How to Reduce Solar Heat Gain in Smart Glass Facades

Introduction

There are only three things that can happen to solar radiation as it strikes a window:

1. It is reflected away (R)
2. It is absorbed by the glass (A), and partially re-radiated as secondary heatflow (SH)
3. It is transmitted (T) through the window, with or without a change in direction

This derives directly from the principle of Conservation of Energy in physics.

Based on the variables in the image:

Sol = R + A + T + SH

Solar Gain

The total solar energy entering a building through the windows causes a rise in temperature and can be measured by one of two variables:

• SHGC: Solar Heat Gain Coefficient, a term used in North America (ranging from 0 to 1) 
• SF: Solar Factor, the equivalent term in Europe (shown as a percentage). It is also known as ‘g-value’ and ‘Total Solar Energy Transmittance (TSET)’.

In this article, we will refer to all these terms as simply ‘Solar Gain’, which we calculate as follows:

Solar Gain (SG) = Solar Transmittance (T) + Secondary Heatflow (SH)

Combining the above two equations then results in:

Sol = R + A + SG

• A low Solar Gain (e.g. SHGC=0.2 or SF=20%) implies less heat coming in through the window. It is beneficial in warmer climates since it helps to reduce cooling costs. 
• A high Solar Gain (e.g. SHGC=0.8 or SF=80%) is beneficial in colder climates since it provides passive solar warmth, reducing heating costs.

The architect will typically specify a suitable value for solar gain based on building codes, local climate, facade orientation and HVAC loads.

Secondary Heatflow

Example insulated glazing unit (IGU) showing a double glazed window and an internal cavity

The energy absorbed by the glazing (mainly surfaces #1 and #2 in the diagram) and partially re-radiated as long-wavelength infrared is called ‘secondary heatflow’ and increases cooling costs.

This is why a low-emittance (low-e) layer is deposited on an interior glass surface (normally #3 or #4 in the diagram above). The low-e layer reflects the secondary heat outwards and stop it from entering the building.

Secondary heatflow can also reduce the lifespan of the materials which emit it.

This is especially problematic for ‘smart tint’ technologies (located on surface #2), since they have multiple layers and are highly heat-absorbing, as we will find out next.

Secondary Heatflow in Smart Glass

The interlayers inside smart glass complicate the thermal dynamics, as these materials absorb and re-emit heat through both radiation and convection.

Examples of interlayer materials include some of the following:

• PVB (Polyvinyl Butyral)
• EVA (Ethylene-Vinyl Acetate)
• PET (Polyethylene Terephthalate)
• transparent conductors, normally indium tin oxide (ITO) used in electrically-driven smart glass
• electrical connectors (called busbars), made of copper foil and silver paste (although these have in fact little impact on overall thermal performance)

Some of these materials are used in SPD smart glass as well as in thermochromic, photochromic, electrochromic, electrophoretic, photovoltaic and liquid crystal based glass.

When smart glass transitions into its dark or privacy state, the Solar Transmittance (T) decreases because the interlayers absorb or scatter visible light, UV and infrared. 

Consequently, the glass temperature rises (often reaching 60ºC under direct sun). This elevated temperature exacerbates the secondary heatflow.

Let’s look more deeply at the materials we find in smart glass technologies:

Electrochromic (EC) Film

Electrochromic smart glass uses multi-layer ceramic thin films, where lithium ions migrate between an ion storage layer to an electrochromic layer. When we apply a voltage, the stack transitions from clear to tinted by absorbing near-infrared (NIR) and visible light.

Because this process relies on absorption (rather than reflection), a fully tinted EC panel gets very hot. If located on the indoor pane, the glass releases radiative energy inward, intensifying secondary heatflow.

The secondary heating effect of EC layers can thus be very high.

Polymer Dispersed Liquid Crystal (PDLC) Film

Image of dyed PDLC liquid crystal glass

PDLC features micro-droplets of liquid crystal dispersed inside a polymer matrix. In its unpowered state, it scatters light to provide privacy.

