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Fenestration includes all intentional visual or actual openings in a building envelope, particularly in exterior walls, including doors, windows, curtainwalls, storefront systems, and even operable glass walls. Its purpose is first to allow and control (by virtue of opening or operating) the flow of desired elements through the walls, such as daylight, ventilation, access, and connectivity. At the same time, since fenestration forms a part of the building enclosure, there is a need to restrict the flow of unwanted things (i.e., in the “closed” position), such as weather, water, the inefficient transfer of heat between inside and outside, or unwanted air infiltration. Since any fenestration system will have varying degrees to which all of these things can be accomplished, it becomes incumbent on architects and other design professionals to understand the realistic and steadily improving capabilities of different types of fenestration. While none are capable of matching the overall weather resistance and energy tightness of an opaque, well-sealed, and well-insulated wall, it is important to be able to assess how a particular fenestration product can help or hinder any particular building project—whether new or renovation.
Photo courtesy of Graham Architectural Products
Fenestration can take many forms and be used in many building types, such as the adaptive reuse of this former tobacco factory into a high-tech research facility in Winston-Salem, North Carolina.
Recognizing this need, there has been literally decades of research carried out on fenestration. A significant part of that research has been conducted or influenced by Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California. A well-known building scientist, Stephen Selkowitz headed LBNL’s building technologies department from 1985 to 2011. Stephen and his colleagues have worked diligently to investigate the many variables that can affect the performance of fenestration products in terms of energy conservation in buildings, daylight, weather resistance, and indoor environmental quality. He was also the driving force behind a “plug-and-play” testing complex dubbed the Facility for Low-Energy Experiments (FLEXLab). This facility allows researchers to mock up and evaluate the performance of proposed designs with actual building components, such as cladding, windows, lighting, and mechanical systems. The test results can be used to determine the relative difference between different designs in real-world terms and be made available to others to learn from those test results.
Of course, not everyone can undertake full-scale testing, so for the rest of us, there is computer modeling. Selkowitz and LBNL took on that challenge too with funding from the California Energy Commission to create the COMFEN software tool.1 This is a free resource that provides architects and engineers with the ability to assess the energy consequences of building design decisions. While the underlying simulation engines were developed over time as part of the U.S. Department of Energy’s (DOE) national windows and daylighting program, the specific design features of COMFEN were evolved over a several-year period by consulting with a series of largely California-based architectural and engineering firms. These firms provided important guidance and feedback on desirable features in the software program and then again on its functionality once the features were implemented. It is based on the premise that achieving a highly efficient or net-zero energy building cannot be done solely by improving the efficiency of the engineering systems (HVAC, lighting, equipment). It also requires consideration of the essential aspects of the design of the building including fenestration and the ability to compare different design options from the earliest phases of the design process.
With all of the above in mind, we will delve into several specific types of commonly available fenestration in terms of their performance in the context of some of the latest improvements currently available.
Windows
Manufactured windows are typically made from identifiable parts (e.g., frame, glazing, lite, hardware, sash) by a manufacturer that can create finished units in standardized or customized sizes and styles. While the variability of choices and options gives designers a lot of freedom to work with, the standardized nature of the manufacturing allows for a good degree of predictability on performance. In particular, manufactured window units can be physically tested and assessed for the full variety of conditions that affect their performance. One form of testing is done by the independent National Fenestration Rating Council (NFRC) in the same laboratory type of controlled conditions that LBNL and others use. The results of the testing are certified by NFRC, and a label attesting to the results can then be affixed to a window by the manufacturer to show its overall and specific performance characteristics. The ENERGY STAR program administered by the U.S. DOE also tests and rates window units, specifically to determine if they perform high enough above minimum standards to earn the ENERGY STAR label.
