“Free energy” (in the form of sunlight) is right outside the window of every commercial building. The question for building owners and facility managers, however, is whether utilizing that sunlight within the building creates a net gain or loss from an energy standpoint. After all, more sunlight equals more heat, right?
In recent years, increased attention to the intentional use of sunlight in commercial spaces has given rise to the terms “daylighting” and “daylight harvesting.”
Daylighting is the process of designing a structure to obtain maximum illumination benefits of natural lighting. Daylight harvesting is the practice of reducing energy consumption by using photosensors to adjust electric lights in response to changing levels of daylight.
It’s easy to see how allowing sunlight into the building enables automated building control systems to reduce the load on artificial lighting. But uncontrolled, sunlight also brings with it some tradeoffs: glare, UV exposure, and summertime solar heat (with increased cooling costs).
Energy-efficient window film, which is professionally installed on the inside surface of existing or new windows, addresses all of those concerns and can often reduce a building’s energy costs by as much as 5-15%. In warm weather, window film allows daylight to enter while reducing the entry of solar heat and damaging UV rays—helping to control cooling costs. In fact, occupants are much less likely to use blinds or shades to maintain comfortable conditions or to reduce glare once window films have been installed. So, window films can actually provide improved opportunities for daylighting and daylight harvesting. In the wintertime, the film helps keep interior warmth from escaping through window panes.
Window films can also reduce glare without a darkly tinted appearance. There’s even a safety benefit: window films are designed to hold glass shards in place in the event of breakage (added peace of mind on skylights).
While sunlight is free, utilizing that sunlight fully in a commercial building may entail an investment. So how do the economics of daylighting and daylight harvesting play out? It’s difficult to make generalizations on this point because of variations in climate, building characteristics, and local utility rates. However, a controlled study published by the Energy Center of Wisconsin concluded that a comprehensive daylighting strategy could potentially result in a savings of 24¢/sq. ft. for a facility with annual operating costs of $1.13/sq. ft.
To calculate ROI on window films in particular, an online energy savings calculator can be useful.
The bottom line appears to be that implementing daylighting techniques creates a net “plus” for building owners and facility managers—provided that the sunlight is controlled in a way that prevents uncomfortable glare or increased HVAC costs from solar heat.