Daylight Redirecting Window Film
Window Glazing: Daylight Redirecting Film vs. No film
Window film on exterior windowpanes that redirects daylight upward towards the ceiling with minimal glare to occupants, using a diffusing film on the interior pane as needed (3M).
Item ID: 513
Building Envelope--Windows & Skylights
Technical Advisory Group: 2014 Commercial Building TAG (#9)
Average TAG Rating: 3.21 out of 5
TAG Ranking Date: 03/17/2014
TAG Rating Commentary:
- This technology can potentially reduce glare and improve daylight utilization in conventional perimeter spaces as well as expanding the daylight zone to 20-30 feet - thus dramatically increasing savings. Works best on southerly orientations and in sunnier climates but has a role elsewhere. We have developed new tools to predict performance and tested several versions.
- I am not familiar with the 3M technology. From past studies on light redirecting technologies, it seems that they working well when then sun is at the right place, but not as well when it is not.
3M offers a daylight redirecting film and SerraLux offers their SerraGlaze redirecting film. Both involve a thin passive acrylic film that contains micro-structured louvres to re-direct incident daylight arriving on the upper portion of the window toward the ceiling of a room. Daylight is then reflected down to light the room. The redirecting films can be incorporated into windows for new construction or installed as retrofits. Electricity savings occur as lighting can be turned off or dimmed. This technology provides little savings during cloudy, overcast days.
Daylight redirecting films (DRF) can be installed on south, east, and west facing building windows. Generally, the film is installed above a sill height of 6 feet to keep redirected sunlight out of the eyes of occupants. Performance is a function of angle of incidence of sunlight (depends on latitude, orientation of the building façade, season, time of day) and sky conditions. Outdoor views are not possible through the DRF--the appearance is similar to frosted glass. The building façade must not have deep recessed windows, trees, and nearby buildings that block the sunlight. Windows should not have tinted glass or sun control window films.
The films will work as intended but several modifications must be made can reduce cost-effectiveness and/or technical potential. First, the indoor space must have minimal vertical obstructions (for 20 feet to 25 feet). Windows area must be available above six feet from the floor. Ceilings and walls must be light colored to promote reflectance and a dimmable lighting control system is required for affected lights. Sites with vertical shades may have to install horizontal shades just below the DRF level. HVAC cooling and heating loads may increase or decrease based upon the magnitude of the solar gain versus the reduction in heat rejection from lighting sources. Users have found that a diffusing film must also be used to eliminate introduction of glare problems.
The Department of Defense ESTCP program conducted a cost and performance study for DRFs at six sites nationwide including the Bremerton Naval Hospital. The simple payback for the northwest application (windows for examining rooms) ranged from 11 to 35 years for south, east, and west-facing windows assuming an electrical energy rate of $0.086/kWh. Savings were found to be heavily dependent upon successful use of lighting controls, plus building location and orientation. Savings are also dependent upon the light type (32 W fluorescent versus LED) and the existing light level that is maintained in the affected space.
Baseline Description: Typical flourescent office lighting in newer office building.
Baseline Energy Use: 3.7 kWh per year per square foot
The SMUD study determined baseline energy use to be 1.2 Watts per square foot of floor area. (Perry, 2012) This translates to 3.7 kWh per square foot per year with typical office hours.
Manufacturer's Energy Savings Claims:
So many variables exist that can impact performance that manufacturer's do not suggest a "typical savings" or even an "up to" savings value.
Best Estimate of Energy Savings:
"Typical" Savings: 16%
Low and High Energy Savings: 6% to 19%
Energy Savings Reliability: 3 - Limited Assessment
In a modeling study by Sacramento Municipal Utility District, the energy savings attributable to adding the DRF to windows adjacent to an area 24 feet deep with daylight harvesting controls on the lighting system already installed, tinted windows, and vertical blinds automatically controlled was approximately 16%. The lower range reflects an deeper study area and the upper range reflects a taller window with film on it. (Perry, 2012).
A Department of Defense study found the savings attributable to adding the DRF was 12-39 kWh per square foot of film area (Note: this is window film area, not square footage of building space). (DOD ESTCP, 2013)
Energy Use of Emerging Technology:
3.1 kWh per square foot per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
This technology is most appropriate for new construction or retrofits in the commercial sector with unobstructed solar exposure for perimeter areas near south-, east-, and west-facing windows over six feet in height above the floor. A rough guess is that 10% to 20% of buildings might be suitable for use of the DRF technology with only 10% to 20% of this population having glazing area at heights that are suitable for DRF application. Of the remaining building stock only those with relatively unobstructed floor plans within 24 feet of the windows and existing daylighting harvesting lighting controls should be considered. And for these buildings, only the 24-foot perimeter of those building areas can be included as building area that is affected by this technology (assume this represents 25% of the building stock).
This technology is appropriate for virtually all commercial building except for warehouses and unconditioned spaces. However, this will be most appropriate, typically for just the perimeter areas near south-, east-, and west-facing windows in the Northwest. The numbers for total commercial building space are taken from preliminary updated numbers from the 2013 update to the Commercial Building Stock Assessment (CBSA) using the estimates for 2014 (before the update was completed -- from early January, 2014), minus warehouse space, multiplied times the percentage of commercial space that is conditioned based on the 2009 CBSA, times our rough estimate of 25% of applicable space, which would count south-, east-, and west-facing perimeter zones. This result is then multiplied by 15% x 15% or 0.0225 to account for solar exposure and window height yielding a total of 14,412,093 sf.
Regional Technical Potential:
| || Total Floor space || -s.f. Warehouse || non-Warehouse || % Conditioned || % Applicable || Applicable Space |
| Source || (NEEA, 2014) || (NEEA, 2009 App C) || (NEEA, 2009 App C) || (NEEA, 2009 App D, Table D-EA1) ||WSU EP || |
| Values: || 3,118,000,000 || 173,000,000 || 2,945,000,000 ||87.0% ||25% ||640,537,500 |
0.01 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: square foot
Emerging Technology Unit Cost (Equipment Only): $0.57
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $1.30
Baseline Technology Unit Cost (Equipment Only): $0.00
The Department of Defense ESTCP Cost and Performance Report indicates a DRF cost of $11 per square foot of window film. Installation costs for the labor and material required to install the DRF film is $25/sf. These costs include applying the film to the inside of existing windows and then adding a third pane of glass with diffusion film applied to it, as well as installing horizontal shades to the windows below the DRF window level. The windows are 25 square feet and the floor area is 480 square feet. The costs per square foot of floor area are then $.57 for equipment and $1.30 for labor.
Simple payback, new construction (years): 10.7
Simple payback, retrofit (years): 35.1
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
For an application in the northwest (Bremerton Naval Hospital), it was determined that simple paybacks based upon lighting energy savings were in the range of 11 to 35 years, based upon building and window orientation, occupancy, and current lighting system operation. This payback is based upon an electrical energy rate of $0.0836/kWh. The DOD report didn't provide kWh savings, they instead used a unit of full-load equivalent hours (FLE) that electric lights could be turned off. The paybacks calculated indicate savings on the order of 12 to 39 kWh per square foot of DRF window film applied. (DOD ESTCP, 2013) Note: Square foot in this discussion is related to the square feet of DRF window film applied (not square feet of building floor area).