The NFRC recognizes that window performance is not limited to any single criterion. Rather, there are a number of factors that need to be looked at, tested, and assessed to determine the true overall performance of a window. U-factors (the inverse of an R-value) are usually what come to mind first, which measure the rate which a material or product transfers heat through it. When observing U-factors, it is important to differentiate U-factor variables for the insulated glass alone (e.g., center-of-glass U) from the window unit as a whole. NFRC utilizes the component modeling approach to calculate the whole U-factor of a window by assessing the glass, the spacer between glass, the sash, and the frame. Of course the size and makeup of the glass are also important—including the number of panes that make up insulated glazing units (IGUs). Ultimately, the total NFRC U-factors for windows are based on actual testing of representative manufactured units and expressed in normal fashion as a decimal value with a lower number showing less heat transfer, therefore greater energy efficiency. If a U-factor is not listed by a manufacturer as part of an NFRC test or as a center-of-glass value, then it may be based on a post-installation assessment and should be identified based on how the specific window performs in a certain wall after a specific installation.
Once the U-factors are identified, it is important to understand that that a U-factor difference of only 0.01 is not insignificant (i.e., in thermal physics, a difference of 0.01 between U-factors does not mean a 1 percent improvement, but something more). The range of U-factors for most windows only varies from 0.10 to 0.40, so an improvement in U-factor from 0.30 to 0.29, for example, can be highly noticeable in an overall large commercial building design and may save significantly on annual energy costs.
Images courtesy of Marvin Windows and Doors
Multiple aspects of window construction need to be addressed and tested in order to determine the true performance of the overall window unit.
The second notable factor in the energy efficiency performance of a window is the rate of air infiltration, particularly in operable windows. The 2015 International Energy Conservation Code sets the bar at 0.20 cubic feet per minute per square foot of window as the maximum allowable air leakage rate for windows, with some exceptions allowing 0.30 cfm/square foot. This is the same rate as for sliding and swing doors in commercial construction. The NFRC takes the air infiltration into account when testing window units so manufacturers pay attention to details of construction in this regard too. Proper weather sealing and tightness of fit all come into play, but so does the type of window. Casement, awning, or hopper type windows can employ multipoint locking hardware that can provide better-performing, positive-sealing pressure between the window sash and frame compared to a double-hung window, which slides in side tracks and relies on the meeting rails in the center to form a seal. European style tilt-and-turn units have become increasingly popular in the United States because they often provide the most energy-efficient option overall with the greatest air sealing capabilities as well as the highest potential for longevity.
Other aspects of the glass used in windows will affect the energy performance. Glass is tested and rated for its solar heat gain coefficient (SHGC), which indicates how much solar heat passes through the glass compared to what is reflected away. In building situations where heating from passive solar gains is desired, a high SHGC is desirable to take advantage of the free solar heat. In other situations where cooling is the primary energy driver in a building, a lower SHGC is needed to prevent unnecessary air-conditioning use because sunlight is heating up a space. Of course, the treatment for some of the energy-efficiency characteristics of the glass can interfere with the ability of people to see clearly through it. Therefore, visible light transmittance is also measured to help identify acceptable levels of clarity for views or the use of natural daylighting in buildings.
Ultimately it is up to a window manufacturer to put all of these things together and the architect or designer to select a product that suits the design and performance needs of a project. Manufacturers offer numerous window and door options with two or three panes of glass, choices in operation types, and a range of glazing options to meet the performance challenges of any climate. Some even use their extensive selection of shapes, sizes, styles, and finishing options to allow virtually unlimited design capabilities and the flexibility to get exactly what a project requires. Kris Hanson, senior manager of Group Product Management at Marvin Windows and Doors, sums it up this way: “We are continuously updating our product offerings to best meet architects’ current needs and to help solve the challenges they face on a daily basis. As design trends continue toward more glass and narrow frames, we work to create large window offerings that continue to deliver superior performance."
Photos courtesy of Marvin Windows and Doors
The type and function of operable windows can affect their energy performance, particularly in terms of air infiltration. The tilt-and-turn type shown here, which offers opening options, is generally the most energy-efficient type available from some manufacturers.
