When architects and homeowners talk about low e glass types, they’re usually thinking about comfort and energy savings. But there’s another critical property that separates basic low-E glass from truly high-performance glazing: spectral selectivity.
Spectral selectivity is the ability of glass to let in plenty of visible light while blocking as much solar heat as possible. In other words, it answers the question: How much useful daylight do I get per unit of heat that comes with it?
This article explains what spectral selectivity is, why it matters, and which low e glass types provide the strongest spectral selectivity for different climates and building types.
What Is Spectral Selectivity in Low E Glass?
Sunlight is not just “light”—it is a mix of different wavelengths:
- Ultraviolet (UV) – causes fading of fabrics and finishes
- Visible light – the part our eyes can see
- Infrared (IR) – largely experienced as heat
Traditional clear glass lets a high percentage of all these wavelengths pass through. That means good daylight, but also high solar heat gain, glare, and UV damage.
Spectrally selective low e glass types are coated so that:
- They transmit a high proportion of visible light, and
- They block or reflect a large share of infrared and UV energy.
This performance is often summarized by the Light-to-Solar Gain (LSG) ratio:
LSG = Visible Light Transmission (VLT) ÷ Solar Heat Gain Coefficient (SHGC)
- High VLT + low SHGC = high LSG → strong spectral selectivity
- The U.S. Department of Energy typically considers LSG ≥ 1.25 as “spectrally selective glazing.”
Why Spectral Selectivity Matters for Modern Buildings
Spectrally selective low e glass types directly support:
- Energy efficiency – less cooling energy in hot or mixed climates
- Comfort – reduced hot spots near windows and fewer glare issues
- Healthy daylight – keep spaces bright without turning them into greenhouses
- Interior protection – lower UV transmission means less fading of furniture, flooring, and artwork
In highly glazed buildings, these benefits are multiplied across the entire façade. That’s why choosing the right low-E glass is now a central part of sustainable design.
For a deeper dive into how coating structure, gas fills, and frame design all contribute to performance, check out:
👉 What Help Low E Glass Types Achieve Higher Efficiency?
Core Performance Metrics That Define Spectrally Selective Low E Glass Types
When you compare data sheets or simulation results, focus on these key values:
1. Visible Light Transmission (VLT)
- Expressed as a percentage
- High VLT (e.g., 60–70%) gives bright interiors and reduces the need for artificial lighting
- Spectrally selective low-E aims for high VLT with strong solar control, not just dark tinting
2. Solar Heat Gain Coefficient (SHGC)
- Ranges from 0 to 1
- Measures how much solar energy (directly and indirectly) enters through the glass
- For strong spectral selectivity, look for low SHGC with high VLT
3. Light-to-Solar Gain (LSG) Ratio
- LSG = VLT / SHGC
- The higher the LSG, the more “light per unit of heat” you get
- High-performance low e glass types used in warm climates often aim for LSG ≥ 1.3, sometimes much higher
4. U-Value
- Measures general heat transfer due to temperature difference (not just sun)
- Lower U-value = better insulation
- Important for overall energy balance, especially in mixed or cooler climates
Low E Glass Types by Coating Technology
Hard-Coat (Pyrolytic) Low E Glass
How it’s made:
The coating is applied at high temperature during float glass production. It fuses to the glass surface, creating a tough, durable layer.
Pros:
- Very robust and resistant to handling damage
- Can often be used as a monolithic glass or exposed surface
- Good for applications where durability is more important than extreme performance
Cons (for spectral selectivity):
- Typically offers moderate solar control
- Less flexible design; harder to fine-tune color, VLT, and SHGC
- Spectral selectivity tends to be lower than advanced soft-coat options
Result: Hard-coat low-E provides useful energy savings, but it’s rarely the first choice where maximum spectral selectivity is required.
Soft-Coat (Sputter-Coated) Low E Glass
How it’s made:
The coating is applied in a magnetron sputter vacuum chamber, layer by layer, onto already formed float glass.
Pros:
- Multilayer stacks with metals (especially silver) and oxides
- Highly engineered optical performance – excellent control over VLT, SHGC, and color
- Capable of very high LSG ratios and strong spectral selectivity
- Lower interior and exterior reflectivity options for more neutral aesthetics
Cons:
- Requires careful handling and must be used in an insulating glass unit (IGU)
- Slightly more sensitive during processing and edge-fabrication
Result: Most of the top-tier, spectrally selective low e glass types on the market are soft-coat products with one, two, or even three silver layers.
If you’re curious about how those multiple layers, including silver, work together to maximize performance, this broader overview can help:
👉 Which Factors Make Low E Glass Types Perform Better?
