Glazing systems: Considerations for the mechanical engineer

Window glazing and shading or louvers have a direct impact on the HVAC load of a building. Mechanical engineers often are tasked with specifying window and shading systems. Know about building envelope, and how it can be managed/altered by window system selection.

By Jerry Bauers, PE; George W. Houk IV, RA, LEED AP; Sebesta, Kansas City, Mo. June 19, 2014

Learning objectives

  1. Understand how glazing systems can affect a building’s heating and cooling loads.
  2. Learn how to design the building envelope to match the designer’s intent.
  3. Understand the items that can compromise an envelope model.

The abundance of sunlight in buildings is a mixed blessing. In the world of green design, natural light has been found to have enormous positive effects on building occupants. At the same time, the glazing systems that provide this natural light consume a significant amount of the energy required to condition these buildings. With this in mind, effective—and integrated—design and construction of these glazing systems is critical in managing both heating and cooling loads and, perhaps at least as important, infiltration loads in our buildings.

The increasing use of daylighting (the use of natural light in lieu of electric lighting) is an important element of sustainable design in architecture. In addition to the benefits of natural light on the building occupants, managing the operating cost of building lighting systems can have significant positive effects on a building’s energy profile. With building lighting systems alone representing one-third of the average building’s annual energy consumption at the end of the 20th century, reduction in lighting system operating hours can significantly reduce its contribution to building cooling costs.

Finally, creative solutions to managing solar radiation incident on the glazing systems through sun screens leverage the solar heating effect to reduce both heating costs in the winter and cooling costs in the summer.

Given the obvious benefits to each of these strategies, the first challenge engineers often face with the building envelope is accurately modeling its performance. The second and perhaps greater challenge is constructing the envelope elements to match the designer’s intent.

Problems and challenges

Radiant heat gain: A building’s exterior enclosure absorbs short-wave radiation from sunlight, converting that solar energy to heat in the envelope assembly. Short-wave radiant energy passes through the glazing into the space, warming the objects inside. Longer-wave radiation in the sunlight is absorbed by the glass itself, heating the glass surfaces and re-radiating that heat into the building. This phenomenon—the “greenhouse effect”—produces solar heat gain in buildings.

We can employ a number of strategies to effectively manage solar heat gain in design. One of the most popular of these strategies is to apply a coating to the glass to reduce solar gains and/or glare. An uncoated glass surface has a thermal emissivity of 0.84 to 0.91, meaning that it absorbs and re-radiates up to 91% of the radiant thermal energy to which it is exposed. By selectively reducing the emissivity of the glass surfaces, by reducing the amount of energy that is both absorbed and transmitted through the glass, we reduce the amount of solar heat gain through the glazing by as much as 75%. 

Solar glare: The glass that brings natural light into the building can also create a problem with solar glare, when the glazing is oriented toward the sun. Sunlight that reaches work surfaces where visual tasks are performed (both direct and reflected sunlight) can reflect off those working surfaces and into the eyes of the occupant, interfering with the ability to see the task. The most effective way to control solar glare problems within the building is by blocking direct sunlight before it reaches the exterior face of the glazing. Architects have increasingly employed sun shading devices that are designed to block direct sunlight from entering glazing systems in a variety of solar orientations. The design of solar glare controls can affect the engineering of both air conditioning and lighting systems.

Conductive heat loss: Glazing itself adds to building loads and comfort challenges by increasing conductive heat gains and losses. And, while these heat gains are significant, the heat gain/loss through thermal bridging (interruptions or penetrations of the insulation barrier by structural elements of the building envelope) is one significant controllable element of the envelope system and the structures in which glazing is mounted. 

For example, gaps in the exterior wall construction for insulation are specifically called for in many new school buildings constructed in the mid-Atlantic region. Where glazing is to be installed, concrete masonry walls are thickened to allow window framing to be secured to backup walls. Insulation is often omitted at these window openings, creating gaps in the thermal insulation barrier. Glazing systems often are deeply recessed into wall openings or are installed so that the glass is flush with the surrounding exterior wall material. Either design decision can create a condition in which the thermal plane of the glazing (the insulated glass) is not contiguous with the thermal barrier in the surrounding wall system. Laboratory testing has shown a decrease in thermal performance of up to 5% can result from gaps in the insulation coverage equal to 1% of the wall area.

Many solar shading devices are supported on the structural framework at the outer walls. Whether integral to or independent of a curtainwall system, such shading devices typically require structural support members to penetrate the exterior wall systems to enable direct attachment to building structural members. These connections create conductive heat losses via “thermal bridges” that pass through the insulation barrier. These thermal bridges are often seen in infrared images of interior and exterior envelopes.

