Understanding the numbers that determine thermal performance, solar heat gain and daylight, and why there is no single “best” glass

Choosing glazing for a building is often reduced to a few familiar terms: U-value, SHGC, VLT, Low-E and solar-control glass.

But these terms describe different aspects of window performance. A glass that performs well for one objective may not be the optimum choice for another. Reducing solar heat gain, for example, can also reduce visible light transmission; improving thermal insulation does not automatically determine the appropriate solar-control strategy.

For architects, façade consultants, builders, developers and window manufacturers, the real question is therefore not:

“Which glass has the lowest U-value?”

It is:

“Which combination of thermal, solar and optical properties is appropriate for this façade, this climate and this building?”

Research on glazing performance shows that window energy performance varies with climate, orientation, window type, building characteristics and façade conditions (Singh and Garg, 2009). More recent reviews similarly emphasize that U-value, solar heat gain and visible transmittance are interconnected performance parameters rather than independent measures of whether a glazing system is “good” or “bad” (Cuce and Riffat, 2015; Li and Wu, 2025).

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1. The Short Version

The three numbers to understand first are:

• U-value: how readily heat is transferred through the glazing or fenestration assembly.
• SHGC: how much incident solar energy ultimately enters the building through the fenestration.
• VLT: how much visible light passes through the glazing.

In simplified terms:

Lower U-value → greater resistance to heat transfer

Lower SHGC → less solar heat admitted

Higher VLT → more visible daylight transmitted

But these directions should not be interpreted as a universal ranking.

A cooling-dominated building with large sun-exposed façades may need strong solar control. A heating-dominated building may benefit from allowing useful solar gains during appropriate periods. A highly glazed office may need to balance daylight with glare and cooling loads. A shaded façade can behave differently from an unshaded façade using the same glass.

This is why glazing selection must be considered at the fenestration and façade level, rather than by looking at one glass number in isolation.

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2. Why Choosing Glass Is Not Just About “Low-E”

Low-emissivity, or Low-E, glazing has become a major part of modern high-performance fenestration because reducing long-wave thermal radiation can substantially alter the thermal behaviour of glazing.

Low-E coatings work by providing surfaces with low emissivity, reducing radiative heat transfer across glazing cavities. Depending on the coating and glazing configuration, they can also influence the amount and spectral distribution of solar radiation transmitted through the window (Jelle, Kalnæs and Gao, 2015).

However, Low-E is a technology category, not a complete performance specification.

Two Low-E products can have different:

• U-values
• SHGC values
• VLT values
• solar reflectance
• visible transmittance
• spectral characteristics
• coating positions
• glazing configurations
• overall fenestration performance

The same principle applies to tinted, reflective and solar-control glass.

A glass should therefore be specified by its measured or calculated performance parameters, not simply by a product label such as “Low-E,” “solar control,” “high-performance” or “energy efficient.”

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3. U-Value: How Easily Heat Moves Through the Window

The U-value describes thermal transmittance.

It represents the rate at which heat is transferred through a construction for a given temperature difference. It is normally expressed in:

W/m²·K

A lower U-value indicates lower thermal transmittance.

For windows, this distinction is important because the thermal performance of the complete fenestration product is not necessarily the same as the centre-of-glass performance.

A window can contain:

• glass
• coatings
• gas-filled cavities
• spacers
• frame members
• edge components

All of these influence the final thermal performance.

Centre-of-glass U-value vs window U-value

A specification may provide a U-value for the glass alone, commonly called centre-of-glass U-value, while a building-performance calculation may require the U-value of the complete fenestration assembly.

The latter can include:

glass + frame + spacer + edge effects + other assembly components

This distinction matters particularly in buildings with large areas of glazing, where the frame can represent a significant portion of the total fenestration area.

Modern calculation and rating frameworks therefore distinguish between material/glazing properties and whole-window performance.

The 2024 Indian Energy Conservation and Sustainable Building Code, for example, specifies fenestration U-factor requirements at the assembly level, including the sash and frame, for relevant compliance pathways.

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4. What Actually Controls U-Value?

U-value is influenced by the construction of the glazing system rather than simply by whether the glass is single, double or triple glazed.

