When designers talk about low e glass types, they usually focus on numbers like U-value, SHGC, and visible light transmission. But there’s a quieter detail that can dramatically change how the glass behaves in real buildings: which surface of the insulating glass unit the coating is on.
Move the same low-E coating from one surface to another and you’ll see different results for solar control, insulation, comfort, and even appearance. In this article, we’ll unpack why low e glass types change performance by surface location, and how to choose the right configuration for your climate and façade.
How Glass Surfaces Are Numbered in Low E Glass Types
Before we talk about performance, we need the numbering system used for insulated glass units (IGUs).
For a double-glazed unit, surfaces are counted from the outside in:
- Surface #1 – Exterior face of the outer pane (exposed to weather)
- Surface #2 – Interior face of the outer pane (facing the air or gas cavity)
- Surface #3 – Exterior face of the inner pane (facing the cavity)
- Surface #4 – Interior face of the inner pane (facing the room)
In triple glazing, there are six surfaces, but the logic is the same: start counting from the outside and move inward.
Most modern low e glass types place coatings on one or more of these interior surfaces. Where you put the coating changes how the glass interacts with:
- Solar radiation (shortwave energy)
- Longwave heat radiation from inside or outside
- Conductive and convective heat inside the cavity
Why Surface Location Matters: Shortwave vs Longwave Energy
To understand performance differences, separate two kinds of energy that pass through glass:
- Shortwave solar radiation
- Includes ultraviolet, visible, and near-infrared light
- Comes mainly from the sun
- Drives solar heat gain and glare issues
- Longwave infrared radiation (heat)
- Emitted by warm surfaces inside and outside the building
- Governs how well windows hold heat in or keep heat out when there is a temperature difference
Different low e glass types are tuned to interact with these wavelengths in specific ways:
- Solar control low-E: optimized to block or reflect incoming shortwave radiation while still allowing daylight in.
- Insulating low-E: optimized to reflect longwave heat back toward the side it came from, improving U-value.
Where the coating sits in the IGU decides which side of the system sees that effect most strongly.
Surface #2: Best for Solar Control Low E Glass Types
For buildings in warm or mixed climates where cooling loads are high, solar control is a top priority. Placing the low-E coating on Surface #2 offers clear advantages.
How Surface #2 Placement Works
When the sun hits the glass:
- Solar energy first encounters the coated Surface #2.
- A significant portion of shortwave energy is reflected back outside before it can heat the cavity or interior pane.
- Some energy is absorbed by the outer pane and re-radiated, but a smaller fraction makes it indoors.
This configuration tends to:
- Lower the Solar Heat Gain Coefficient (SHGC) more effectively
- Reduce interior glass temperatures, improving comfort near windows
- Still allow optimized visible light transmission for daylight
That’s why many spectrally selective low e glass types designed for hot regions are explicitly engineered for surface #2 installation.
To see how coating design, silver layers, gas fill and frames combine to boost efficiency, this article is a useful companion read:
👉 What Help Low E Glass Types Achieve Higher Efficiency?
Surface #3: Best for Insulating Low E Glass Types in Cold Climates
In colder climates, the main issue isn’t solar heat streaming in; it’s valuable indoor heat escaping out. That’s where a thermal insulation low-E coating on Surface #3 shines.
How Surface #3 Placement Works
- Indoor surfaces (walls, floors, people, heating equipment) emit longwave infrared radiation.
- When the coating is on Surface #3, it faces this indoor heat across the cavity.
- The low-E coating reflects a large portion of that longwave energy back into the room, reducing heat loss.
Results:
- Lower U-value and improved insulation
- Warmer interior glass surfaces, reducing condensation risk and drafts
- Better comfort for occupants sitting close to windows, even on very cold days
In such cases, SHGC might be kept moderate or even relatively high to allow beneficial solar gains in winter. That’s a completely different design goal from a hot climate façade, and the surface location of the coating reflects that.
Comparing Surface #2 vs Surface #3 for Low E Glass Types
You can think of the choice like this:
- Surface #2 solar control low-E
- Priority: minimize solar heat gain
- Ideal for: hot or sunny climates, highly glazed façades, cooling-dominated buildings
- Main effect: lower SHGC, lower interior glass temperatures
- Surface #3 insulating low-E
- Priority: improve insulation & retain indoor heat
- Ideal for: cold climates, heating-dominated buildings
- Main effect: lower U-value, warmer inner glass surface
Some advanced IGUs combine these strategies in triple glazing: for example, a solar control low-E on surface #2 and a high-insulation low-E on surface #5. That’s one way high-performance low e glass types hit very low U-values while still finely tuning solar gain.
For a deeper overview of the many other variables (silver layers, tint, gas type) that influence overall performance, see:
👉 Which Factors Make Low E Glass Types Perform Better?