Custom Replication Windows
It is easy to think of selecting windows creatively in terms of new construction, but existing buildings, and historic buildings in particular, bring some additional considerations. Typically, windows in these cases need to fit some preexisting conditions or incorporate custom features to achieve results that match the design, function, or historic needs of the existing building. Therefore, the design challenge is to work with a window manufacturer who can help maintain the original aesthetics of the building while improving overall efficiency using modern materials and glazing. Often, the exact look or design an architect is trying to achieve does not exist off the shelf as a premanufactured product. Fortunately, there are window companies that will work with the architect and design team to either modify or create new shapes to satisfy the desired look and meet historic requirements.
A specific challenge can be producing windows with designs that faithfully replicate the original steel windows used in many older buildings using alternative, higher-performing materials such as thermally broken aluminum. At least one manufacturer has developed such a steel replication window specifically for use in historic buildings to help meet the requirements of state or national historic preservation agencies. This design includes narrow (less than 2-inch) frames that mimic historic steel profiles and slim-line integral and fixed-stack mullions for minimal sightlines. It is possible to specify historic profile true muntins as well. In addition to meeting these visual needs, applied-muntin grids with 1-inch insulating glass allows for the windows to be upgraded in terms of thermal performance compared to the original, which are most often single-glazed clear glass. That means that retrofitting older buildings can be done using modern materials and window product designs so the overall building energy performance is also inherently improved.
Putting this in context, those engaged in historic replication projects will usually attest that they are not for the faint of heart. Each job of this nature has its own unique challenges, which means it often requires a combination of architectural attention to detail along with a window manufacturer that can provide a highly engineered solution to deliver the necessary blend of strengths, capabilities, and aesthetic demands. It is important to select a company that has a track record of meeting the demanding scrutiny of historic preservation jurisdictions. It is equally important that the company can demonstrate how it recaptures original aesthetics while providing improved thermal efficiency and structural integrity to the buildings it has been involved in. That might include projects like renovating old factories or enabling the repurposing of an old mill to bringing the highest levels of performance, beauty, and operability to new construction.
The companies involved in successfully producing replication window solutions find it is a particular source of pride for those involved. Their focus is on overcoming the limitations of steel windows with single panes of glass and being able to design and deliver aluminum replications using state-of-the-art thermal technology that meets the often conflicting needs of both the National Park Service and the Department of Energy. Bill Wilder is Graham Architectural Product’s director of technical sales and comments, “That’s what makes our niche hard to do. That’s the fight we face in this industry of architectural windows: how to morph current technology into antiquated design while meeting today’s demanding standards for energy efficiency.”
Images courtesy of Graham Architectural Products
Older, historic buildings that need new windows can find new life by incorporating custom replication windows that can meet historic preservation guidelines and modern energy performance needs as shown here in the Firestone Triangle Building in Akron, Ohio.
Aluminum Storefronts and Curtain Walls
In many commercial buildings, the exterior building enclosure isn’t made up of an opaque wall with manufactured windows punched into it but instead of an aluminum and glass fenestration system, such as a storefront system (suitable for light-duty, first-, or second-floor installations) or a curtain wall (suitable for higher-design conditions and multistory applications). Similar to windows, however, their overall performance is comprised of and determined by the variable combination of different key components. These include the aluminum framing system, glass types, glass spacers, thermal break types, and gaskets. Of course, the specific characteristics of these items will vary based on whether a storefront system or curtain wall system is being considered and certainly will vary somewhat between manufacturers. Therefore, it is important that architects consult and collaborate with the product manufacturer and their technical staff early in the design phase. This way, the proper understanding of available product options can be obtained and the best means determined to achieve the desired results, including the overall U-factor, solar heat gain coefficient (SHGC), visible transmittance (VT), and condensation resistance factor (CRF). “Early collaboration between the design team and manufacturer becomes essential when specifying fenestration systems that are driven by performance,” says Mario Maggio, sales director at CRL-U.S. Aluminum. “As codes grow more stringent and building envelope trends evolve, we help select the most effective systems by remaining abreast of code changes and proactively coordinating with the architect during the design phase.”
