Window Glazing and Curtain Wall Systems Guide: Types, Thermal Performance, and Structural Considerations | Projul

If you have worked on a commercial building in the last decade, you have dealt with glazing specifications that read like a physics textbook. U-factors, SHGC values, thermal break requirements, structural silicone, pressure equalization. The language can be dense, but the concepts matter. Glazing and curtain wall systems define how a building looks, how it performs thermally, and whether it keeps water out for the next 50 years.

This guide breaks down the major types of window glazing and curtain wall systems, the performance metrics that drive specification decisions, and the installation considerations that contractors need to understand.

What Is a Curtain Wall?

A curtain wall is a non-structural exterior wall system. It does not carry any of the building’s floor or roof loads. Instead, it hangs from the structural frame (usually steel or concrete) like a curtain, carrying only its own weight and transferring lateral wind and seismic loads back to the structure.

Most curtain walls are made from aluminum framing members (mullions) filled with glass panels, though they can also include opaque panels made from metal, stone, or composite materials.

Curtain walls are found on mid-rise and high-rise commercial buildings, office towers, hospitals, airports, and institutional buildings. They allow architects to create large expanses of glass that would not be possible with traditional punched window openings in a load-bearing wall.

Types of Curtain Wall Systems

Stick-Built Curtain Wall

Stick-built systems are assembled on-site. Individual mullions (vertical and horizontal framing members) are anchored to the building structure, and glass panels are installed into the frames piece by piece.

Advantages:

• Lower shipping costs since components are compact
• Easier to accommodate irregular building geometries
• Good for low-rise and mid-rise applications
• Changes can be made in the field more easily

Disadvantages:

• Slower installation since everything is assembled at height
• More dependent on field labor quality and weather conditions
• Higher risk of installation defects at joints and seals

Unitized Curtain Wall

Unitized systems arrive on-site as pre-assembled panels, typically one story tall and one mullion spacing wide. Each panel is factory-glazed, sealed, and tested before shipping. On-site, a crane hoists panels into position, and workers clip them to anchors on the structural frame.

Advantages:

• Much faster installation, especially on tall buildings
• Factory-controlled quality for glazing and sealing
• Less exposure to weather during assembly
• Reduced on-site labor

Disadvantages:

• Higher upfront cost
• Requires careful coordination for shipping and staging
• Less flexibility for field modifications
• Panels are large and heavy, requiring crane access

Point-Supported (Spider) Glazing

Point-supported glazing uses individual stainless steel fittings (often called spiders) to clamp glass panels at their corners or edges. The glass is structural in this system, resisting wind loads through its own stiffness and transferring forces through the point connections to a supporting structure (cables, rods, or a steel frame behind the glass).

This system creates the most transparent appearance because there are no visible mullions. You see it in high-end lobbies, atriums, and feature walls.

Advantages:

• Maximum transparency and visual impact
• Dramatic architectural effect

Disadvantages:

• Expensive to engineer and install
• Requires very tight tolerances
• Glass replacement is more complex
• Limited to lower wind load applications in most configurations

Structural Glazing

In a structural glazing system, the glass is bonded to the aluminum frame with structural silicone sealant rather than being held in place by mechanical pressure plates and caps. From the outside, you see a smooth glass surface with minimal visible framing.

This approach requires careful engineering of the silicone joint, strict adherence to sealant manufacturer specifications, and controlled application conditions.

Understanding Glass Types

The glass you put in a curtain wall or window system has a massive impact on building performance. Here is what you need to know about the major types.

Annealed Glass

Basic float glass with no additional heat treatment. Annealed glass breaks into large, sharp shards. It is the least expensive option but is rarely used in curtain walls without further processing because of safety and strength concerns.

Heat-Strengthened Glass

Heated and cooled in a controlled process that makes it roughly twice as strong as annealed glass. Heat-strengthened glass breaks into larger pieces than tempered glass, which is actually an advantage in certain curtain wall applications because the broken pieces tend to stay in the frame rather than falling.

Tempered (Fully Tempered) Glass

Heated and rapidly cooled to create high surface compression. Tempered glass is about four times stronger than annealed glass and breaks into small, relatively harmless granules. Building codes require tempered or laminated glass in certain locations (near doors, at low heights, in overhead applications).

The downside of tempered glass in curtain walls is that when it breaks, the entire panel shatters into small pieces that can fall out of the frame. This is why many curtain wall specs call for heat-strengthened laminated glass instead.

