Energy Star Windows with Smartglass

Energy Star windows with smart glass would enable a faster transition to Net Zero by allowing more transparency and choice to consumers looking for improvements in energy efficiency.

In a Nutshell

This article introduces the Energy Star scheme as applies to windows and proposes the addition of criteria suitable for smart glass. 

Let’s dive in:-

According to reports from the EU and the NREL, buildings consume about 40% of all energy worldwide and produce a third of all carbon emissions. 

In the EU, 75% of buildings have substandard energy performance and 10% -15% of building heat is lost through the windows (mostly through the edges).

Both the US Energy Star programme and the EU ENERG scheme aim to solve this issue by improving energy-efficiency and reducing carbon emissions across various product categories.

The advantage of both these programmes is the transparency and choice that they bring to consumers when deciding which products to purchase.

Whereas Energy Star covers buildings, appliances and glazing, the EU ENERG scheme is more limited in its remit. In fact, building facades are covered by entirely separate EU legislation.

This article compares both programmes, and calls for an international energy labelling scheme that includes the properties applicable to smartglass. 

Examples include variable light transmittance, power consumption and in the case of photovoltaic smartglass, power generation.

Energy Star Windows

The Energy Star programme implements the 2005 Energy Policy Act and is managed by the US Environmental Protection Agency (EPA). 

The EPA is also responsible for the ‘Energy Star Portfolio Manager®’, a benchmarking tool for commercial buildings (based on a rating scale of 1 to 100), which helps building owners to:-

  • identify under-performing buildings
  • set capital investment priorities
  • report performance data to stakeholders, and
  • implement performance improvement plans to reduce carbon emissions.

Compliance with Energy Star also allows builders to qualify for US Federal Income tax credits and deductions.

Since 2011, the EPA requires third-party certification for all Energy Star products. This involves testing in an EPA-recognised laboratory and review by an EPA-recognised certification body.

Energy Star Logo

Energy Star service mark, [United States Environmental Protection Agency, Public domain, Wikimedia Commons]

Furthermore, the Energy Star NextGen™ Certified Homes and Apartments programme, launched in November 2023, is an option for properties that incorporate more advanced technologies. Examples include heat pumps, smart water heaters and on-site electric vehicle charging.

NextGen™ certified homes need to meet the most stringent parts of the Energy Star programme, providing 20% better energy-efficiency than properties built to the 2021 International Energy Conservation Code.

Crucial to the glazing sector, Energy Star covers windows, doors, skylights and curtain walls. Manufacturing is conducted only by certified partners, and the National Fenestration Rating Council (NFRC) conducts independent verification according to and EPA guidelines regarding energy efficiency.

Performance criteria depend on US climate zones, which include Northern, North-Central, South-Central, and Southern, as indicated by the following map:-

Energy Star Climate Zones US

The criteria include the U-Factor and the Solar Heat Gain Coefficient (SHGC), but this assumes static optical and thermal characteristics. 

Herein lies the inherent limitation for more advanced technologies such as smartglass.

Energy Star Certification Criteria

Energy Star Certification Criteria for U-Factor (U-Value) and SHGC, based on US Climate Zone

Energy Star and Smartglass Windows

It is clear that adoption of any smartglass technology would allow buildings to adapt to diverse climate zones worldwide. This would allow facades to respond better to changing seasons and changing conditions throughout the day.

To take one example, dye-doped liquid crystal (DLC) smartglass varies in visible light transmittance (VLT) from 1% to 68%. The technology has corresponding variations in infrared and UV rejection across its dynamic range.

When placed in an Insulated Glass Unit (IGU), the smartglass can adapt to the environment by simply changing its operating voltage. The IGU typically also incorporates an argon gap, low-e layer and warm edge spacers.

Dyed Liquid Crystal smartglass with low-e soft coat

Dye-Doped Liquid Crystal (DLC) Smartglass DGU with argon gas fill and low-e coating (example configuration)

Furthermore, an AI-controlled smartglass system can respond to changes in the weather, building occupancy or power grid pricing.