Just like for windows, the NFRC has a process to rate and certify storefront and curtain wall systems, but it is based on computer software rather than physical testing since the combination of possibilities is vast and testing is simply not practical. Therefore, architects need to understand that all of the different components in a system can and need to be considered, assessed, and specified individually to produce the best combination for a particular building in a specific climate zone. Comparing different options and combinations based on computer analysis is the best way to identify a system solution that can achieve the design intent, meet the performance requirements, and stay within budget.
Aluminum and glass fenestration systems are designed based on transferring their own weight and structural forces back to the building’s main structural system. However, they do need to maintain their own structural integrity as a system through the use of aluminum frames and related components. At the same time, those frames need to address thermal performance since aluminum is a very good conductor of heat with an undesirably high U-factor. Thermal breaks in the frame are meant to do just that—stop or slow the flow of heat through the frame by separating the inside portion of the metal frame from the outside portion around the entire perimeter of the unit. In order to maintain the integrity of the window unit, the two halves still need to be joined, just not with metal. Rather, a low heat-conductive material is used with enough rigidity to be effective but enough insulation value to reduce heat flow. When it comes to framing systems, those with more than one thermal break are the most effective at maintaining high thermal performance. The more thermal interruptions between the exterior and interior of a building, the better the fenestration system is at mitigating heat transfer.
There are two common ways that thermal breaks are incorporated into curtain wall and storefront system frames. The first is to create an extrusion that has a “pocket” in the middle of the frame perimeter where a polyurethane material is poured in. Once cured, the portion of aluminum that connects the inner and outer frame is cut away, thus eliminating the thermal bridge between the inside and outside of the frame. This method is appropriately called “pour and debridge” and is common. The other method is to cast aluminum frame pieces that can accept polyamide (nylon) structural insulating strips that create an appropriate but strong thermal break. The size of these strips can vary, usually with wider ones being regarded as more effective.
Storefront and curtain wall systems will all have the same variables as other fenestration when it comes to glass options. Specifically, are double or triple IGUs being incorporated, what inert gas is selected between the panes (Argon and Xenon), what coatings (i.e., low-e) are being incorporated, etc. They also have the same IGU spacer options such as the common but conductive aluminum spacers or more advanced composite spacers that create a “warm edge” around the glass. By identifying all of these specific details into the makeup of a system, the computer analysis and the NFRC ratings can identify the specific U-factors and other attributes of a system. Some manufacturers may have some standard configurations and can offer them as starting points for comparison with certifications from NFRC accordingly. Note that changing any individual component will affect overall performance, either positively or negatively, and should all be assessed carefully in concert with the manufacturer.
Photos courtesy of CRL-U.S. Aluminum
Aluminum storefront systems (left) and curtain wall systems (right) are subject to many variables that need to be selected and specified in order to determine specific performance.
Operable Glass Walls
In some building situations, there is a design intent or user need to provide extreme flexibility in the use of fenestration to the point of being able to open or close large wall areas completely. Operable glass walls are a product that do just that by going beyond the capabilities of conventional windows and instead using large, door-sized glass panels that can be readily opened or closed on demand. Like any other type of building fenestration, this system does not carry any structural load from the building but is reliant on being appropriately attached to the building and operates within a structurally supported opening.
When operable glass walls are used as part of the building enclosure, the performance of these systems is clearly critical. Comparing manufacturers and specifying operable glass walls that can show documented capabilities to appropriately withstand the challenges of wind, water, extreme temperatures, forced entry, impact, acoustics, and structural integrity is a critical first step. This review should include attention to details such as multipoint locking entry doors that may be equipped with tamper-resistant locking rods between panels to ensure they meet or exceed forced-entry testing for commercial-grade door panels. It may also include built-in adjustment and compensation points to ensure continued ease of operation if any building settling occurs.
