Daylight Redirecting Window Film Insert installation name  Several studies have shown that better use of daylight can reduce energy demands by 20-40%, while reducing  cooling load from interior lighting. The effective use of daylight for occupant comfort and energy efficiency presents unique problems for building and energy managers. Daylighting represents a significant opportunity for installation energy managers to improve efficiency across their portfolio of buildings A recent ESTCP project has demonstrated daylight redirecting window films that can help installation energy managers lower lighting energy consumption and increase occupant comfort.

Technology Description A window film that redirects daylight further into interior spaces Reflects sunlight toward the ceiling to distribute light more uniformly Can reflect natural light up to 40 feet from the window This daylight redirecting film project presents a potential solution for retrofit and new construction applications. The demonstrated window film redirects daylight further into interior spaces and can reflect natural light up to 40 feet from the window and extends the daylight zone up to 8 feet for every vertical foot of film. The thin translucent film is covered with microscopic prisms and fused to a layer of pressure-sensitive adhesive that attaches to the inside surface of a window.. Image source: 3M

What it Does Here is an annotated fisheye view

How Does It Work .The thin translucent film is covered with microscopic prisms and fused to a layer of pressure-sensitive adhesive that attaches to the inside surface of a window. The film’s microstructures maximize reflection of sunlight towards the ceiling to allow the ceiling surfaces to redistribute the light more uniformly in the space. This can reduce glare so occupants can work with less eyestrain from the daylight and reduce the need for artificial light.

How Does It Work Continued The product was produced in a roll-to-roll format that consisted of acrylic micro-prismatic elements on a clear polyester (polyethylene terephthalate [PET]) substrate and coated with a pressure sensitive adhesive (PSA) on the backside. The microstructures are designed to maximize reflection of incident sunlight towards the ceiling to allow the ceiling surfaces to redistribute the light more uniformly in the space. The thin translucent film is covered with microscopic prisms and fused to a layer of pressure-sensitive adhesive that attaches to the inside surface of a window. The film’s microstructures maximize reflection of sunlight towards the ceiling to allow the ceiling surfaces to redistribute the light more uniformly in the space. This can reduce glare so occupants can work with less eyestrain from the daylight and reduce the need for artificial light

The Six Demonstration Locations The films were installed in six different DoD buildings across three different climate zones. The sites were selected based on user profile, building location, access, window design and structure as well as availability of similar, if not identical, space that could be designated as “control” space in order to perform a side-by-side comparison. So, what do I need to use this product at my installation? To realize energy savings, daylight redirecting film should be paired with advanced lighting controls. Otherwise, lighting levels will not dim in response to the increase in daylight. The ESTCP Project EW- 201012 is a good example of these type of controls. The technology is best suited for existing buildings with unshaded windows facing south, west, or east where the view is not required, particularly windows with a high visible light transmittance and a clerestory area beginning at least 7 feet above the floor. The technology is most effective in spaces with high-reflectance ceilings and flush mounted light fixtures that do not impede light transmittance on the ceiling. Sunny climates also offer the greatest opportunity for energy savings. However, facilities that do not meet all these conditions can still realize cost savings.

Benefits of the Technology Helps meet regulatory requirements for energy reduction Lowers lighting energy consumption Increases interior daylight, which can improve productivity Rejects 99.9% of ultraviolet rays Payback of 10 years on average This technology offers multiple benefits compared to standard window film. The film’s depth of daylight penetration is 40 feet while other systems such as lightshelves, is 15 feet and 10 feet for the automated shades and blinds.  By increasing the daylit zone, it decreases lighting energy consumption by reducing the need for electric lighting. An increase in daylight can also stimulate healthy circadian rhythm, reduce stress, and improve productivity and attentiveness. In addition, the film rejects 99.9% of ultraviolet rays.  

Potential Challenges and Solutions Installation Daylight redirecting film installation is highly dependent on the current building configuration. It is generally less expensive in the new construction situation than the retrofit scenario. Application The maximum savings will be accomplished by using this film in conjunction with advanced lighting controls using daylight sensing and diming strategies. (See ESTCP Project EW-201012 for example ) Blast-Rating Current daylight redirecting films are not blast-rated, so windows may require an additional layer of blast film There are a couple of potential limitations to this technology. The first is the installation costs are variable depending on the complexity of the current structure. It is easier to install for a “New construction” situation than a retrofit.. The second limitation is the maximum benefit is gained by having control systems that integrate daylighting sensors into the system. An additional limitation is that currently the film is not blast rated so it is possible installations may need to install an additional layer of blast film.

Sample Financial Considerations Current Commercial Product developed from Demonstration: DRF Film Installed New construction $25/ft2 Retrofit Application $25/ft2 -$40/ft2 No operation or maintenance costs Simple payback period 10 years (average) SIR 1.4 – 2.58 Lifetime 15 years For the demonstration, the prototype daylight redirecting film cost about $11 per window square foot and installation costs were about $25 per window square foot. The current commercialized product averages about $25/ft 2 of window area for new construction and $25 to $40 per ft 2 for retrofit solutions depending on the existing building configurations. Comparing these cost to other systems the DRF averaged a 10 year simple payback with a range of 3 – 35 years over the six installations. The simple payback for some other commercial daylighting system is approximately 25 years, for lightshelves and 58 years for automated shades and blinds. The ESTCP project savings-to-investment ratio ranged from 0.38 to 3.75 and all the calculations depend on the local electricity rates and climate.

Bottom Line Pros Cons Lowers lighting energy consumption Increases interior daylight Rejects ultraviolet rays No maintenance requirements Increases occupant comfort Cons Retrofit costs Most effective requires Advanced Lighting Controls Not Blast-Rated

Where to Learn more Mr. Tim Tetreault, ESTCP Energy and Water Program Manager, 571- 372-6397 SERDP-ESTCP Website, https://serdp-estcp.org