Important factors include:

• number of panes
• cavity width
• gas filling
• coating emissivity
• coating position
• glass thickness
• spacer characteristics
• frame construction
• thermal bridging
• overall window geometry

The development of low-emissivity coatings has been particularly important because the radiative component of heat transfer can be reduced without making the glazing opaque (Jelle, Kalnæs and Gao, 2015).

Gas-filled insulating glass units can further alter heat-transfer characteristics.

However, a low U-value does not tell you how much solar radiation enters the building.

That is where SHGC becomes important.

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5. SHGC: The Number That Controls Solar Heat Gain

The Solar Heat Gain Coefficient (SHGC) represents the fraction of incident solar radiation that ultimately enters the building through the fenestration.

It includes two principal contributions:

1. solar radiation transmitted directly through the glazing; and
2. solar energy absorbed by the glazing and subsequently transferred inward.

In simplified form:

SHGC = directly transmitted solar energy + inward-flowing fraction of absorbed solar energy

SHGC is dimensionless and normally expressed between 0 and 1.

A lower SHGC means that a smaller fraction of incident solar energy is admitted indoors.

This makes SHGC especially important for façades exposed to substantial solar radiation.

Li and Wu (2025) emphasize that SHGC must be interpreted together with the other glazing-performance parameters. A lower SHGC can reduce unwanted cooling loads, while a higher SHGC can potentially be beneficial where useful solar gains are desirable.

It is also worth noting that SHGC is not the only name this property goes by. North American practice generally uses the term SHGC, while European and other international glazing practice commonly refers to the same underlying property as the g-value, or total solar energy transmittance. Abolghasemi Moghaddam, Simões and Gameiro da Silva (2023) explicitly describe the property as “SHGC, or g-value” in their review of experimental determination methods, and note that its value depends on the calculation or testing methodology and boundary conditions used, rather than being a single context-free number. For an audience spanning Indian, European and North American practitioners and manufacturers, it is worth checking which term and which measurement basis a given specification is using before comparing products.

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6. Why SHGC Is Not Simply “Lower Is Better”

Consider two buildings.

Building A

A highly glazed commercial building in a cooling-dominated environment has:

• large areas of exposed glass
• strong solar exposure
• substantial air-conditioning demand
• limited external shading

In such a case, solar heat entering through the façade can contribute significantly to cooling loads.

A glazing system with appropriate solar control may therefore be advantageous.

Building B

A building in a heating-dominated environment has substantial heating demand and receives useful winter solar radiation.

Aggressively reducing solar admission may reduce useful passive solar gains.

The appropriate SHGC can therefore differ between these two buildings.

This is why the correct question is not:

“What is the lowest SHGC available?”

It is:

“What SHGC produces the desired solar balance for this façade and building?”

The relationship is demonstrated particularly clearly by Singh and Garg (2009), whose analysis of ten window types across five Indian climatic zones found that window energy performance varied with window type, orientation, climate and building characteristics.

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7. VLT: Daylight Without Confusing It With Solar Heat

Visible Light Transmittance (VLT) describes the proportion of visible light transmitted through glazing.

It is generally expressed as a percentage or decimal fraction.

For example:

• VLT 0.70 ≈ 70% visible transmittance
• VLT 0.40 ≈ 40%
• VLT 0.20 ≈ 20%

Higher VLT generally means more visible light can pass through the glazing.

But VLT is not the same thing as SHGC.

Solar radiation spans a much wider spectral range than the visible portion of the spectrum. A glazing system can therefore be designed to transmit a significant amount of visible light while limiting a portion of solar energy.

This is one of the fundamental objectives of modern solar-control glazing.

Kiran Kumar et al. (2018), for example, experimentally investigated the spectral behaviour of different tinted and reflective window glasses across the approximately 300–2500 nm range and calculated properties including solar transmittance, reflectance, absorptance and SHGC.

This illustrates why evaluating glass only by how dark it looks can be misleading.

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8. VLT Is Also a Daylight and Glare Parameter

VLT has consequences beyond the amount of light entering a room.