Thermal break technology, now commonplace in window and curtainwall framing, effectively isolates the metal (highly conductive) exterior surfaces from the interior spaces. But the installation of the framing required to support the glazing and shading devices penetrates those breaks, creating thermal bridges that compromise the insulating effect an engineer expects from the envelope assembly.

In buildings with curtainwall systems, the curtainwall framing on the interior side of the thermal break can be isolated from the exterior surfaces for both curtainwall and framing elements to inhibit thermal conduction and cold (uncomfortable) interior surfaces. Where the curtainwall system extends above the roof to form a parapet, the back of the curtainwall system is sometimes not insulated. It may even be inadvertently vented to the interior. Heat loss and uncontrolled condensation occur in such areas during cold weather.

In such cases, the engineer’s analysis of condensation potential may not take into account the actual surface temperature of the curtainwall frame members that are exposed to interior environmental conditions. The condensation resistance factor (CRF) of the curtainwall system is a measure of performance where the system is fully exposed to the interior on one side and to the exterior on the other side. Because a portion of the curtainwall system extending above the roof is exposed to outdoor conditions on both sides, the conductive efficiency of the framing will allow outdoor temperatures to control surface temperatures on frame surfaces immediately below the roof. These conditions should be considered in an energy model, through computational fluid dynamics (CFD) or other analytical tools.

Developing a model of the energy performance of a building envelope generally relies on the gross details of construction of the envelope. It has become increasingly important for designers to understand those details of envelope construction that require that thermal and vapor barriers are interrupted by the structural requirements of the envelope system. Further, in details like the parapet wall example above where improper envelope construction can result in direct communication (either by conduction or air leakage), it is critical that the designed detail be accounted for in the model. More importantly, by identifying these areas of potential failure of the envelope to meet the designer’s intent, the designer can and should pay particular attention to the construction of these details.

Air leakage: Uncontrolled air passage through the building envelope significantly increases the amount of energy required to heat and cool buildings, often dwarfing other envelope losses in poorly sealed buildings. Excessive air leakage significantly undermines the effectiveness of a thermal insulation barrier, vapor barriers, and envelope energy management strategies. It is a primary cause of poor humidity control and condensation within envelope assemblies. Managing air infiltration is central to contending with the risks of humidity, condensation, and mold in buildings. In addition, poor air barrier installation will always compromise any thoughtful building energy model. 

Glazing systems are part of the air barrier assembly, a continuous barrier to the movement of air through the building enclosure. In most buildings today, the glazing system itself is can typically provide good resistance to airflow across the pressure plane intended for the envelope. However, if the intersection of the glazing system with the balance of the envelope is not sufficiently detailed or inspected to provide effective resistance to airflow at the transition between solid and glazed envelope assemblies, air infiltration will occur. In fact, the effectiveness of the air barrier at this transition should become the subject of intensive inspection and performance testing by the designers to assure compliance with the design intent.

The junctions between glazing systems and walls or roofs are among the most common sources of uncontrolled heat loss and air leakage in buildings. The most common envelope-related air infiltration failures are typically the result of poor coordination of the multiple trades that are responsible for the wall assembly construction. Defining the appropriate limits of work and responsibilities for each trade at the junction of materials and assemblies and the proper sequencing of installation is essential to effective installation in the field. Intersections of dissimilar materials and systems (glazing, curtainwall framing, masonry construction) give rise to material compatibility issues. Commonly used materials such as silicone and polyurethane caulking may resist efforts to seal to curtainwall materials and to each other. 

  • Window-to-wall joints: The construction sequence in many buildings calls for the glazing system to be installed after the roofing and exterior wall systems have been constructed. This limits the methods that can be used to seal an air barrier to the perimeter frame of the window or curtainwall. The details used to seal this critical joint are often not clearly delineated in construction drawings, and the trade contractor is sometimes left to construct a joint with no specified standard. Air and water leakage through the enclosure is often the predictable and preventable result.

  • Curtainwall-to-roof joints: Curtainwalls are often designed to extend upward past the roof line of a building. Because the vertical mullions are tubes, it is important to connect the air barrier of the roof to the glazing cavity of the curtainwall to prevent uncontrolled exfiltration of air at the parapet. It is also important to consider how insulation is installed to prevent an open cavity inside the curtainwall that communicates with the above-ceiling spaces, particularly if those above-ceiling cavities are negative pressure plenum returns.

Remedial measures

Glazing coatings: Coatings are applied to window glass to reflect short-wave solar radiation, and/or long-wave radiation from heat sources either around the building or within its interior. Reflective coatings on glass reflect or absorb most of the direct solar short-wave radiation, and are typically used to reduce heating load on buildings. Emissivity-reducing glass coatings have been developed to reflect long-wave radiation while maximizing the transmission of visible sunlight. A low-E coating will reflect nearly all of the long-wave radiation that would otherwise increase heat gain.

