When developing a new building (or renovating an existing building), one of the most important aspects of the building envelope to consider is the fenestration. Some projects may enjoy large views through extensive glazing, but depending on the location, orientation, and efficiency of your building envelope, this can have a detrimental effect on your energy savings if the proper performance values for your windows are not selected. Understanding how your fenestration affects your building’s performance is critical to developing a compliant envelope, and the two values which define this efficiency (the U-factor and SHGC) can have a positive or negative impact depending on the scenario.

The U-factor is essentially the conductivity of a given type of material. Per the California Energy Commission, the average window has approximately 25% of the effective thermal resistance of its surrounding walls. This means that the conductivity of the window will have the largest overall effect on the amount of heat transfer taking place between the conditioned interior and the unconditioned exterior. You can think of the U-factor as the inverse of an R-value, i.e. if a higher R-value is better, then a lower U-factor would also be better. Lowering the amount of conductivity in the window is the most effective way to bring about greater savings for your building.

The Solar Heat Gain Coefficient (SHGC) is the amount of solar radiation passing through the window. Unlike the U-factor, this value can be less or more effective depending on the climate zone. For example, if your building is located in a hotter climate zone such as Los Angeles or Palm Desert, then a lower SHGC value will provide a large benefit to the project as it lowers the overall cooling load needed for the conditioned spaces. However, if the project is in a colder climate zone such as Crestline or Salinas, the SHGC value will assist in lowering the annual heating load and increase the overall savings. In other words, a lower SHGC value is better in hotter regions and a higher SHGC value is better in colder regions.

Selecting the right windows for your project can produce significant savings on first costs as well as utility savings down the line. Early energy modeling is the best way to mitigate any unforeseen budgeting issues within the envelope. VCA Green provides energy modeling services for Title 24, LEED, GreenPoint Rated, and other related programs. For more information on how to optimize the energy usage of your new or existing development, contact Moe Fakih below.

Contributing Writer: Burke Boydell, CEA

Moe Fakih, Principal

714-363-4700 x 501