Laminated Glass

Two or more layers of glass bonded with a plastic interlayer (typically PVB or SGP). If the glass breaks, the interlayer holds the fragments in place. Laminated glass provides safety, security, sound reduction, and UV filtering.

In curtain walls, laminated glass is standard for overhead glazing (skylights, sloped glazing) and is increasingly specified for all positions because of its post-breakage performance.

Insulated Glass Units (IGUs)

An IGU consists of two or more glass panes separated by a spacer and sealed to create an air or gas-filled cavity. This cavity provides thermal insulation.

Key IGU components:

• Spacer bar: Separates the glass panes. Warm-edge spacers (stainless steel, foam, or composite) perform better than traditional aluminum spacers because they reduce heat conduction at the glass edge.
• Gas fill: Argon is the most common fill gas, providing about 30% better insulation than air. Krypton is more effective but significantly more expensive and is used in thinner cavities.
• Sealant system: A dual-seal system (inner seal of polyisobutylene and outer seal of silicone or polysulfide) keeps moisture out and gas in.

Standard commercial IGUs use two panes (double glazing). Triple-glazed IGUs are becoming more common in cold climates and high-performance buildings.

Thermal Performance Metrics

When you read a glazing specification, you will see several performance numbers. Here is what they mean and why they matter.

U-Factor

U-factor measures the rate of heat transfer through the glazing assembly. It is expressed in BTU/hr·ft²·°F. Lower numbers are better.

• Single-pane clear glass: approximately 1.0
• Standard double-pane IGU: approximately 0.47
• Low-E double-pane with argon: approximately 0.27
• Triple-pane low-E with argon: approximately 0.18

Energy codes (ASHRAE 90.1 and IECC) set maximum U-factor requirements based on climate zone and window-to-wall ratio. In colder climate zones, you need lower U-factors, which means better-insulated glazing.

Solar Heat Gain Coefficient (SHGC)

SHGC measures the fraction of solar radiation that passes through the glass, both directly transmitted and absorbed/re-radiated. Values range from 0 to 1.

• Clear single pane: approximately 0.86
• Standard double pane: approximately 0.70
• Low-E double pane: approximately 0.25 to 0.40

In cooling-dominated climates (the South and Southwest), a low SHGC saves significant energy on air conditioning. In heating-dominated climates, a higher SHGC can be beneficial because it allows passive solar heating.

Visible Light Transmittance (VLT)

VLT measures the percentage of visible light that passes through the glass. Higher VLT means more natural daylight.

The trick is balancing VLT with SHGC. You want enough light for occupant comfort and daylighting energy savings, but not so much solar heat that the cooling system cannot keep up. A good target for most commercial buildings is a VLT-to-SHGC ratio (called the Light-to-Solar-Gain ratio or LSG) of 1.25 or higher.

Condensation Resistance Factor (CRF)

CRF rates the ability of a window or curtain wall assembly to resist condensation on interior surfaces. Scores range from 0 to 100, with higher scores indicating better resistance. This is especially important in cold climates and in buildings with high interior humidity (pools, hospitals, museums).

Thermal Breaks: Why They Matter

Aluminum is an excellent conductor of heat. Without a thermal break, an aluminum curtain wall frame acts as a direct path for heat to flow between inside and outside, creating cold interior surfaces, condensation, and energy waste.

A thermal break is a strip of low-conductivity material (usually polyamide nylon) placed between the interior and exterior portions of the aluminum frame. This interrupts the thermal path and dramatically improves the frame’s insulating value.

Every modern curtain wall system for conditioned buildings should include thermal breaks. The depth and material of the thermal break directly affect the system’s overall U-factor and condensation resistance.

Structural Considerations

Wind Load Design

Curtain walls must resist positive and negative wind pressures. Design wind loads are determined by ASCE 7 based on building location, height, exposure category, and the specific position on the building face. Corner zones and upper floors see higher pressures than mid-wall areas at lower heights.

Glass thickness, mullion size and spacing, and anchor design all respond to wind load requirements. The glazing engineer sizes these components to keep deflection within acceptable limits (typically L/175 for mullions, where L is the span length).

Seismic Performance

In seismic zones, curtain walls must accommodate inter-story drift without breaking. This means the connections between the curtain wall and the building structure need to allow movement. Unitized systems handle this naturally because each panel is independently attached and can rack slightly relative to its neighbors. Stick-built systems need specially designed slip connections.

