How To Read A Window Glass Performance Guide

How to Read Performance Guide

U Factor: (inverse of R Value)- The heat flow rate through a given construction expressed in Btu/hr/ft2/F. Values are normally given for NFRC/ASHRAE winter conditions of 0 degrees outdoor temperature, 70 degrees indoor temperature, 15 mph wind and no solar load. U-values can be expressed in terms of the glass alone (Ucog) or for the entire window (Uwindow) which includes the effect of the frame and the spacer material. The lower the number the better the insulating value.

R Value: Higher number the better

SHADING COEFFICIENT (SC) — The ratio of solar heat gain through a glazing to the solar heat gain through a single lite of 1/8″ (3.0mm) glass. Dimensionless and varying between 0 and 1, the smaller the number, the better the glazing is at preventing solar gain. The shading coefficient is specific to normal incidence angle and is being phased out in favor of the solar heat gain coefficient value. The lower the number, the less solar heat gain.

SOLAR HEAT GAIN COEFFICENT (SHGC) — The fraction of incident solar radiation which enters a building as heat. It is based on the sum of the solar energy transmittance + the inwardly flowing fraction of absorbed solar energy on all lites of the glazing. Note that the smaller the number, the better the glazing is at preventing solar heat gain. This value is preferred over the shading coefficient since it can be used for solar incidence angles other than normal to the glass surface. It can be expressed in terms of the glass alone (SHGCcog) or total unit value (SHGCwindow).

RELATIVE-HEAT GAIN (RHG) — The total amount of heat gain through a glazing system at NFRC/ASHRAE specified summer conditions, incorporating the U-value and the Solar Heat Gain Coefficient. The conditions are 230 Btu/hr/ft2, outdoor temperature of 89 degrees F, indoor temperature of 75 degrees F, and 7.5 mph wind. This value is expressed in terms of Btu/hr/ft2. The lower the number, the less heat gain.

VISIBLE LIGHT TRANSMITTANCE — In the visible spectrum (380 to 720 nanometers), the percentage of light that is transmitted through the glazing relative to the C.I.E. [Commission International de I’Eclairage (International Commission on Illumination)] standard observer. The higher the number, the more clarity of the glass.

UV TRANSMISSIONS (Tuv) — The percentage of ultra violet rays that enter a room through the glazing system. It is a predictor of potential fading damage. Lower percentages indicate less fading potential. (UV rays are those with a wavelength ranging from 0.30 to 0.38 nanometers). The lower the number the less fade potential.

LBL DAMAGE FUNCTION (Tdw-K) — This function is another way of expressing UV Transmission. It is a better predictor of potential fading damage than UV Transmission. Lower values indicate less fading potential. The LBL Damage Function is a weighted value which takes into account that as the wavelength of the UV rays get shorter, the fading damage potential increases. Therefore, two glazing systems with the same UV transmission may have different LBL Damage Function values because one allows more “shorter” wavelength rays to pass through than the other.