This type of flexible fenestration has been popularly used in a wide range of building types, including restaurants, retail, hospitality, education, sports venues, and even residential projects, both single and multifamily. In restaurants and retail, for example, opening up the entrance makes the entire street front a welcoming door by eliminating barriers. This creates a seamless transition between the street or mall and the store or restaurant, helping to attract customers inside and increasing sales. Passersby easily see what is inside and are immediately engaged in the activity and ambience of the space. By opening up the inside to the outside, additional seating space can be readily accessible whether for restaurants, hotels, or other gathering places. This type of system can also provide comfortable and attractive four-season outdoor dining by enclosing a covered patio with energy-efficient, NFRC certified operable glass walls. When it is time to close up for the day, the glass wall continues to showcase the interior and provides a secure, energy-efficient, transparent facade that seals tight as a dust-control measure after hours. Restaurants are also able to benefit from operable glass walls that can increase seating capacity and boost revenue by opening a restaurant’s interior to surrounding outdoor spaces such as the street, a patio, or balcony. In mixed-use conditions, it is possible to create unique and memorable indoor/outdoor dining atmospheres for shoppers to enhance and extend their shopping experiences.
In hotels and other hospitality projects, similar benefits exist in using an operable glass wall that creates large, inviting entrances in the lobby area and throughout the hotel, producing a memorable first and lasting impression. It is also possible to incorporate beautiful views, hotel landscaping, natural daylight, and fresh air into the registration, dining, and guest room areas when the walls are open. When closed, the operable glass walls protect against the weather but also dampen sound transmission for the desired acoustical performance needed in hospitality and other settings. They can also be used as interior divisions in hotels and restaurants to allow personnel to quickly and easily incorporate or close off adjacent retail space, dining areas, bars, terraces, or meeting rooms. Such a separation might be appropriate between a time-specific breakfast area and the hotel public space after the food service has concluded. Or it might be a way to quickly create private banquet rooms, meeting rooms, or retail spaces that can still transmit light but significantly reduce sound transmission.
Matt Thomas is the marketing manager for NanaWall Systems and offers this perspective on the need for both design flexibility and high performance: “For us as a manufacturer, it’s important to supply a product that enables the architect to fulfill their vision but also to provide a product that lasts. These points are as important to the architect as the aesthetic since we provide the product they are specifying on behalf of their client.”
Photo courtesy of NanaWall Systems
Operable glass walls can be used to provide complete flexibility of building design by allowing indoor and outdoor spaces to be combined when the walls are opened or sealed off and protected when closed.
Rolling Doors
In many buildings, there is a need for a large fenestration opening that has less to do with moving people or creating a particular aesthetic but is more about moving equipment, goods, or machinery. This can be true in industrial settings, warehouse spaces, commercial enterprises, institutions, or others. In these cases, the design issue is usually centered on the type of door to provide and how that door operates. Roll-up, or coiling, doors have improved in recent years. New high-performance models operate at higher speeds, can cycle (open and close) more frequently without wearing out, and have been designed with energy efficiency and aesthetics in mind. This means that high-performance rolling steel doors can find their place as a part of a robust energy efficient building envelope—something that might not have been considered previously possible.
The biggest advances in rolling steel door design have focused on dramatically reducing air infiltration. In the past, rolling steel door products have not generally been known as being very air tight, which many people assume is because of the slatted design of the door (i.e., air gets through in between each slat of the door). In reality, that’s not actually the case since the slats in higher-performance rolling doors interlock and don’t account for much air infiltration. Instead, the primary locations of air leakage come from the perimeter of the door—particularly the sides and top of the door. This is due to the design of the guides (the steel channels on each side of the door) plus the top of the door where it coils up on itself. By addressing these areas with advanced sealing and gasketing techniques, manufacturers are able to demonstrate notably reduced air infiltration rates when the door is closed.
