Insulated Glass Units (IGUs)
Insulated Glass Units (IGUs) are engineered glass assemblies consisting of two or more glass panes separated by a sealed space filled with air or inert gas, designed to significantly improve thermal performance, reduce energy consumption, and enhance occupant comfort in buildings.
Composition
An Insulated Glass Unit (IGU) consists of two or more glass panes separated by a spacer and hermetically sealed around the perimeter. The space between the glass layers (typically 6mm to 20mm) is filled with dry air or inert gases such as argon or krypton to improve thermal performance. The spacer, traditionally aluminum but increasingly warm-edge materials like stainless steel, structural foam, or composite materials, contains a desiccant to absorb any moisture that might penetrate the seal. The perimeter is sealed with primary and secondary sealants (typically polyisobutylene and silicone or polysulfide) to maintain the hermetic barrier. The glass panes can be various types including annealed, tempered, laminated, or coated glass (such as low-emissivity or solar control coatings) depending on performance requirements. Advanced IGUs may incorporate additional features such as integrated blinds, photovoltaic cells, or switchable privacy glass.
Properties
Thermal Performance (U-Value)
0.2 to 0.5 W/m²K (varies by configuration)
U-value measures heat transfer rate through the glass assembly, with lower values indicating better insulation. Double-glazed IGUs typically achieve 0.24-0.36 W/m²K with argon fill and low-e coating, while triple-glazed units can reach 0.15-0.22 W/m²K. This represents a significant improvement over single glazing (U-value ~5.8 W/m²K).
Solar Heat Gain Coefficient (SHGC)
0.2 to 0.7 (varies by glass type and coatings)
SHGC measures the fraction of solar radiation admitted through the glass that becomes heat. Lower values indicate less solar heat transmission. IGUs can be configured with various coatings to achieve desired SHGC values based on climate and building orientation.
Visible Light Transmission (VLT)
30% to 80% (varies by glass type and coatings)
VLT measures the percentage of visible light that passes through the glass. Higher values indicate more natural light transmission. IGUs can be designed to balance light transmission with thermal performance through selective coatings.
Sound Attenuation
STC 28-38 (varies by configuration)
Sound Transmission Class (STC) measures the IGU's ability to block sound. Higher values indicate better acoustic performance. Factors affecting sound attenuation include glass thickness, asymmetrical glass panes, gas fill, and laminated glass layers.
Condensation Resistance
CR 50-80 (varies by configuration)
Condensation Resistance (CR) measures the ability to resist condensation formation on the interior surface. Higher values indicate better performance. IGUs significantly reduce condensation compared to single glazing by maintaining higher interior surface temperatures.
Service Life
15-30 years (depends on quality and conditions)
The effective service life of an IGU depends on the quality of materials, manufacturing process, installation, and environmental conditions. The primary failure mode is seal failure, which allows moisture ingress and results in fogging between panes.
Applications
Residential Windows and Doors
IGUs are the standard for energy-efficient residential windows and doors in most climates. In homes, they significantly reduce heating and cooling costs while improving comfort by minimizing cold spots near windows. Double-glazed units are most common in moderate climates, while triple-glazed units are preferred in severe cold climates. Various configurations are available to optimize performance based on orientation and climate, including different low-e coatings for north, south, east, and west-facing windows. IGUs can be incorporated into various window styles including casement, double-hung, sliding, fixed, and specialty shapes, as well as patio doors and entry door lites.
Commercial Curtain Walls and Storefronts
In commercial buildings, IGUs are essential components of curtain wall systems, storefronts, and window walls. These applications often require large glass areas while maintaining energy efficiency. Commercial IGUs typically incorporate high-performance coatings to manage solar heat gain while maximizing natural light. Structural silicone glazing systems use IGUs with specialized edge seals designed to withstand the stresses of structural applications. Point-supported systems use IGUs with special fittings and heat-strengthened or fully tempered glass to accommodate the concentrated loads at connection points. Many commercial applications require custom IGU specifications to meet specific energy codes, daylighting goals, and aesthetic requirements.