VLT influences daylight availability and can therefore affect electric-lighting demand, visual comfort, glare, interior appearance and views. VLT itself does not directly set electric-lighting demand; it is the daylight availability it produces, together with how lighting is controlled and operated, that determines that relationship.

Higher VLT can provide more daylight, but more daylight does not automatically mean better visual comfort.

For example, a highly exposed façade with very high visible transmittance can produce excessive luminance contrast or glare if solar control and shading are poorly coordinated.

Conversely, an excessively low-VLT glazing specification may reduce useful daylight and increase dependence on electric lighting.

The correct VLT is therefore related to the building’s daylight strategy rather than being an isolated “higher is better” metric.

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9. Solar-Control Glass: What Is Actually Being Controlled?

The term solar-control glass covers glazing designed to modify the transmission, reflection and absorption of solar radiation.

This can be achieved through different technologies, including:

• body-tinted glass
• reflective coatings
• spectrally selective coatings
• low-emissivity coatings with solar-control characteristics
• multilayer coated glass
• combinations of glazing and coatings

These technologies can produce different relationships between:

SHGC ↔ VLT ↔ reflectance ↔ thermal performance

That relationship is important.

A heavily reflective glass may have relatively low visible transmittance and a different aesthetic appearance from a spectrally selective coating designed to maintain comparatively high visible transmission while reducing solar heat gain.

Kiran Kumar et al. (2018) demonstrated experimentally that different tinted and reflective glasses have substantially different spectral properties and resulting SHGC values. Their study also showed that the energy consequences varied according to climate and orientation.

The result from one glass type in one simulation should therefore not be interpreted as a universal ranking of all solar-control glasses.

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10. The Critical Trade-Off: U-Value vs SHGC vs VLT

The three parameters answer different questions.

Parameter	Main question	Typical interpretation
U-value	How easily does heat move through the fenestration?	Lower = less thermal transmittance
SHGC	How much solar heat enters?	Lower = less solar heat admitted
VLT	How much visible light enters?	Higher = more visible light transmitted

The mistake is to treat these three values as though they form a simple scorecard.

They do not.

A glazing specification may have:

• low U-value
• low SHGC
• moderate VLT

Another may have:

• low U-value
• moderate SHGC
• high VLT

Another may have:

• moderate U-value
• very low SHGC
• lower VLT

Which one is appropriate depends on the building.

This is the central principle of modern glazing selection.

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11. Why Climate Changes the Answer

Climate determines whether the dominant problem is primarily:

• unwanted heat gain
• heat loss
• both at different times of year
• daylight and glare
• or a combination of these factors

Cuce and Riffat (2015) reviewed a wide range of advanced glazing technologies and showed how modern glazing systems have developed specifically to address the thermal and solar limitations of conventional windows.

Singh and Garg (2009) provide particularly useful evidence from India. Their study evaluated different glazing types in New Delhi, Jodhpur, Chennai, Bangalore and Shillong, representing different climatic conditions.

The study found that energy performance depended on the combination of:

• climate
• orientation
• window type
• building dimensions
• wall and roof transmittance

This is why a glass selected for a hot-and-dry project should not automatically be copied into a cold-climate project.

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12. Orientation Changes the Answer

The same glass can behave very differently depending on where it is installed.

Solar exposure varies with:

• façade orientation
• latitude
• season
• time of day
• surrounding buildings
• overhangs
• fins
• balconies
• vegetation
• adjacent structures

A south-, east-, west- or north-facing façade can therefore require different solar-control strategies.

This is particularly important for large glazed façades.

A glass specification that works on one elevation may not be optimal on another.

Indian building requirements increasingly reflect this principle. ECSBC 2024 provides different SHGC requirements according to orientation and climate, and also recognizes the effect of permanent external shading in determining equivalent solar performance.

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13. Window-to-Wall Ratio Matters

A glazing system cannot be evaluated independently of how much glazing the building contains.

Consider two buildings using exactly the same glass:

Building A

WWR = 20%

Building B

WWR = 70%

The second building exposes substantially more façade area to the thermal and solar behaviour of glazing.

Therefore, the impact of the glass on the overall building energy balance can be very different.