Water Management

Water infiltration is the single most common performance failure in curtain wall systems. There are two main approaches to managing water:

Pressure-equalized rain screen: The most reliable approach. The exterior face of the curtain wall has openings that allow air pressure to equalize between the outside and the cavity behind the outer seal. With equal pressure on both sides, water has no driving force to push through. A secondary inner seal (the air barrier) provides the actual weather seal.

Face-sealed: All weather resistance depends on the outer sealant line. If that sealant fails anywhere, water gets in. Face-sealed systems are less reliable long-term and are generally not recommended for high-performance buildings.

Thermal Movement

Aluminum expands and contracts significantly with temperature changes. A 20-foot aluminum mullion can change length by nearly 1/4 inch over a 100°F temperature swing. Curtain wall designs must account for this movement with expansion joints, slip connections, and properly sized sealant joints.

Installation Best Practices

Whether you are installing a stick-built or unitized system, these practices apply:

Mock-up testing: On any significant curtain wall project, build and test a mock-up panel before production begins. The mock-up should be tested for air infiltration (ASTM E283), water penetration (ASTM E331 and AAMA 501.1), and structural performance (ASTM E330). Fix problems in the mock-up, not on the building.

Anchor placement accuracy: Curtain wall anchors must be placed within tight tolerances (typically +/- 1/4 inch). Survey and set anchors carefully. Correcting misaligned anchors after concrete is poured is expensive and time-consuming.

Sealant application: Follow manufacturer specifications exactly. This means proper joint dimensions, correct backer rod size and placement, surface preparation (cleaning and priming), and application within the specified temperature range. Bad sealant work is the number one source of water leaks.

Protection during construction: Protect installed curtain wall panels from damage by adjacent trades. Welding sparks, concrete splatter, and scaffold impacts all damage glass and finishes.

Quality control: Inspect every connection, seal, and panel as installation progresses. Catching a misaligned gasket or missed sealant bead during installation is simple. Finding it after the building is occupied is not.

Working With Glazing Subcontractors

If you are a general contractor managing a project with a significant curtain wall scope, here are some practical tips:

• Start coordination early. Curtain wall systems have long lead times (12 to 20 weeks for custom unitized systems) and require extensive shop drawing review.
• Make sure the structural engineer and the curtain wall engineer are communicating about anchor locations, loads, and deflection criteria.
• Schedule the mock-up test early enough that results can influence production.
• Coordinate with the waterproofing contractor on transitions between the curtain wall and adjacent wall assemblies. These interfaces are where most leaks happen.

Using project management tools that give you visibility into submittals, schedules, and field reports makes coordinating this kind of multi-trade work much more manageable. Projul’s scheduling and communication features are built for exactly this kind of coordination.

Energy Code Compliance

Current energy codes (ASHRAE 90.1-2019 and IECC 2021) set prescriptive requirements for glazing performance based on climate zone and window-to-wall ratio (WWR). As WWR increases, the required U-factor and SHGC values become more restrictive.

For most commercial projects, meeting code with a WWR above 40% requires high-performance glazing: low-E coatings, argon fill, thermally broken frames, and sometimes triple glazing. The cost of the glass goes up, but the energy savings and the ability to downsize HVAC equipment can offset the premium.

Many jurisdictions are adopting more aggressive energy codes. If you are bidding work, check the locally adopted code version. What was acceptable three years ago may not meet current requirements.

Cost Considerations

Curtain wall costs vary widely based on system type, glass specification, and project complexity:

• Basic stick-built storefront: $25 to $45 per square foot installed
• Standard stick-built curtain wall: $45 to $85 per square foot installed
• Unitized curtain wall: $70 to $150+ per square foot installed
• Point-supported glazing: $100 to $250+ per square foot installed

These numbers include materials, labor, and typical general conditions but vary by region and market conditions. High-performance glass with multiple low-E coatings, lamination, and triple glazing can add $10 to $30 per square foot over standard IGUs.

Key Takeaways

Window glazing and curtain wall systems are complex assemblies that affect a building’s appearance, energy performance, occupant comfort, and long-term durability. Getting them right requires:

1. Understanding the differences between system types and choosing the right one for your project
2. Specifying glass that meets energy code requirements while balancing light, heat, and cost
3. Insisting on thermal breaks in aluminum framing
4. Designing for water management with pressure-equalized systems
5. Testing with mock-ups before production
6. Coordinating early and often between structural, architectural, and glazing trades

If you are managing commercial projects that involve curtain wall or glazing systems, having a solid project management system makes the coordination much easier. Take a look at Projul’s features or request a demo to see how it works for commercial contractors. You can also compare pricing plans to find what fits your operation.