Low-Emissivity (Low-E) Glass
Low-Emissivity (Low-E) Glass is a technologically advanced glazing material featuring microscopically thin, transparent metal or metallic oxide layers that selectively filter solar radiation, allowing visible light to pass through while blocking infrared and ultraviolet radiation to improve energy efficiency and interior comfort.
Composition
Low-E glass consists of standard glass (typically float glass) with one or more microscopically thin coatings of metal or metallic oxide layers. These coatings, often silver, tin oxide, or other specialized materials, are measured in nanometers (billionths of a meter) and are virtually invisible to the human eye. The coatings are applied through various methods, with the two primary types being pyrolytic (hard coat) and sputter-coated (soft coat). Pyrolytic coatings are applied during the float glass manufacturing process while the glass is still hot, creating a durable layer that fuses with the glass surface. Sputter coatings are applied in a vacuum chamber after the glass is manufactured, requiring protection within an insulated glass unit. The specific composition and number of layers vary by manufacturer and performance requirements, with advanced low-e coatings often featuring multiple silver layers separated by dielectric materials to achieve specific performance characteristics for different climate zones and building orientations.
Properties
Emissivity
0.02 to 0.20 (varies by coating type)
Emissivity measures a material's ability to radiate absorbed energy, with lower values indicating better performance. Standard clear glass has an emissivity of approximately 0.84, while low-e coatings reduce this to between 0.02 and 0.20, significantly reducing radiant heat transfer through the glazing system.
Solar Heat Gain Coefficient (SHGC)
0.20 to 0.70 (varies by coating type)
SHGC measures the fraction of solar radiation admitted through glass that becomes heat. Low-e coatings can be engineered to provide high solar gain (passive low-e, SHGC ~0.60-0.70) for heating-dominated climates or low solar gain (solar control low-e, SHGC ~0.20-0.40) for cooling-dominated climates.
Visible Light Transmission (VLT)
30% to 80% (varies by coating type)
VLT measures the percentage of visible light that passes through the glass. Modern low-e coatings can maintain high visible light transmission while controlling infrared radiation, with typical values ranging from 30% for highly reflective coatings to 80% for high-transmission variants.
U-Value Improvement
30% to 50% reduction compared to uncoated glass
When incorporated into insulated glass units, low-e coatings typically reduce U-values (heat transfer rate) by 30-50% compared to uncoated insulated units, significantly improving thermal performance. The exact improvement depends on the specific coating, glass configuration, and gas fill.
UV Filtration
Blocks 75% to 99% of UV radiation
Low-e coatings typically block 75-99% of ultraviolet radiation, protecting interior furnishings, artwork, and finishes from UV damage and fading. This property is particularly valuable in museums, retail environments, and residential applications.
Durability
Varies by coating type (pyrolytic vs. sputter)
Pyrolytic (hard coat) low-e has excellent durability and can be used in single-glazed applications or exposed within an IGU. Sputter-coated (soft coat) low-e offers superior performance but must be protected within an IGU as it can be damaged by handling or environmental exposure.
Applications
Residential Windows and Doors
Low-e glass is now standard in most residential windows and doors, with specific coatings selected based on climate zone and orientation. In cold climates, passive low-e coatings with higher solar heat gain are often used to allow beneficial solar heating while reducing heat loss. In hot climates, solar control low-e coatings with lower solar heat gain are preferred to minimize cooling loads. Many high-performance homes use different low-e coatings on different elevations to optimize performance based on solar exposure. In insulated glass units, low-e coatings are typically applied to the #2 surface (inside surface of the outer pane) for solar control or the #3 surface (outside surface of the inner pane) for thermal insulation, with advanced systems sometimes using coatings on multiple surfaces.
Commercial Curtain Walls and Facades
Commercial buildings extensively use low-e glass in curtain walls, window walls, and storefronts to meet energy codes while maintaining desired aesthetics. These applications often use high-performance solar control low-e coatings to manage solar heat gain in large glass areas while maintaining appropriate visible light transmission for daylighting. The coatings can be specified with various appearance options, including neutral, blue, green, or reflective finishes to achieve architectural design goals. Advanced commercial projects may use dynamic glazing that combines low-e coatings with electrochromic or thermochromic technologies to adapt to changing environmental conditions. Low-e coatings are critical for meeting increasingly stringent commercial building energy codes and achieving green building certifications.