This is why modern façade design considers glazing performance together with:

• WWR
• orientation
• shading
• wall performance
• roof performance
• HVAC loads
• daylight requirements
• occupancy
• internal heat gains

The Indian ECSBC 2024 framework explicitly incorporates WWR into its fenestration provisions, illustrating that glazing cannot be separated from façade geometry.

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14. External Shading Can Change the Glazing Strategy

Glass should not always be expected to perform the entire solar-control function.

External shading devices can intercept solar radiation before it reaches the glazing.

Examples include:

• horizontal overhangs
• vertical fins
• balconies
• verandahs
• canopies
• external louvers
• architectural projections

This can fundamentally change the solar-load problem.

ECSBC 2024 specifically recognizes permanent external projections when determining equivalent SHGC for compliance.

For façade design, this leads to an important principle:

Glass and shading should be designed as a system.

Selecting extremely low-SHGC glass may not always be the most appropriate way to compensate for poor façade design.

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15. The Importance of Spectral Selectivity

Solar radiation is not a single wavelength.

It covers a broad spectral range, while visible light occupies only a portion of that spectrum.

This makes it possible to design glazing with different spectral responses.

A spectrally selective glazing system can aim to:

• transmit useful visible light
• reject a portion of unwanted solar radiation
• maintain an appropriate visual appearance
• control solar heat gain

This is one reason modern coated glass can achieve performance that cannot be understood simply by looking at colour or darkness.

Jelle, Kalnæs and Gao (2015) reviewed low-emissivity materials and coatings and discussed their interaction with thermal radiation, solar radiation and optical properties.

Kiran Kumar et al. (2018) similarly demonstrated experimentally that different solar-control glasses have different spectral transmission, reflection and absorption characteristics.

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16. Low-E Glass and Solar-Control Glass Are Not Synonyms

The terms are sometimes used interchangeably in commercial discussions, but they describe different concepts.

Low-E primarily refers to low-emissivity surfaces that reduce radiative heat transfer.

Solar-control glazing refers more broadly to glazing designed to manage solar radiation.

A modern coated product can combine both functions.

For example, a double-glazed unit may incorporate:

• insulating glass
• inert-gas filling
• a Low-E coating
• solar-control characteristics
• a selected VLT
• a selected SHGC

Therefore, the specification should identify the actual performance values, rather than relying only on the technology name.

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17. Glass Numbers vs Complete Window Numbers

One of the most important specification issues is the distinction between glass-only data and complete fenestration data.

A glass manufacturer may provide centre-of-glass values.

A façade or building-energy calculation may require values for the complete window.

The final assembly can be affected by:

• frame material
• frame fraction
• spacer
• edge conditions
• glazing configuration
• operable versus fixed construction
• thermal bridges

For this reason, architects and façade consultants should ask:

Are these values for the glass or for the complete fenestration product?

This distinction is especially important when comparing products from different manufacturers.

A window with excellent centre-of-glass performance can have a significantly different whole-window result once the frame and edge are included.

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18. Why Standardized Measurement Matters

Performance numbers are useful only when they are determined using appropriate methods and clearly defined boundary conditions.

The international standards landscape has also continued to evolve.

ISO 9050:2026, published in July 2026, is the current edition for determining the luminous and solar characteristics of building glazing. It covers conventional glazing as well as absorbing and reflecting solar-control glazing and provides methods for characteristics used in lighting, heating and cooling calculations.

The 2026 edition replaces ISO 9050:2003.

For thermal and fenestration calculations, standards such as ISO 15099 provide the framework for calculating thermal and optical transmission properties of windows and doors.

The practical implication is straightforward:

Do not compare two glazing products unless you know what was measured, how it was calculated, and whether the reported value refers to the glass or the complete fenestration assembly.

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19. What the 2026 Glazing Landscape Looks Like

For 2026-era façade design, a more useful question is not simply:

“Which glass has the best thermal performance?”

but:

“Which glazing system produces the required building performance?”

