Descriptions of Commonly Used Fenestration Terminology

Solar Optical Properties (TRA Values):  The solar optical properties of a glazing or product are the transmittance, reflectance and absorbence. Generally referred to as the TRA values.

The solar optical properties are further broken down into three categories, total solar, visible and UV.

Total solar properties are used for the analysis of the product/glazing for the entire spectrum related to solar radiation. These values would be used when one is concerned with the solar heat gain (heat) into a space.

The visible properties are the same TRA values but for the visible range of the solar spectrum only. This is the solar spectrum range that the average human eye responds to. These properties would be used for analysis of day lighting, glare, etc.

The UV transmittance is the transmittance of the product/glazing for the UV range of the solar spectrum. This value is used as a rating to determine the product/glazing effectiveness in guarding against UV degradation. The reader should be aware that the UV portion only accounts for approximately 1/3 of the damage. The environmental UV radiation and the heat produced by absorbed solar radiation both impact the degradation to carpet and furnishings.

Shading Coefficient (SC): A value that represents the quantity of solar heat through the product/glazing in question as compared to the solar heat gain through the ASHRAE reference glazing (1/8" clear single glazing; SC = 1.0), under the same conditions. 

Solar Heat Gain Factor (SHGF): A solar radiation level (Btu/hr.ft² or W/M²) for a given geographic location, surface tilt and orientation, time and day that would enter (transmitted and the inward flowing fraction of the absorbed portion of solar radiation) the space through the ASHRAE reference glazing. This value accounts for the change in transmittance, reflectance and absorbence due to the changing incident angle on the glass.

Solar Heat Gain Coefficient (SHGC): This value was developed to replace the Shading Coefficient due to the fact that the Shading Coefficient is not constant. Specifically for products that have strong angular solar optical properties. ie: louvered shade screen. -- The ratio of solar heat gain (transmitted and the inward flowing fraction of the absorbed portion of solar radiation) through the product/glazing to the incident solar radiation striking the surface for a given condition.

U-factor: The U-factor (Btu/hr.ft².F or W/M².C) is the heat transmission value for the product/glazing/window which occurs to the indoor/outdoor temperature differential. this can be a heat gain or loss depending on the indoor/outdoor temperatures.

R-value: The R-Value (F/Btu/hr.FT²) is the inverse of the U-value. The value represents the resistance to heat flow due to the indoor/outdoor temperature differential.

Daylight Efficacy Value (K_e): The daylight Efficacy Value is an indication of the amount of light (footcandles) that will enter through a fenestration system as compared to the amount of heat (Btu/hr.ft² or W/M²). K_e = Visible Transmittance / Shading Coefficient.