Skylights and Overhead Glazing
IGUs for skylights and overhead glazing applications must meet specific safety and performance requirements. These typically incorporate laminated glass on the interior pane to prevent fall-through hazards if breakage occurs. Solar control coatings are often used to manage heat gain, as overhead glazing receives direct solar exposure. Special consideration is given to condensation management and drainage details. Some skylight IGUs incorporate fritted or translucent glass to diffuse direct sunlight and reduce glare. Advanced skylight systems may use photochromic or electrochromic IGUs that change tint based on light conditions or user control.
Specialty Glazing Applications
IGUs are used in numerous specialty applications with specific performance requirements. In museums and retail displays, IGUs with high UV filtration protect sensitive artifacts and merchandise. In sound studios and performance spaces, specialized acoustic IGUs with asymmetrical glass thicknesses and laminated components provide enhanced sound isolation. For security applications, IGUs can incorporate laminated security glass or ballistic-resistant components while maintaining thermal performance. In healthcare facilities, IGUs may include switchable privacy glass or antimicrobial coatings. Marine applications use specialized IGUs designed to withstand the harsh saltwater environment and meet maritime regulations.
Cold Storage and Refrigeration
IGUs are essential for cold storage applications, refrigerated display cases, and freezer doors where extreme temperature differentials exist. These specialized units often use triple or quadruple glazing with multiple low-e coatings and krypton gas fill for maximum thermal resistance. Anti-fog coatings or heated glass may be incorporated to prevent condensation on viewing surfaces. The spacer systems and sealants are specially designed to withstand the extreme temperature differentials without failure. These high-performance IGUs allow visibility into cold storage areas while minimizing energy loss and preventing frost formation.
Passive House and Net-Zero Buildings
In ultra-high-performance buildings such as Passive House or net-zero energy buildings, specialized IGUs are critical components of the building envelope. These often feature triple or even quadruple glazing with multiple low-e coatings, krypton or xenon gas fills, and insulated spacers to achieve U-values as low as 0.10 W/m²K. The glass specifications are carefully balanced to optimize solar heat gain based on orientation while maintaining high visible light transmission. These high-performance IGUs are typically paired with thermally broken frames and careful installation details to minimize thermal bridging. The exceptional performance allows for larger glass areas while maintaining the stringent energy requirements of these building standards.
Advantages
- Significantly improved thermal insulation compared to single glazing (up to 70% reduction in heat loss)
- Reduced energy consumption for heating and cooling (typically 15-30% energy savings)
- Enhanced occupant comfort by minimizing cold spots and drafts near windows
- Reduced condensation on interior glass surfaces
- Improved sound attenuation compared to single glazing
- Can incorporate additional performance features (low-e coatings, solar control, etc.)
- Compatible with various window and curtain wall systems
- Contributes to green building certification (LEED, BREEAM, etc.)
Limitations
- Higher initial cost compared to single glazing (typically 15-40% more expensive)
- Heavier than single glazing, requiring stronger framing and hardware
- Seal failure can lead to condensation between panes (fogging)
- Limited lifespan of seals (typically 15-30 years)
- Not easily repaired if seal fails; entire unit must be replaced
- Requires special handling during transportation and installation
- Thermal stress can cause breakage if not properly specified
- Challenging to recycle due to composite construction
Sustainability Profile
Insulated Glass Units have a mixed sustainability profile. On the positive side, they significantly reduce building energy consumption by minimizing heat loss in winter and heat gain in summer, potentially reducing carbon emissions over the building's life cycle. High-performance IGUs can reduce a building's heating and cooling energy by 15-30% compared to single glazing. However, IGUs require more materials and energy to manufacture than single glazing, and their composite construction makes recycling challenging. The hermetic seals have a limited lifespan (15-30 years), after which the entire unit typically needs replacement. Some manufacturers have developed take-back programs and improved recycling processes to address end-of-life issues. When specifying IGUs, look for manufacturers with environmental product declarations (EPDs) and those who have implemented sustainable manufacturing practices. Consider the embodied carbon alongside operational energy savings when evaluating overall environmental impact. In most climates and applications, the energy savings over the IGU's lifespan outweigh the additional embodied carbon from manufacturing.