This includes consideration of:

Thermal performance

• U-value
• insulating glass configuration
• Low-E coating
• gas filling
• frame performance

Solar performance

• SHGC
• solar transmittance
• solar reflectance
• external shading

Optical performance

• VLT
• visible reflectance
• colour
• glare
• daylight availability

Façade design

• orientation
• WWR
• shading
• façade geometry

Building operation

• heating demand
• cooling demand
• lighting demand
• occupancy
• HVAC system

Compliance and verification

• applicable building code
• standardized calculation method
• accredited testing or certification
• glass versus complete-window data

This systems approach is consistent with the direction of recent glazing research, which increasingly considers thermal, solar and daylight performance together rather than treating them as isolated properties (Li and Wu, 2025).

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20. A Practical Glazing Selection Matrix

The following is a decision framework, not a universal ranking.

Project condition	U-value focus	SHGC focus	VLT focus	Additional consideration
Cooling-dominated, highly exposed façade	Low	Often lower	Balance daylight and glare	External shading
Heating-dominated building	Low	May need useful solar admission	Balance daylight	Seasonal solar gains
Large WWR	Low	Strongly evaluate	Important for daylight	Whole-building simulation
Strong external shading	Low	Evaluate together with shading	Daylight remains important	Equivalent solar performance
High daylight requirement	Low	Avoid excessive solar gain	Often higher	Glare control
Highly exposed east/west façade	Low	Strong solar-control evaluation	Balance with glare	Orientation-specific design
Mixed-climate building	Low	Seasonal balance	Daylight + glare	Annual simulation
High-performance façade	Very important	Very important	Very important	Whole-system optimization

The table deliberately does not assign one universal “best” combination.

That would contradict the evidence.

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21. How to Read a Glass or Window Specification

When evaluating a proposed glazing system, ask for at least the following:

1. U-value

Is it:

centre-of-glass U-value

or

whole-window/fenestration U-value?

2. SHGC

Is the SHGC:

glass-only

or

complete fenestration assembly?

3. VLT

Is VLT reported for:

glass

or

complete window including the frame?

4. Glazing configuration

Specify:

• single / double / triple
• glass thickness
• cavity width
• gas filling
• coating position
• coating type
• spacer type

5. Solar-control properties

Where relevant, ask for:

• solar transmittance
• solar reflectance
• SHGC
• VLT

6. Frame

Specify:

• aluminium
• thermally broken aluminium
• uPVC
• timber
• composite
• other system

7. Orientation

State the intended façade orientation.

8. Shading

State whether external shading exists and its geometry.

9. WWR

Specify the approximate window-to-wall ratio.

10. Test/calculation basis

Identify the applicable standard and whether the result has been independently tested, calculated or certified.

Without this information, comparing two glazing products purely by a brochure’s headline number can be misleading.

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22. What Not to Do

Don’t choose glass only by U-value

A very low U-value does not tell you whether the solar heat gain is appropriate.

Don’t choose glass only by SHGC

Very low solar admission can also reduce useful daylight and, depending on the building and climate, useful solar gains.

Don’t choose glass only by VLT

High VLT can increase daylight but does not automatically guarantee visual comfort or low cooling loads.

Don’t assume darker glass is automatically better solar-control glass

Colour and appearance do not provide a complete description of spectral solar performance.

Don’t compare glass-only data with whole-window data

The comparison may not be equivalent.

Don’t ignore orientation

A glazing specification can perform differently on different façades.

Don’t ignore shading

External shading can materially alter the solar-load problem.

Don’t copy a glazing specification from another project without checking the design conditions

Different climates, orientations, WWRs and operating conditions can produce different outcomes.

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23. How Can Glazing Performance Be Optimized?

The most robust approach is to optimize the fenestration system, rather than a single glass property.

A typical design sequence is:

Step 1: Establish the building conditions

Determine:

• climate
• orientation
• WWR
• occupancy
• internal loads
• heating/cooling strategy

Step 2: Establish façade constraints

Determine:

• external shading
• façade geometry
• frame type
• daylight requirements
• visual requirements

Step 3: Set thermal requirements

Establish the appropriate target for:

U-value

Step 4: Set solar requirements

Establish the required range for:

SHGC

based on orientation, climate and shading.

Step 5: Set daylight requirements

Establish an appropriate:

VLT

rather than simply maximizing it.

