Cooling Load - Window/Skylight

The heat loads form the skylights and windows can be broken up into (2) types of loads, conductive and radiation loads. The conductive loads for skylights and windows use the same formula as that of the roofs and windows, shown below again.

Conductive loads

Cooling Load Window Equation for the Mechanical PE Exam

The radiation loads or solar transmission is calculated by multiplying the area of the window or skylight by the shading coefficient and the solar cooling load factor.

Heat Transfer - Solar Cooling Load Factor for the Mechanical PE Exam

The shading coefficient is the ratio of the specific window or skylight's solar transmission compared to 1/8" clear glass. The shading coefficient is typically specific to the glass manufacturer and can be found in the manufacturer's product data. During the exam, this value along with the solar cooling load factor should be given. The solar cooling load factor is given in the ASHRAE 1997 Fundamentals book and similarly to CLTD it serves as a simplified approach to calculating heat gain. In addition, SCL is organized similarly by skylight/window, orientation, month, latitude and hour.

In lieu of SC, the term Solar Heat Gain Coefficient (SHGC) is being used by window/skylight manufacturers. This term is simply found by dividing the SC by 1.15. A lower SHGC or SC means that the glass lets in less solar gain and a higher SHGC or SC means that the glass allows more solar gain through.

The National Fenestration Rating Council (NFRC) rates glass and certifies the SHGC and U-Factor. Additional values like Visible Transmittance, Air Leakage and Condensation Resistance are also tested and certified.