A brief explanation of two approved methods for calculating energy consumption rates in building structures when installing impact windows and doors.
After the 2010 Florida Building Code went into effect last March 15th, 2012, owners of new construction projects were required to comply with new energy-consumption guidelines, including maximum U-factor and Solar Heat Gain Coefficient (SHGC) values. This post is written from the perspective of specifying and selecting the right glazing makeup of impact windows and doors that meet the Code.
To comply with energy requirements, designers have two methods they can choose from: the prescriptive or the performance path.
With the prescriptive or pre-determined path, designers must ensure that the proposed impact windows and doors have a maximum U-value of 0.75 and a maximum SHGC of 0.30. Windows and doors must bear the label with the energy ratings independently confirmed by National Fenestration Rating Council (NFRC). Section 402.3.2 of the code specifies that when the U‐factor varies between the fenestration products, the use of area‐weighted averaging is allowed to satisfy the U‐factor requirements. Fenestration products missing SHGC labels are assigned default values from Table 303.1.3(3).
As an alternative to using the prescriptive path described above, designers can opt for employing the performance path to meet the energy preservation code. With the performance path, designers need to use an energy compliance software tool approved by the Florida Building Commission. This analysis only includes heating, cooling and service water heating. Using this software allows architects or mechanical engineers to perform a trade-off between U-values and SHGC ratings. When using trade‐offs from Section 405, the area‐weighted average maximum allowed for fenestration SHGC is 0.50. Why is this trade-off important? To achieve low U-values, insulated-laminated glazing is almost always needed, an option that is more expensive than just laminated glass.
The information presented above is not intended to replace the code. Equally important, I cannot guarantee this information is completely accurate. Instead, it is a quick reference for our potential clients who are designing new construction projects and are evaluating the available options for impact windows. We want to ensure that they know they have two ways of meeting the energy requirements. Hence, they should consult with the competent professional to ensure they can build cost efficiently.
For our potential clients, we provide the following information that could be useful when analyzing our proposal of impact windows and doors.
The Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from sunlight. The SHGC is the fraction of the heat from the sun that enters through a window. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.
Facts about R‐values and U‐values
For additional information about this topic, you can call us at 1-305-328-3198.