures.

Fenestration alignment example #1: Multi-family wood-framed building with flanged windows

In Table C402.1.3 of the 2021 International Energy Conservation Code (IECC), construction with wood-framed walls allows the option to forgo continuous exterior cavity insulation and only provide batt insulation between studs. Presently, many buildings are still designed and constructed this way, but wood-framed buildings with continuous insulation are likely to become more common either by virtue of design choice or jurisdiction-specific mandates (Figure 5).

Fenestration alignment example #2: Curtain wall with supplemental steel shelf angle at sill

In ground-floor applications such as retail and entry fenestration, design professionals often choose to elevate the fenestration on a curb with a robust cladding, such as dimension stone (e.g. granite), to improve water-penetration resistance and durability. The concrete curb backup is classified as a mass wall, and if the prescriptive path of the energy conservation code is used, exterior cavity insulation is often required. The cladding thickness, along with an air gap or grout collar joint, further increases the depth of the exterior wall relative to the fenestration frame. If the design seeks to align the exterior face of the fenestration with the underlying cladding, a structural shelf encapsulated by a membrane flashing in the rough opening is a reasonable approach, subject to assessing the interdependencies described previously (e.g. thermal bridging). With respect to thermal bridging at the steel shelf angle, designers can consider specifying solid plastic shims (provided the shelf angle is designed for eccentric loading) or exploring thermally broken steel components or non-conductive structural elements for this purpose.

If the design seeks to inset the exterior face of the fenestration relative to the cladding, exterior trim is required.

Figure 6 illustrates these examples using a captured curtain wall product type. In both cases, careful attention to the continuity of the primary seal and maintaining a drainage path to the exterior (e.g. wept rainscreen seal) is important.

Fenestration alignment example #3: Window set within precast concrete panels

Precast concrete wall panels function as a barrier wall, serving as the substrate for the primary seal, the air/water barrier, the finished exterior wall surface, and part of the fenestration’s load path. Insulation is installed only on the interior of the panel, and strategies (e.g. offsetting interior wall studs) can be used to achieve reasonable continuity between floor slabs. Contemporary architectural design intent often aims for an inset appearance between the panel’s exterior face and the fenestration. Also, to align the fenestration’s thermal break with the thermal insulation (commonly closed-cell sprayfoam or foil-faced insulation), the fenestration is typically partially set within the precast and partially cantilevered towards the interior, with a structural steel shelf or engineered cold-formed metal framing supporting the sill condition as necessary. The use of sprayfoam as the insulating material requires that an interior cavity closure serve as a “spray stop” for the insulation. Figure 7 illustrates this example using a window.

Closing

Many of the considerations discussed above related to fenestration alignment are intertwined and, in some cases, have competing interests. The fact that the fenestration portion of building enclosure designs is often a moving target adds to the challenge. There is never only one perfect solution. But with an understanding of the relevant design considerations and a framework to tackle this portion of the design, the authors hope that design professionals will be empowered to leverage the concepts presented above to arrive at an appropriate combination of alignment and perimeter detailing.

Notes

¹ Refer to “FGIA Glossary.” Fenestration & Glazing Industry Alliance (2024), page 87.

² Refer to “FGIA Glossary.” Fenestration & Glazing Industry Alliance (2024), page 58.

³ Review “Window Walls: Blurring the Line Between Glazing Products.” The Construction Specifier (November 2017).

⁴ See “Back to Some Basics.” Door & Window Manufacturer (October 2011), page 18-20.

⁵ Refer to ASTM E331-00 (2016), Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference.

Authors

John N. Karras, P.E., is a principal at Simpson Gumpertz & Heger Inc. (SGH) in the Washington, D.C., office. He has more than 20 years of experience in building enclosure consulting and construction management, including design, consulting, investigation, and construction-phase services. He can be reached at jnkarras@sgh.com.

Emily V. Beam, P.E., is a senior project manager at Simpson Gumpertz & Heger Inc. (SGH) in the Washington, D.C., office. She has over 12 years of experience in building enclosure consulting, including new design, field investigation, condition assessment, and repair/rehabilitation design projects. She works with clients to provide expertise and deliver building enclosure solutions for below-grade waterproofing, exterior wall cladding systems, air and water barriers, fenestration systems, and roofing. She can be reached at evbeam@sgh.com.

Maria Raggousis, P.E., CPHC, is a senior consulting engineer at Simpson Gumpertz & Heger Inc. (SGH) in the Chicago, Ill., office. She consults on building enclosures and specializes in thermal and hygrothermal analysis to develop performance-based solutions in predicting, mitigating, or reducing moisture-related damage to building enclosures. She can be reached at mraggousis@sgh.com.