Retrofits and Historic Preservation
Low-e glass plays an important role in energy retrofits of existing buildings and historic preservation projects. For historic windows where replacement is not desirable, low-e storm windows or panels can significantly improve energy performance while preserving original elements. Specialized restoration glass with low-e coatings can replicate the appearance of historic glass while providing modern thermal performance. In commercial retrofits, replacing older glazing with low-e glass is often one of the most cost-effective energy improvements. Some low-e films can be applied to existing glass as a retrofit measure, though these typically offer less performance improvement than replacement with new low-e insulated units.
Skylights and Overhead Glazing
Skylights and overhead glazing benefit significantly from low-e coatings, as these applications receive direct solar exposure and can create substantial heat gain and glare issues. Solar control low-e coatings help manage heat gain while maintaining appropriate light levels. Many skylight applications combine low-e coatings with fritted or translucent glass to diffuse direct sunlight. In cold climates, low-e coatings help prevent heat loss through skylights, which can otherwise be significant thermal weak points. Advanced skylight systems may combine low-e coatings with dynamic tinting technologies to adapt to changing daylight conditions throughout the day and seasons.
Passive Solar Design
Low-e glass is a critical component in passive solar building design, where glazing is strategically used to capture or reject solar energy. South-facing windows (in the northern hemisphere) often use passive low-e coatings with higher solar heat gain to maximize winter solar heating, while east, west, and north-facing windows use solar control low-e to minimize heat loss and manage glare. The selective properties of low-e coatings allow designers to fine-tune the performance of different glazing areas based on orientation and desired performance. In well-designed passive solar buildings, properly specified low-e glazing works in concert with thermal mass, shading devices, and building orientation to significantly reduce or eliminate mechanical heating and cooling requirements.
Specialty Applications
Low-e coatings are used in numerous specialty applications beyond standard windows. In refrigeration display cases, low-e coatings improve energy efficiency while maintaining product visibility. In automotive and transportation applications, low-e windshields and windows reduce solar heat gain and improve climate control efficiency. Museums use specialized low-e glass with enhanced UV protection to safeguard sensitive artifacts while maintaining visibility. Cold climate applications may use low-e coatings on interior surfaces of triple-glazed units to maximize thermal performance. Some specialized applications use low-e coatings on interior glass partitions to improve the energy performance of interior zones with different temperature requirements.
Advantages
- Significantly improved thermal performance compared to uncoated glass
- Reduced heating and cooling costs (typically 15-35% energy savings)
- Enhanced occupant comfort by reducing cold/hot spots near windows
- Blocks 75-99% of UV radiation, protecting interiors from fading
- Available in various performance levels for different climate zones
- Maintains high visible light transmission while controlling infrared radiation
- Compatible with various window systems and insulated glass configurations
- Contributes to green building certification (LEED, BREEAM, etc.)
Limitations
- Higher cost than uncoated glass (typically 15-40% price premium)
- Sputter-coated (soft coat) low-e requires protection in an IGU
- Some coatings may impart a slight tint or reflectivity
- Can interfere with cellular and Wi-Fi signals in some configurations
- May create potential for thermal stress breakage if improperly specified
- Requires careful handling during processing and installation
- Performance degradation possible if seals fail in insulated units
- Some low-e coatings may not be suitable for certain climates or orientations
Sustainability Profile
Low-E glass offers significant sustainability benefits primarily through operational energy savings. By reducing heat transfer through windows, low-e coatings can decrease a building's heating and cooling energy consumption by 15-35%, directly reducing carbon emissions over the building's lifecycle. This operational energy saving typically far outweighs the slight increase in embodied carbon from the manufacturing process. The coatings also block 75-99% of ultraviolet radiation, protecting interior furnishings and potentially extending their useful life. From a life-cycle perspective, the environmental impact of low-e coatings is minimal compared to the glass substrate itself, as the coatings are microscopically thin and use very small amounts of material. The manufacturing process for sputter coatings does require vacuum chambers and specialized equipment, which has energy implications, but manufacturers have made significant improvements in process efficiency. When specifying low-e glass, it's important to select the appropriate coating type for the specific climate and building orientation to maximize energy benefits. Look for products with Environmental Product Declarations (EPDs) that document their full environmental impact. At end of life, low-e glass can be recycled, though the coatings may slightly complicate the recycling process compared to uncoated glass.