Glass Skylights: Choosing the Right Glazing Specification

Glass skylights are more than just "windows for your roof." They are high-performance

architectural elements that can significantly impact your building's energy bill and occupant comfort. When specifying glazing for a skylight, you aren't just choosing glass; you are managing heat, light, and energy transfer. To get the specification right, you must look beyond aesthetics and focus on two critical technical metrics: SHGC and U-value.

1. Understanding the U-Value: Your Thermal Shield

The U-value (or U-factor) measures the rate of non-solar heat transfer through the skylight. Think of it as a measure of how well the unit insulates against the outside air temperature.

• What it measures: How much heat escapes during winter or enters during summer

  via conduction.

• The Goal: Lower is always better. A low U-value means the skylight is a better

  insulator, keeping your indoor environment stable.

2. Solar Heat Gain Coefficient (SHGC): The "Solar Gate"

While U-value handles air-to-air heat, SHGC manages the energy coming directly from the sun’s rays.

• What it measures: The fraction of solar radiation admitted through the glass, either transmitted directly or absorbed and released as heat inside.

• Climate Matters: Unlike U-value, the "ideal" SHGC depends on your location:

  • Hot Climates: Seek a low SHGC to block excessive solar heat and reduce air

    conditioning loads.

  • Cold Climates: A higher SHGC can be beneficial, allowing "passive solar gain"

    to help heat the building naturally during winter.

  • Mixed Climates: Aim for a balanced SHGC (around 0.50 to 0.60) to provide

    year-round efficiency.

3. Glass Skylight Size and Thickness Requirements

Size is a critical factor in skylight performance, because it determines the total amount of

energy entering or leaving a space. While SHGC and U-values are rated per unit of area, the physical dimensions of the unit change the overall thermal impact on the building.

In addition to thermal performance, the physical size of a glass panel is the primary driver for determining its required thickness. In skylight applications, glass must be thick enough to resist deflection and prevent breakage from self-weight and wind loads.

Here are a few points on how panel size dictates glass thickness:

• Weight-to-Thickness Ratio: Glass is heavy! For very large custom skylights, the glass

  must be thick enough to support its own weight without sagging in the centre of the frame, especially when installed at low pitches.

• Standard Manufacturing Limits: Standard glass thicknesses (e.g., 6.38mm, 8.38mm,

  10.38mm) have maximum recommended spans. Once a design exceeds these

  dimensions, the specification must jump to the next thickness tier.

• Deflection Limits: Larger panels are more prone to visible flexing. Building codes set

  strict deflection limits for overhead glazing. To stay within these limits on a wide

  span, thicker glass is required to provide the necessary stiffness.

4. Key Components for a High-Performance Specification

To achieve these target values, your specification should include these specific technical

details:

• Single or Double Glazing is essential: Double glazing can achieve lower U-values

than single glazing.

• Low-E Coatings: Low-emissivity coatings reflect infrared energy (heat) while letting

visible light through, helping to lower both U-values and SHGC.

• Thermal Breaks: Ensure the frame itself has a "thermal break / air gap", an

insulating barrier that prevents the frame from acting as a heat bridge.

By correctly balancing SHGC and U-values, you ensure that your skylight provides beautiful natural light without turning the below area into a greenhouse or a walk-in freezer. Whether you are specifying for a residential or commercial project, explore our full range of skylights to find the right solution for your space. Reach out to RSI and we’ll help you specify the perfect skylight!