What About Surfaces #1 and #4?
You’ll rarely see low-E coatings deliberately applied on Surface #1 or #4 in standard architectural systems.
Surface #1 (Exterior Exposed)
- Directly exposed to weather, dirt, cleaning tools, and mechanical wear
- Soft-coat low-E layers are too delicate to live here long-term
- Pyrolytic “hard-coat” low-E can sometimes be used on surface #1, but this is more niche and usually offers less refined spectral control
Surface #4 (Interior Exposed)
- Exposed to occupants, cleaning, and interior humidity
- Could create glare or aesthetic issues if highly reflective
- Most manufacturers want the low-E to sit behind at least one pane for durability and stable performance
So while it’s technically possible, standard low e glass types for façades and windows are almost always designed for surfaces #2 and/or #3.
How Surface Location Changes Spectral Selectivity
Spectral selectivity is all about how much visible light you get for how much solar heat. Moving the coating changes those numbers:
- A solar control coating on Surface #2 typically delivers stronger SHGC reduction for the same visible light transmission, improving the Light-to-Solar Gain (LSG) ratio.
- A purely insulating coating on Surface #3 might deliver excellent U-value but less aggressive solar control, so LSG is different even if the coated glass is otherwise similar.
Manufacturers design specific low e glass types with different surface placements to target high LSG, neutral color, or specific glare requirements. When you want to understand which products truly offer strong spectral selectivity, it’s helpful to look at both the coating design and where that coating is meant to sit in the IGU.
For more detail on this aspect, you can explore:
👉 Which Low E Glass Types Provide Strong Spectral Selectivity?
Interaction With Gas Fill, Spacer, and Frame Design
Coating location is crucial, but it’s only one piece of the puzzle. The whole window system—not just the glass—decides final performance:
- Gas fill (argon, krypton) changes conductive and convective heat transfer in the cavity.
- Warm-edge spacers help reduce edge-of-glass heat loss and improve comfort.
- Thermally broken frames prevent heat from bypassing the glass completely.
The best low e glass types are almost always paired with:
- Argon-filled cavities
- High-quality warm-edge spacers
- Frames designed for the same performance class as the glazing
If any one of these elements is weak, the benefit of carefully placing the coating on surface #2 or #3 gets diluted.
Surface Location, Façade Orientation, and Urban Context
The way a building interacts with its surroundings also affects which surface configuration makes sense:
- East and west façades in sunny climates take intense low-angle sun; coatings on surface #2 with strong solar control are often favored here.
- North façades in some climates may prioritize insulation over solar control, making a surface #3 insulating low-E attractive.
- Dense urban environments with reflective neighboring buildings may need carefully selected products to manage both glare and privacy.
In modern planning, high-performance low e glass types work together with shading elements, street orientation, and urban greenery to create comfortable microclimates. If you’re working on larger developments or smart city concepts, glazing strategy needs to connect with public space shading, vegetation, and energy planning overall. A helpful cross-discipline read on this topic is:
👉 Define Landscape Solutions in Smart Cities
Practical Tips for Specifying Low E Glass Types by Surface Location
When you’re choosing products or writing specifications, use this checklist:
- Clarify climate and energy priorities
- Cooling-dominated? Start by asking for solar control low-E on surface #2.
- Heating-dominated? Look for insulating low-E on surface #3 with a suitable SHGC.
- Check manufacturer documentation
- Confirm which surface location the product is designed for.
- Many product names include hints like “2nd surface coating” or specify placement in the technical sheet.
- Review performance metrics in context
- Compare U-value, SHGC, VLT, and LSG together, not in isolation.
- Simulate performance in your energy model with the correct surface locations.
- Consider orientation-specific glazing
- You don’t need to use the same low-E configuration on every façade.
- South, east, west, and north façades may each benefit from different low e glass types and surface placements.
- Coordinate with fabricators and installers
- Make sure everyone understands which side of the glass faces outside.
- Mis-orienting the coated pane can seriously degrade performance.
Conclusion: Small Change in Surface, Big Change in Performance
The same coating, used on a different surface, is effectively a different product. That’s why low e glass types change performance by surface location:
- Surface #2 emphasizes solar control and lower SHGC, ideal for cooling-dominated designs.
- Surface #3 emphasizes retaining indoor heat and lowering U-value, ideal for heating-dominated climates.
- Combined strategies in triple glazing can fine-tune both goals at once.
By understanding how surface numbering works and how each location interacts with shortwave and longwave energy, architects, engineers, and homeowners can select low e glass types that truly match their climate, façade orientation, and energy goals—rather than relying on generic “low-E” labels.
When you pair the right coating technology with the right surface location, plus quality frames and thoughtful urban design, low-E glazing becomes one of the most powerful tools for creating comfortable, low-energy buildings.