In contrast with glass, polished aluminum has a thermal emissivity of 0.03; polished silver, with an emissivity rating of 0.02, reflects 98% of radiant thermal energy. Reflective coatings, applied to new glass to increase reflective properties and drop emissivity levels, are typically multi-layer combinations of reflective metals. Modern low-E coatings on glass are composed of 12 or more layers of metals and ceramics in an extremely thin coating, applied by specialized coaters inside vacuum chambers. Current low-E coating technology incorporates three silver layers and multiple ceramic layers, formulated to pass only visible light while reflecting the infrared wavelengths that produce heat gain. Factory-applied coatings are now capable of providing a center-of-glass solar heat gain coefficient (SHGC) up to 0.04 (meaning the coated glass is capable of reflecting 96% of long-wave infrared radiation). This performance contrasts sharply with standard clear glass, which has an emissivity of 0.84 over the long-wave portion of the spectrum.

Site-applied films are often considered for use in older buildings, to reduce the solar heat gain of existing glazing systems. SHGC for window films can vary significantly (from 0.17 to 0.71, according to testing from one manufacturer). Many films block the short-wave visible light radiation more effectively than the long-wave infrared radiation that creates most heat gain. Some films simply block the full spectrum of solar radiation, absorbing heat, which is then released into the enclosed space or reflected into the glass.

SHGC is the accepted industry standard for measuring the effectiveness of coatings applied to glass in blocking radiant solar energy. Comparing the effectiveness of heat gain reduction in glazing compared with a theoretical black body, SHGC expresses the solar performance of the glass as a ratio; the lower the value, the less solar heat is transmitted. Shading coefficient (SC), used to quantify energy transmittance through glass, has been replaced by SHGC as the standard indicator of a glazing system’s shading performance.

Solar shading: Exterior sun-shading devices supported on curtainwall framing require attachments to the supporting structure that create thermal bridges in the exterior envelope. It is important to incorporate structural thermal break products to inhibit conductive heat loss where frame members exposed to exterior conditions pass through the thermal enclosure. The engineer should verify that these thermal breaks are clearly shown on the architectural details while preparing an energy model of the building.

Major buildings completed in recent years have been constructed with screen-walls, often perforated metal panels, which provide shading of the glass and the exterior walls. The reduction of incident sunlight on the envelope reduces cooling loads and improves building energy efficiency. Screen walls also improve daylighting conditions by increasing the effective shading coefficient of the glazing system and by reducing glare in workspaces within the building. Such systems are prominent features in the designs of recently completed buildings such as Cooper Union’s new academic building and Harvard University’s Oxford Street data center (see Figure 2). 

Thermal insulation and thermal breaks: Thermally improved window and curtainwall framing members have nonconductive materials separating the interior framing from surfaces exposed to exterior temperatures. These thermal breaks typically align with the plane of the glass. Glazing systems should be installed so that the exterior surfaces of the framing are on the outside of the insulation and cannot conduct cold winter temperatures through the thermal barrier. Installation details should call for the insulation to extend to the window framing. 

The mechanical engineer designing the HVAC system should verify that the architectural detailing calls for a continuous thermal enclosure that terminates against the interior side of the glazing system frame.

Air barrier systems:
Recognizing that the actual rate of air leakage through the envelope is critical to acceptable building performance, the industry has begun to implement requirements for inspection and testing of the air barrier system. Regulatory requirements for buildings have been revised over the past 14 years to address the importance of air leakage control in building enclosures. The International Energy Conservation Code included a requirement for sealing penetrations and openings in its 2009 edition. ASHRAE Standard 90.1 requires a continuous air barrier and mandates that it be clearly identified and detailed on construction drawings. ASHRAE Standard 189.1 carries air barrier requirements a step further, directing that the air barrier assembly is inspected during construction and tested after it is complete to demonstrate successful performance. 

To the mechanical engineer designing the HVAC system, identifying an air leakage rate that is both appropriate to the building use and realistically achievable is critical to the success of the project. The air leakage rate used as the basis of design should be reviewed by the entire design team, to verify that the systems comprising the enclosure are appropriately selected.

Of no less importance is the need to verify air barrier performance in the building prior to initial occupancy. The two general approaches to air barrier performance verification are quantitative testing and qualitative testing. The standard test procedure for verifying the rate of air leakage in the entire air barrier assembly, popularly referred to as the “blower door test,” requires that designed air leakage paths such as air exhausts, fresh-air intakes, and plumbing vents be sealed prior to air leakage testing. Construction sequencing may make testing of the completed installation impractical. A valid alternative for smaller projects is to isolate and test a representative section of the completed air barrier. This test requires the construction of a containment chamber that fully isolates the intended test specimen. Such testing can be difficult to perform on curtainwall systems, in which the framing consists of aluminum tubes that must be either sealed at the edges of the test chamber or fully contained within it.

