The Glass G Value Calculator is a building performance tool used to determine how much solar energy passes through a glass surface, considering both direct transmission and secondary heat release. The g-value, also known as the Solar Heat Gain Coefficient (SHGC), is important in energy-efficient building design because it affects indoor temperatures, cooling requirements, and overall comfort. By calculating the g-value, architects and engineers can select glazing systems that balance daylight entry with solar heat control. This helps in achieving energy savings and ensuring thermal comfort for occupants.
This tool falls under the category of solar performance and building energy efficiency calculators.
formula
Standard definition (measurement / standards form)
g = (τe × Gdir + qi) ÷ Gdir
Where:
g: Total solar energy transmittance (dimensionless, 0…1)
τe: Direct solar spectral transmittance integrated across the solar spectrum (dimensionless)
Gdir: Incident solar irradiance used as reference (W/m²)
qi: Portion of absorbed solar energy released inward (dimensionless)
Since Gdir cancels out when τe and qi are computed per unit incident irradiance, the formula is often written as:
g = τe + qi
Notes on Standards
ISO 9050, ISO 52022-3, and EN 410 define methods for calculating τe and qi from spectral measurements. τe is derived from how much solar energy passes directly through the glass, while qi is calculated based on the fraction of absorbed energy that radiates or convects into the interior. This requires laboratory measurements or simulation using thermal models.
General G-Value Reference Table
Below is an approximate reference for typical glass types and their g-values.
Glass Type | G-Value (Approx.)
Clear single glazing | 0.85
Tinted single glazing | 0.65
Double glazing clear | 0.75
Double glazing low-E | 0.55
Triple glazing low-E | 0.40
Solar control coated glass | 0.30
Note: These are common ranges. Actual values depend on glass thickness, coatings, and spacer types.
Example
Suppose we have:
τe = 0.60
qi = 0.15
Step 1: Use simplified formula
g = τe + qi
g = 0.60 + 0.15
Step 2: Result
g = 0.75
This means that 75% of the solar energy hitting the glass will be transferred indoors as heat, either directly or indirectly.
Most Common FAQs
The g-value determines how much solar heat enters a building through glass, which directly impacts cooling needs, indoor comfort, and energy costs.
Not necessarily. In cold climates, a higher g-value can help capture free solar heat, reducing heating costs. In hot climates, a lower g-value is better to reduce cooling loads.
The g-value may change slightly due to dirt accumulation, aging of coatings, or seal degradation, but high-quality glazing systems maintain stable values for many years.