What is the difference between U Value and Solar Heat Gain Coefficient?

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© Timber arched Low-E glass upgrade with ComfortPlus™ Neutral and New Beads By Adelaide Glaziers

The U-value and the Solar Heat Gain Coefficient (SHGC) are critical factors to consider when evaluating the energy efficiency of windows and other glazed structures. They each describe different aspects of how a window responds to environmental conditions. Here’s a detailed explanation of each and how they differ: 

U-Value (or U-Factor)


The U-value, also known as the U-factor, quantifies the rate of heat transfer through a window or glazing system. It essentially measures how well a window can insulate.


It is calculated as the amount of heat lost or gained per hour per square meter of window area for every degree Celsius difference in temperature between the inside and outside.


It is usually expressed in watts per square meter per degree Celsius (W/m²°C) or British thermal units per hour per square foot per degree Fahrenheit (Btu/hr ft² °F) in some regions.


  • Lower U-Value: Indicates better insulation properties. It means the window prevents heat from escaping in winter and keeps heat out during summer.
  • Higher U-Value: Suggests poor insulation properties, with a higher rate of heat transfer through the window.


It’s a crucial factor to consider for energy efficiency, especially in climates with significant temperature variations between seasons.

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Solar Heat Gain Coefficient (SHGC)


The Solar Heat Gain Coefficient quantifies the fraction of solar radiation that is transmitted through a window or glazing system. It essentially measures how well a window blocks heat caused by sunlight.


It is calculated as the ratio of the solar heat gain entering the space through the window to the incident solar radiation.


It is dimensionless, usually expressed as a number between 0 and 1.


  • Lower SHGC: Indicates that the window is effective at blocking unwanted solar heat gain, beneficial in hot and sunny climates to keep interiors cool.
  • Higher SHGC: Suggests that the window allows more solar heat to pass through, which can be advantageous in colder climates to utilize passive solar heating.

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It is a vital consideration in climates with high solar radiation, helping to manage the cooling loads in summer and, in some cases, aiding in heating during winter.

Comparative Analysis

  • Aspect of Measurement:
    • U-value focuses on the general heat transfer through the window, considering all forms of heat transfer (conduction, convection, and radiation).
    • SHGC primarily concerns the heat gain from solar radiation.
  • Climatic Relevance:
    • U-value is a critical parameter in regions with significant temperature variations, helping in selecting windows that aid in temperature insulation.
    • SHGC is essential in regions with high solar radiation, assisting in managing solar heat gain through windows.
  • Energy Efficiency Planning:
    • U-value helps in planning the insulation aspects of a building.
    • SHGC aids in devising strategies for managing solar heating, which includes considerations for shading devices and window orientations.


Understanding the differences between U-value and SHGC allows homeowners and builders to make informed decisions about selecting windows and glazing systems that suit the specific climatic conditions of a region, enhancing the energy efficiency and comfort of a building. Seeing this topic often resolves back to Double Glazing if you want to study up on many topics about IGU’s then check my other blog solely about Double Glazing aptly Called DoubleGlazing.au


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