In quantitative air leakage testing, the air barrier is placed under both positive and negative pressure relative to the exterior atmosphere to replicate the actual conditions to which the building will be exposed. Based on the area of the envelope, the testing authority will establish the number, capacity, and placement of fans needed to create the desired air pressure differential. Using fan pressurization or depressurization, the tester will determine the airflow rate through the building envelope. From the relationship between the airflow rate, the surface area under evaluation, and the pressure differential, the air leakage of the building envelope will be evaluated.

When an air pressure differential is established across the assembly, qualitative testing can also be performed. Qualitative testing consists of verifying the performance of critical areas of the air barrier assembly by using infrared thermography and/or smoke tracer to visually verify air leakage pathways. The results of qualitative testing can often be used to implement effective procedures for repairing conditions in existing buildings and correcting deficiencies in new construction.

Design considerations

The design of the HVAC system must be based on an assumed air leakage rate that is realistically achievable as well as consistent with the facility’s performance requirements. Identifying the acceptable air leakage rate in a project and determining the expected rate in a given building envelope assembly are among the most difficult challenges in the design phase of a project. The Air Barrier Association of America (ABAA) recommends a maximum air leakage rate of 0.04 cfm/sq ft at 0.3 in. WC (0.2 L/s per m2 at 75 Pa) for air barrier assemblies, but this maximum allowable air leakage rate should not be applied to all buildings. Laboratories, natatoriums, and cold storage facilities will require far less air leakage in order to deliver successful and cost-effective performance. On the other hand, a recreational facility housed in a pre-engineered metal building system should be expected to allow a far greater air leakage rate than the ABAA recommendation.

Accurate measurements of air tightness, verified by laboratory testing, are readily available for individual building components. While such test information is useful in selecting appropriate systems for the building, it is of little value in determining appropriate design air leakage rates for the junctions between systems.

Maximum air leakage rates for system interfaces can be verified by building and testing prototype assemblies in a testing facility. A test specimen is assembled and a chamber is constructed on the interior or exterior of the test specimen. Using the chamber, the rate of air leakage through the fenestration assemblies is determined at the project-specified pressure differential induced across the assemblies. The air leakage rates determined are compared against the acceptable rates identified for the project.

The data in the table above has been extracted from model building codes and national standards, including the following:

  • U.S. Army Corps of Engineers, Air Leakage Test, Protocol for Building Envelopes, Version 3May 11, 2012

  • Washington State Nonresidential Energy Code, 2009

  • National Model Building Code of Canada, NBC 2010, Chapter 5

  • International Energy Conservation Code (IECC), 2012
  • ASHRAE Standard 189.1-2013, amendments

Energy modeling

To accurately model the energy performance of a new building, the interfaces of dissimilar systems such as walls and glazing systems should be carefully reviewed while developing the model. Energy modeling software in current use for most building projects does not accurately calculate the actual U-values of system interfaces. There is little consistency in the design and construction of glazing system interfaces from one building to another.

Energy modeling software often does not consider the thermal bridges in building enclosure systems. Engineers must estimate how much the thermal bridges will degrade the overall U-value of the envelope. For air leakage, energy modeling software is typically run with an estimated average airflow through the envelope. The use of an average leakage rate inadequately models actual performance: in reality, nearly all air leakage flows through small pathways in the interfaces between the work of separate trades. Critical elements of the envelope design may need to be analyzed using CFD, with specific materials and sections modeled to accurately predict performance. In producing an energy model for a proposed building, the following items should be taken into consideration:

  • Verify the SHGC of all glazing types used on the building, including total glass area and solar orientation for each glazing type.
  • Identify each distinct glazing system interface detail, including terminations at the base of the glazing, intersections with dissimilar walls, and roof terminations. Quantify the total length of each detail, and verify that appropriate termination details for the thermal and air barriers have been developed by the design team. Select specific details that warrant further analysis in order to accurately predict performance.
  • Review structural penetrations of the building envelope, such as supports for sun shading systems, protecting roof canopies, balconies, and other cantilevers to determine whether thermal bridges are present.

Jerry Bauers is vice president and national director of commissioning at Sebesta. He is a member of the Consulting-Specifying Engineer editorial advisory board. George Houk is a registered architect at Sebesta with approximately 33 years of diverse experience in architecture, building inspections, and construction management. Through his diverse background in architecture and construction, he has developed significant expertise in the technical aspects of exterior envelope systems, design and construction, and BIM implementation, planning, and consulting.