Of course, any door has the issue of creating air exchange when the door is open so in settings where frequent openings and closings are needed, the key becomes to minimize the time it takes to operate the door. Some high-performance rolling doors have open speeds of up to 24 inches per second. Other types of high-speed doors can open at speeds of up to 48 inches per second. This may sound like a large difference, but the average height of a door is 10 feet or less, meaning that the difference between the two opening speeds yields a net difference of less than 2 seconds. The best speeds for high-performance rolling steel products are often achieved using an electric operator with a variable-frequency drive that ensures a soft start and stop—reducing wear and tear on both the door and the operator. This type of direct drive design also means there’s no sprocket and chain to wear or replace.
Beyond door speed, activation devices play an important role in energy efficiency. If the sensors are set to open the door too soon or delay too long before closing, the door will remain open longer than needed. Properly placing and setting the sensors/activation devices will help assure that the timing of door opening and closing is optimized to balance both building operation needs and energy efficiency. In this way, rolling steel doors can address both without compromising on either.
Of course, there are other reasons to take a look at rolling steel doors too. They are known to be durable, often with a positive life-cycle assessment, and they typically meet the security needs of building owners and users in their facilities. Some are tested and rated for 300,000 cycles of generally maintenance-free performance—maximizing uptime for building operations and improving productivity. Since springs are the most frequent point of failure for high-cycle products, doors that feature direct drive operation without the need for springs eliminate that maintenance concern. They are also available with insulation in the doors, which will improve the overall U-factor of this fenestration type to be likely higher than some others.
Images courtesy of CornellCookson, Inc.
Rolling steel doors are useful in many buildings, but air leakage around the perimeter of the doors needs to be addressed to assure that they are energy efficient as well as functional.
When considering the use of rolling steel doors in a project, there are four common design criteria to consider. First, how many times does the door need to open each day? Occasional use will suggest a standard door compared to a very high usage demand necessitating a high-performance door system. Directly related, the second thing to consider is the distribution of traffic or traffic patterns, including how it’s spaced throughout the day. This may suggest that different doors are subject to different usage patterns. Third is the degree of productivity impact that the doors can have. Higher speeds of opening with the correctly placed activation devices can shave seconds off of each pass through—which can turn into hours and then days of productivity gained. Finally, consider the nature of the opening in terms of usage. If it’s critical, then a springless design that can operate reliability throughout the life cycle of the door is likely the preferred choice.
In terms of appearance, there are a variety of powder coating, graphics, and custom design options that are available for rolling steel products. In some cases, custom solutions have been created for high-cycle applications that blend right into the facade of the building without it being obvious that a rolling door is even there. Manufacturers are quite willing to work with architects to make the door function requirements meet the design vision for the building.
Architect Marc Chavez, AIA, a construction specifier at ZGF Architects in Washington state, is proud of specifying high-performance products for his clients that comply with strict energy code requirements there. When looking at energy models for their projects, he previously found that the building envelope performance suffered from sectional or coiling doors that would leak air. “Selecting a high-performance door system is important as well as making sure it is not value engineered out of a project,” he explains. “Increased performance values are available in rolling doors that decrease air infiltration by as much as 95 percent.” Needless to say, he is convinced that others need to pay attention to these details too in order to achieve these results.
Photos courtesy of CornellCookson, Inc.
The successful operation of rolling steel doors depends on a variety of factors, including the opening and closing speed, the proper activation timing, and the ability to address the details on the inside and outside of the door.
Conclusion
Incorporating new or replacement fenestration into a building can make a dramatic difference on the overall performance, longevity, and appearance of that building. Understanding the performance principles and standards for different types of fenestration, as we have discussed, can make a significant difference on the operation of the building. Staying up to date on available products, systems, and other manufactured fenestration choices provides designers with a broad palette to create buildings that are better designed, more appealing to users, more energy efficient, and more cost effective to operate.
End Note
1COMFEN. Lawrence Berkley Laboratory. January 2013. https://windows.lbl.gov/software/comfen/comfen.html.
Peter J. Arsenault, FAIA, NCARB, LEED AP, is a practicing architect, green building consultant, continuing education presenter, and prolific author engaged nationwide in advancing building performance through better design. www.linkedin.com/in/pjaarch
