Florida Demands Optimal Windows

By Bruce Lang

Florida home builders know that windows in Florida must block solar heat to reduce air conditioning operating cost. They also know that high light transmission is important to homeowners who don’t want to tradeoff darkened interiors for heat reduction. What they may not know is that glass also capable of superior insulation performance helps reduce the cost of air conditioning.

Here’s why:

Home cooling requirements are not just a function of reducing radiant solar energy. The same insulation performance in windows that keeps heat indoors during winter by impeding heat transfer via conduction and convection also keeps heat out year-round in Sunbelt climates like Florida’s.

It’s easy to think that direct solar energy from the sun radiating on windows is the only source of heat. A major source, yes. The only source, no. Physics 101 tells us that heat always moves from where it is hot to where it is not. In Florida, night time ambient air with a temperature of 90 F will pass through single pane glass to warm interiors that are being kept 15 degrees cooler by air conditioning. Heat transfer will also take place via poorly insulated window frames. To the extent windows permit this heat transfer via conduction and convection, they will not maintain an adequate temperature difference required to optimally reduce air conditioning operating cost no matter how well they block radiant solar heat.

Why is this important? Because 25-35 percent of the energy used in homes and buildings is wasted due to inefficient glass that is responsible for over 10 percent of the total carbon emissions in the U.S. annually. Given this reality, Florida home builders should know that a double pane window, either with visible tint or an invisible solar heat-blocking coating, will perform better at reducing heat transfer from solar radiation, conduction and convection than a single pane window tinted to reflect solar energy.

Windows From Worse to Better

Let’s review glass options in terms of which best supports a Florida home’s cooling requirements.

 • Single pane glass without a coating to reflect solar energy and providing an insulation value, or U value of 1, leaves the homeowner defenseless against all types of heat. Given today’s need to save energy on AC and reduce carbon emissions, a single pane window is totally unacceptable.

A double pane window, either with visible tint or an invisible solar heat-blocking coating, will perform better at reducing heat transfer from solar radiation, conduction and convection than a single pane window tinted to reflect solar energy.

 • Single pane glass with a solar control coating is better in terms of reflecting direct solar radiation but, as noted above, does not adequately prevent heat transfer caused by conduction and convection.

 • Standard insulating glass with a solar control coating and a U value of ~0.5 should be the minimum baseline for Florida homes. The coating reflects solar energy and the sealed air space impedes heat transfer.

 • Insulating glass with dual coatings that simultaneously reflect radiant solar heat and ambient heat via conduction and convection is even more effective in saving energy and improving occupant comfort. For most of the US, this version of low – e glass with a U value of 0.35 is the Energy Star standard for homes in which both summer cooling and winter warming are important. As we have explained, considering the complexities of optimal cooling, this type of window should be de facto requirement for Florida homes as well.

Many might think this is where the story ends. However, proposed revisions to the Department of Energy’s Energy Star® glass performance standards scheduled to debut as early as 2009 should make clear that even the most energy efficient low-e glass no longer represents a level of energy efficiency required to “transform the market,” a key charter of the agency’s Energy Star program.

With advances in glass coating technology expected to provide minimal performance improvement, the focus has now shifted from coatings to cavities.

Generic low-e insulating glass, consisting of two pieces of coated glass separated by a sealed, gas-filled air space (or cavity), achieves a maximum U value of 0.25. With further advances in glass coating technology expected to provide minimal performance improvement, the focus has now shifted from coatings to cavities. Just as the introduction of single-cavity insulated glass provided a breakthrough in performance beyond monolithic glass, the introduction of multi-cavity constructions, consisting of two or even three insulating cavities, is providing the next performance breakthrough for insulating glass. As we have demonstrated, this is just as important for keeping Florida homes cool as it is for keeping northern homes warm.

A superior alternative to generic low-e glass available now consists of suspending a low emissivity and solar reflective film inside of an insulating glass unit. Film can create two, three or even four insulating cavities that maximize light transmission and provide conservation performance ranging from U 0.16 to an amazing U 0.05 to provide optimal cooling for both residential new construction and renovation projects.

Such internally-mounted film does not replace low-e glass. It leverages the benefits of film-based and glass-based technologies to create a lightweight, multi-cavity insulating glass that offers a new level of performance. Most units fabricated today utilize low-e coated glass to minimize solar heat gain, while using film to maximize insulation performance, block UV radiation, reduce noise, and increase occupant comfort more effectively than low-e glass alone. For protection against hurricane force winds, laminated glass can be used in conjunction with internally mounted film to achieve both energy conservation and enhanced security.

Windows, window glass and the building envelope must be viewed as an integrated system when assessing such issues as energy efficiency, greenhouse gas emissions, occupant comfort and well being. An optimum window for Florida should reflect a maximum amount of radiant solar heat, transmit high levels of visible light (subject to coastal turtle code requirements), provide maximum insulation against heat transfer via conduction and convection and enhance protection against wind blown debris. Film-based, multi-cavity insulating glass can do all of that and more to maximize energy savings in thousands of homes throughout the Sunbelt.

Bruce Lang is Vice President of Marketing & Business Development at Southwall Technologies, Inc., in Palo Alto, CA. He can be reached at blang@southwall.com.

Return to Previous Page | Email This Article