Multiple bounces of scattered light can increase absorption by the glass and the polymer. 

The secondary heating effect of PDLC layers can thus be moderate to high.

Suspended Particle Devices (SPD) Film

SPD utilizes a thin layer of rod-like nano-particles, suspended in a matrix. With no voltage, the particles are randomly oriented and absorb up to 99% of incoming light.

The dark particles act as a thermal sponge in the glazing assembly, creating secondary heat dissipation inward.

The secondary heating effect of SPD layers can thus be very high.

Indium Tin Oxide (ITO) Transparent Conductors

ITO is the transparent conductive oxide layer which applies electrical voltages across EC, PDLC and SPD smart layers.

ITO has a high thermal conductivity (4 to 10 W/m.K) compared to glass (1.4 W/m.K) , acting as a heat sink and dispersing hot spots.

However, ITO inherently possesses a mild low-emissivity property to far-infrared radiation. While it generates minimal heat when conducting electrical current, it can affect heat radiation depending on which face of the glass it is deposited on.

Since ITO is normally on surface #2, heat generated by the ITO can become trapped in the cavity due to the reflective low-e layer on surface #3. This can cause thermal stress in the cavity and possible expansion of the gas leading to seal failure.

One solution is to keep the ITO on surface #2, but place the low-e layer on surface #4. This would allow heat to pass safely through the IGU into the room, without stressing the window cavity.

We can mitigate the overall thermal effect by choosing low-current, low-power electrochromic (and electrophoretic) smart glass.

PVB and EVA Laminating Films

These are the plastic bonding layers used to encapsulate liquid crystal (LC) or SPD films into laminated safety glass.

PVB and EVA have lower thermal conductivity than float glass. 

When smart interlayers absorb solar energy and heat up, the PVB/EVA acts as a temporary thermal barrier. 

It slows down conductive heat transfer, but also traps heat inside the glass assembly, forcing the outer skin of the glass to reach higher temperatures and radiate secondary heat.

The secondary heating effect of ITO layers can thus be moderate.

How to avoid Secondary Heating Effects in Smart Glass

To prevent these interlayers from dumping their absorbed secondary heat into your room, windows are typically Insulated Glazing Units (IGUs), rather than a monolithic arrangement. 

IGUs utilize specific structural positioning to reduce secondary heatflow mentioned above:

We can thus minimize secondary heating effects by placing:

• the absorbing ‘smart’ layer (e.g. EC, PDLC or SPD) on the outermost panel
• an insulating gap (either vacuum, air or inert gas) to reduce convection
• a low-e (heat-reflecting) coating on the internal panel

This configuration ensures that, when the smart interlayer gets hot, the low-e layer reflects any secondary heat away from the building interior.

Are SHGC and Solar Factor the same thing?

Short answer: not exactly. 

There are multiple international standards which show how to calculate the Solar Gain through a glazed facade. 

Some are simple and aimed at cost estimation in the early stages of build, whereas others are focussed on modelling building physics for complex architectural energy calculations.

The simpler standards result in mathematical models from which we derive the Solar Factor, whereas the more complex standards result in SHGC.

Simpler Energy Models

These simpler standards include:

ISO 9050:2003: “Glass in building — Determination of light transmittance, solar direct transmittance, total solar energy transmittance, ultraviolet transmittance and related glazing factors”, and;

EN 410:2011: “Glass in building – Determination of luminous and solar characteristics of glazing”. This is a harmonised variant of ISO 9050.

In both cases, these documents:

• focus on center-of-glass (COG) measurements only (ignoring the window frame)
• assume a simple perpendicular angle of incoming light
• are used by manufacturers and builders to compare glass brands simply and objectively

Solar Factor (used mainly in Europe) focuses on centre-of-glass performance and is calculated from ISO 9050 or EN 410.

More Complex Energy Models

These more complex standards include:

ISO 15099:2003: “Thermal performance of windows, doors and shading devices — Detailed calculations”

NFRC 200-2023: “Procedure for Determining Fenestration Product Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence”. This is based on ISO 15099.