Step 6: Compare complete fenestration systems

Evaluate:

glass + coating + cavity + gas + spacer + frame + shading

rather than glass alone.

Step 7: Verify the building-level result

Where project scale warrants it, evaluate the glazing within the building energy and daylight model.

This approach is more defensible than selecting a product from a single performance number.

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24. The Indian Context: Why One Specification Does Not Fit Every City

India provides a particularly useful example of why glazing selection cannot be reduced to a single national “best glass.”

Indian buildings encounter substantially different climatic conditions, from hot-and-dry environments to warm-humid, composite, temperate and cold conditions.

Singh and Garg (2009) evaluated window performance across five Indian climatic zones and demonstrated that the relative energy performance of window systems changes according to climate and orientation.

The Energy Conservation and Sustainable Building Code (ECSBC) 2024 also reflects this climate- and orientation-sensitive approach. Its vertical-fenestration provisions specify U-factor and SHGC limits by climatic zone and orientation, while also incorporating WWR and provisions for permanent external shading.

That is an important lesson for Indian projects:

A glazing specification should be developed for the building and façade, not simply selected from a generic “best glass” list.

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25. The Bottom Line

There is no universally best window glass.

U-value, SHGC and VLT measure different aspects of fenestration performance:

• U-value addresses heat transfer.
• SHGC addresses solar heat admitted to the building.
• VLT addresses visible light transmission.

The objective is not to minimize one number and maximize another in isolation.

The objective is to find a compatible combination of thermal, solar and optical properties for the actual building.

That combination depends on:

climate + orientation + WWR + shading + façade design + building loads + daylight requirements + window construction

Modern glazing technologies make it possible to engineer these properties with considerable precision. Low-E coatings, solar-control coatings, insulating glass units, spectrally selective products and advanced fenestration systems can all be used to achieve different performance combinations.

But the specification should always begin with the building’s requirements, not with a glass marketing category.

The most technically defensible question is therefore not:

“Which glass is the best?”

It is:

“Which glazing performance combination gives this building the required thermal, solar and daylight performance?”

That is the foundation of evidence-based glazing selection in modern façade design.

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References

Cuce, E. and Riffat, S.B. (2015). A state-of-the-art review on innovative glazing technologies. Renewable and Sustainable Energy Reviews, 41, 695-714. https://doi.org/10.1016/j.rser.2014.08.084

Jelle, B.P., Kalnæs, S.E. and Gao, T. (2015). Low-emissivity materials for building applications: A state-of-the-art review and future research perspectives. Energy and Buildings, 96, 329-356. https://doi.org/10.1016/j.enbuild.2015.03.024

Abolghasemi Moghaddam, S., Simões, N. and Gameiro da Silva, M.C. (2023). Review of the experimental methods for evaluation of windows’ solar heat gain coefficient: From standardized tests to new possibilities. Building and Environment, 242, 110527. https://doi.org/10.1016/j.buildenv.2023.110527

Kiran Kumar, G., Saboor, S., Kumar, V., Kim, K.-H. and Babu, T.P.A. (2018). Experimental and Theoretical Studies of Various Solar Control Window Glasses for the Reduction of Cooling and Heating Loads in Buildings across Different Climatic Regions. Energy and Buildings. https://doi.org/10.1016/j.enbuild.2018.05.054

Li, J. and Wu, Y. (2025). A review of complex window-glazing systems for building energy saving and daylight comfort: Glazing technologies and their building performance prediction. Journal of Building Physics, 48(4). https://doi.org/10.1177/17442591241269182

Singh, M.C. and Garg, S.N. (2009). Energy rating of different glazings for Indian climates. Energy, 34, 1986-1992. https://doi.org/10.1016/j.energy.2009.08.013

Standards and codes

ISO 9050:2026. Glass in building – Determination of luminous and solar characteristics of glazing. International Organization for Standardization.

ISO 15099. Thermal performance of windows, doors and shading devices – Detailed calculations. International Organization for Standardization.

Energy Conservation and Sustainable Building Code (ECSBC), 2024. Bureau of Energy Efficiency, Government of India.

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Compiled from published glazing research and current Indian building-energy codes for architects, façade consultants and window manufacturers evaluating glass performance.