WASHINGTON (September 28, 2016) - A hotel’s windows are a huge source of solar heat gain especially when the windows face the sun. We already know that the drapes in a guestroom, when not occupied, are kept closed to reduce the solar heat gain and lower energy costs. What else can a hotel do, especially in common areas, to reduce solar heat gain? There are two options: install new low-E windows or retrofit the existing windows with energy-saving film.

Over the last fifteen years energy-saving film has been used as a way to reduce the excessive solar heat gain that occurs when a hotel’s windows face the sun. Hotel owners and operators worldwide are using energy-saving film as a way to reduce their heating, ventilating and air conditioning load and lower their energy costs by up to 10%.

An energy-saving window film typically consists of a thin (0.025mm, 0.001 inch) polyester film that has a micro-thin, transparent metal coating applied to one side. The metal coating is applied using vacuum-based technologies such as vapor deposition or sputtering. A second layer of polyester film is laminated over the metal coating to protect the metal. A scratch-resistant coating is then applied onto the side of the laminated composite that faces the building interior to protect the film during normal window cleaning. An adhesive layer is applied to the film side that faces the glass and is protected by a removable release liner until just before the film is applied to the window. UV absorbers are added to the polyester film layers, the adhesive layer, or both to protect from UV deterioration.

The appearance of film is determined by the metal coating(s) used. This can include color, the level of visible light transmission and the degree of reflectivity. Typically, all-metal energy films are silver-reflective, gray, silver-gray, bronze or light green in color. Visible light transmissions can vary from very dark (10 percent) to very light (70 percent), and the visible reflectance can vary from the same reflectance as clear glass (8 percent) to highly reflective (60 percent).

Energy-saving window film offers hoteliers a great return on their investment due to the combination of reduced solar heat gain and a low material/installation cost. Reduced solar heat gain equates to lower energy costs and material/labor costs for installation of the film is approximately $3.00-5.00 per square foot. Across all four U.S. climate zones, studies have shown that for each dollar available for window retrofit (energy saving film) or window replacement, energy-saving window film provides a greater energy cost savings than total window replacement with new low-E windows. As expected, the biggest energy savings are in the Southern and South/Central zones but the North/Central and Northern zones also benefit.

Business Case 

A 300-room hotel believes that they may have a potential opportunity to reduce energy consumption by adding energy saving window film to their common area windows. Before taking any action, they want to calculate the return on investment. The computations listed below show the annual savings, investment costs, and the payback period for the installation.  

Energy costs saved are taken from the DOE’s energy-saving calculator. The cost of energy-saving window film depends on the size of the window surface. The average hotel common area has 140 windows that are about 78 square feet. Many local utility companies have energy-saving window incentives in their rebate programs. Be sure to contact your local utility company before starting any project to determine if rebates are offered.

Calculations 

For the calculation, the cost per kwh is $0.13 (national average).  The cost to install one energy-saving window film is $270 per 78 sq/ft including labor. Sunlight is 12 hours per day during the 220 days of cooling for the year. 

Number of Windows Reduction in Wattage Hours Used Daily Total Days Kwhr Multiplier Total Kwh Saved

140     x400w x12 x220 x.001 =147,840

Annual Savings 

     Annual Kwh Electric Savings:  147,840 x $.13 = $19,219 
     Total Annual Savings:              $19,219

Investment 

     Cost per Window Installed:      $270
     Number of Windows :              140

     Total Investment:                     $37,800

Payback 

     Investment:                             $37,800
     Annual Savings:                      $19,219

     Payback:                                2 years

A total window replacement with solar-control low-E windows does provide good energy savings; but is a relatively poor return on investment due to high material and installation costs. As a result, the return on investment for window film is faster than installing low-E windows.

In reviewing the replacement of windows or retrofit of energy-savings film, the application of energy saving window film should be strongly considered in lieu of window replacement only if existing windows are functionally sound. Functionally sound windows should have few or no failed seals, limited air leakage, and the frames should be in good condition.

Resources 

 Learn more about commercial window energy saving film from DOE 

Visit the EPA’s Energy Star site to calculate window replacement in your city 

Review BOMA article on measuring the savings from window film