In both cases, these documents:

• are applicable to multi-component window assemblies under variable environmental conditions
• solve energy balance equations and factor in absorbed solar energy exchange
• include internal convection of the gas inside the cavity and energy exchange between layers
• determine performance metrics at any tilt angle or solar orientation
• used by architectural software algorithms

Solar Heat Gain Coefficient (SHGC), used mainly in North America, focuses on whole-window performance and is calculated from ISO 15099 or NFRC 200.

What looks like a plethora of overlapping standards now hopefully starts to make sense.

Comparing Energy Metrics

This diagram tries to summarise all the terms we find related to solar flows in glazed facades:

We now look at each term in a little more detail.

U-Factor

According to the National Fenestration Rating Council (NFRC), the U-factor is ‘the rate at which a window, door, or skylight transmits non-solar heat flow’.

Whereas some glass suppliers refer to the U-Factor as heat lost by the glass alone, the NFRC U-factor represents the entire window, including frame and spacer materials. 

The lower the U-factor, the more energy-efficient the window, door, or skylight.

U-Factor values between 1.0 and 1.6 W/m²K are considered adequate for modern buildings, while highly insulated structures like Passivhaus require 0.8 W/m²K or less.

Visible Light Transmittance (VLT)

VLT is the visible light (380 to 720 nanometers), weighted by the sensitivity of the human eye, which is transmitted through a window, door or skylight. 

That is to say, VLT is a photometric measurement of light, rather than radiometric.

A product with a higher VLT transmits more visible light, and depends on your building’s daylighting requirements.

Solar Transmittance

The radiometric counterpart to VLT is Solar Transmittance, which measures the total solar energy (UV, visible, and IR) passing through a glazing material.

Light-to-Solar Gain (LSG)

Light-to-Solar Gain (LSG) is the ratio between the VLT and SHGC. It shows the efficiency of glass in transmitting daylight while blocking heat. 

The higher the LSG, the more light is transmitted without adding warmth.

In Conclusion

Sunlight incident on a window comprises energy reflected, absorbed or transmitted into the building.

The energy absorbed and re-radiated is secondary heatflow, and is particularly problematic for smart glass windows due to their multiple laminating interlayers, which include PVB, EVA and ITO.

The best way to deal with secondary heatflow in smart glass windows involves placing the smart tinting layers on the outer window pane in an IGU. 

A low-e layer on the IGU interior reflects reradiated energy outwards, rather than allowing it to enter the building, thus lowering temperatures and energy costs.

Frequently Asked Questions (FAQ)

Does smart glass get hot to the touch?

Yes, especially in its tinted or privacy state. Smart glass works by absorbing or scattering light, which causes the surface temperature to rise, sometimes reaching 60ºC under direct sunlight. This is why proper IGU configuration is essential to manage heat dissipation.

Why is my smart glass making the room feel warmer?

If your smart glass is installed on the interior pane of the window, the absorbed heat has nowhere to go but into your building. This phenomenon is called “secondary heatflow.” To fix this, smart layers should be placed on the exterior pane, with a low-e coating on the interior pane to reflect heat back outside.

What is the difference between SHGC and Solar Factor?

While both measure solar gain, they stem from different standards. “Solar Factor” (SF) is typically used in Europe (based on ISO 9050/EN 410) and focuses on center-of-glass performance. “SHGC” (Solar Heat Gain Coefficient) is common in North America (based on ISO 15099/NFRC 200) and accounts for the whole window assembly.

Can low-e coatings solve secondary heatflow on their own?

Low-e coatings are highly effective, but they work best when paired with correct glass positioning. If a low-e coating is placed on a smart glass panel that is already facing the interior, it can trap heat, creating a “radiator” effect. It is most effective when the smart layer is on the exterior pane and the low-e layer is on the interior pane.

Next Steps

If you are looking for smart glass products with a specific SHGC, VLT or U-Value, use our parameterised search to zoom in on supplier products by product attributes. 

This is much faster than trawling through datasheets surfaced by Google, and does not suffer from AI hallucination, since we verify all data manually.