Multi-Silver Low E Glass Types: The Spectral Selectivity Champions
Within the soft-coat family, multi-silver low-E coatings are typically the strongest performers for spectral selectivity:
- Single-silver coatings – good insulation, moderate solar control
- Double-silver coatings – significantly improved SHGC with good VLT; strong spectral selectivity
- Triple-silver coatings – premium performance, combining:
- Very low SHGC
- High LSG
- Neutral color and minimized reflectivity
By carefully stacking silver layers and dielectric layers, manufacturers can design low e glass types that:
- Pass a large fraction of visible light (even 60–70% or more)
- Cut down solar heat gain to very low SHGC values
- Achieve LSG ratios that far exceed basic low-E or tinted glass
These are the go-to choice for:
- High-rise towers in hot or mixed climates
- Large commercial façades with high window-to-wall ratios
- Projects targeting certifications such as LEED, BREEAM, or WELL where energy metrics and daylighting are critical
The Role of Coating Position: Surface #2 vs Surface #3
In a double-glazed unit, glass surfaces are numbered from outside to inside:
- Surface 1 – exterior face of outer pane
- Surface 2 – interior face of outer pane (facing cavity)
- Surface 3 – interior face of inner pane (facing cavity)
- Surface 4 – interior face of inner pane, exposed to the room
For solar control low e glass types:
- Placing the coating on surface #2 allows the coating to reflect a significant portion of solar energy before it penetrates deeply into the IGU.
- This improves spectral selectivity because less energy is absorbed and re-radiated inward.
For insulation-focused low-E in cold climates, a coating on surface #3 can be better to reflect indoor heat back inside.
When your goal is strong spectral selectivity in warm or mixed climates, verify that the product you choose is designed for surface #2 placement in the IGU.
Matching Spectrally Selective Low E Glass Types to High Solar Gain Homes
Residential projects with large windows or sliding doors exposed to strong sunlight need glass that balances:
- Daylight comfort
- View quality
- Solar heat control
- Budget
For high solar gain homes, the best options are usually:
- Double- or triple-silver low-E soft-coat glass
- Moderate to high VLT (to avoid dark, cave-like interiors)
- Low SHGC to keep indoor temperatures under control
In some climates, it may be desirable to maintain a slightly higher SHGC on certain façades to capture winter sun while still enjoying spectral selectivity.
If you’re evaluating glass for a specific home design, this detailed guide is helpful:
👉 Which Low E Glass Types Work Best for High Solar Gain Homes?
Beyond Glass: How Urban Design and Landscaping Support Spectral Selectivity
Even the best low e glass types operate within a broader environmental context. Smart building and urban design strategies can enhance their benefits:
- External shading (overhangs, fins, louvers) reduces direct solar exposure, allowing you to use glass with slightly higher VLT while maintaining comfort.
- Green spaces and reflective surfaces around the building influence how much radiant energy hits the façade.
- Smart city planning can position buildings, streets, and trees to minimize heat islands and reduce cooling loads overall.
In modern smart cities, high-performance glazing and thoughtful landscape design go hand in hand, shaping comfortable, low-energy microclimates. To understand how landscape strategies integrate with building technologies like spectrally selective glazing, see:
👉 Define Landscape Solutions in Smart Cities
How to Choose Spectrally Selective Low E Glass Types for Your Project
When you’re ready to specify or purchase, use this simple checklist:
- Check the LSG ratio.
- Aim for LSG ≥ 1.25 as a baseline, higher if possible for hot climates.
- Look for multi-silver soft-coat options.
- Double- or triple-silver low-E generally deliver the strongest spectral selectivity.
- Balance VLT and SHGC for your façade.
- High VLT (60–70%) + low SHGC = bright yet cool interiors.
- Adjust slightly by orientation (more control on east/west).
- Verify coating position in the IGU.
- For solar control, confirm the product is designed for surface #2.
- Confirm U-value and frame quality.
- Low-E glass can’t compensate for poor frames or leaky installation.
- Use argon fill, warm-edge spacers, and thermally broken frames where possible.
- Consider aesthetics and reflection.
- Review samples in real daylight to judge color, tint, and exterior reflectivity.
- Modern spectrally selective products can look very neutral while performing extremely well.
- Ensure proper installation and handling.
- Soft-coat glass requires experienced fabricators and installers to protect edges and coatings.
Final Thoughts
Spectral selectivity is what turns low-E glass from a basic energy upgrade into a powerful design tool. By choosing the right low e glass types—especially advanced multi-silver soft-coat products placed correctly in an IGU—you can:
- Keep interiors bright and visually open
- Dramatically cut unwanted solar heat gain
- Reduce cooling loads and operating costs
- Protect interior finishes from fading
- Support overall sustainability goals for the building and the city around it
As glazing technology continues to evolve, understanding spectral selectivity helps you move beyond simple “low-E or not?” decisions and instead select the precise low e glass types that best match your climate, façade orientation, and architectural vision.