For this, we need to add parameters to Energy Star windows, over and above the U-Value and Solar Heat Gain Coefficient (SHGC):-

  • variable transmittance
    • visible light, infrared and ultraviolet
    • results in minimum and maximum parameters for each (on vs off)
  • variable haze for privacy smartglass, such as liquid crystal (LC)-based technologies
  • operating voltage and frequency
    • prioritising lower voltages which imply reduced cabling costs
  • active power factor monitoring for electrically-driven smartglass technologies which are capacitive loads (e.g. SPD, PDLC)
    • reducing the chances of fines from electrical utilities for power factor violations
  • power consumption for electrically-activated smartglass
  • power-generation for 
    • transparent photovoltaic (PV) layers embedded inside smartglass
    • opaque PV modules embedded onto the window frame
    • opaque PV building cladding.

EU Energ Scheme

In the European Union, the equivalent energy efficiency scheme is the EU ENERG label, governed by Regulation 2017/1369/EU. This dictates energy consumption requirements by grading products from A (the best) down to G, as shown below.

However, this covers consumer appliances, such as refrigerators, washing machines and light bulbs (as well as automobiles). Most notably, it does not include buildings, windows, doors, skylights or curtain walls.

Many might be wondering why such an important product category was omitted.

European Union energy label

EU ENERG label for a refrigerator with rating of ‘B’

To understand how the EU controls energy efficiency in buildings, we must look to the EU Energy Performance of Buildings Directive (EPB).

EU EPB Directive

There has been some debate in the EU as to whether to have an EU-wide glazing certification scheme. Some claim that such a scheme would harm the EU glass industry due to the diversity of the EU building stock.

Opponents of an EU-wide energy labelling scheme for windows claim that the Energy Performance of Buildings Directive [EPBD] is the best tool to achieve this.

This legislation is complemented by the Energy Efficiency Directive EU/2023/1791

Both were revised in 2023.

The legislation aims to revitalise renovation, especially for poorly-performing buildings and to improve air quality using energy monitoring systems.

The EU has also introduced energy performance certificates as an independent benchmark for potential customers looking to buy or rent a property.

The Problem with Directives

However, EU Directives, by their very nature, need to be transposed in each country. 

This can lead to differing implementations across the region, leading to increased costs for manufacturers who would need to create product variants for each set of national requirements. 

This will typically raise prices for consumers and put European manufacturers at a competitive disadvantage to other (non-EU) companies who have more unified local markets (e.g. USA and China).

So, it is clear that buildings – and glazing products in particular – are where the industry needs to focus efforts to meet energy efficiency and global carbon emissions targets.

What seems to be missing in the EU is an initiative that harmonises requirements on glazing products and minimises them as the ‘weak link’ in the building envelope.


The US Energy Star programme has reduced energy costs by $500B and carbon emissions by 4 billion metric tons since 1992.

The EU urgently needs a comparable glazing certification programme to help consumers choose greener products. This would help to achieve the EU’s stated goal of a fully decarbonised building stock by 2050.

This article calls for an international product labelling scheme, in line with that for Energy Star windows, which harmonises the requirements for glazing manufacturers. Such a scheme would bring transparency and choice to consumers looking for products that reduce energy bills and carbon emissions.

Since buildings lose substantial energy through their facades, it makes sense to include windows, doors, skylights and curtain walls. This would allow consumers to compare glazing products based on energy efficiency, operational cost, changing climate needs and building occupancy.

Finally, the scheme proposed would widen the criteria to include those applicable to smart glass. Examples include variable transmittance, power consumption and electrical power output.

Such an initiative would allow developers, architects and owners to capitalise on the potential of smartglass to reduce energy costs and carbon emissions across the global building stock.

Need vendor-neutral advice choosing smart glass for your next project?