Electrochromic Glass
Sustainable Built Environments | College of Architecture + Planning + Landscape Architecture
The University of Arizona
Armando Lagunas
Mentor: Colby Moeller AIA, LEED AP
May 4, 2017
Abstract
Electrochromic glass is a type of smart glass, a new technology that has potential to
reduce the amount of sunlight entering a building by changing its physical properties. The
purpose of this study is to understand the properties of electrochromic glass and determine if it is
a viable alternative to conventional single pane and double pane glass in the Tucson area. Using
research and statistics from smart glass production companies, a comparative analysis will be
done using the building simulation software Energy-10. It was found that when compared to
single pane glass, double pane glass had a decrease of 7.21% in energy cost and electrochromic
glass had a decrease of 9.81%. For the used building model, this meant a return investment in 30
and a half years. While electrochromic glass is a new clean method of energy usage reduction, it
currently cannot return the consumers initial investment within a desirable time span.
Table of Contents
Introduction…………………………………………………………………………….....……4
What is Smart Glass?. ……………………………………………………………….........…6
- Three Types of Smart Glass…………………………………………………..…………6
o Electrochromic Glass………………………………………………….…………6
o Suspended Particle Glass………………………………………………….……8
o Polymer Dispersed Liquid Crystal (PDLC) Glass ………………………….…9
Methodology……………………………………………………………………………………10
Literary Review…………………………………………………………………...……………11
- Service Life…………………………………………………………………………………12
- Maintenance……………………………………………………………………………..…12
- Drawbacks…………………………………………………………………………….……13
Data………………………………………………………………………………………..…….14
- Building Model………………………………………………………………………..….…16
- Comparison………………………………………………………………………...…....…15
Discussion………………………………………………………………………………...….…18
Conclusion………………………………………………………………………………………19
Limitations……………………………………………………………………….………………19
Recommendations for Future Study…………………………………………………….……20
References………………………………………………………………………………………22
- Additional Resources………………………………………………………………………22
Introduction
One of the biggest issues the city of Tucson, Arizona faces in becoming a more
sustainable city is the sun. With temperatures reaching over 100 degrees Fahrenheit and limited
cloud cover, the sun impacts all aspects of life from the roads needing constant repair, to the
city’s water supply becoming even more hampered, and more energy has to be used in order to
keep people in an appropriate thermal comfort zone. The Intergovernmental Panel on Climate
Change (IPCC) forecasts the temperature rise from 2.5 to 10 degrees Fahrenheit over the next
century (Climate, 2017). For buildings, two of the biggest issues are regulating the amount of
sunlight and daylight that enters from windows. In order to address this issue, many homeowners
will install windows with multiple panes or tint that may be highly reflective. While a reasonable
thought, such actions may cost them more in the long run than the amount they save from the
reduced energy usage.
Windows are great in the way they allow light and heat to enter buildings when we need
it, yet they are also bad because they allow light and heat in when it should not be. This means
we must provide shading in order to reduce the amount of light entering and use more energy for
air-conditioning. However, smart glass may provide a solution. What makes smart glass special
is electrochromism, which changes the color of the coated thin layer of metal-oxide material
from transparent to opaque when an electrical voltage is applied across the pane.
In the darkened state, smart glass can reflect up to about 98% of all light falling on it,
which drastically reduces the amount of energy used for air-conditioning (Clayton, 2011). Since
they are electrically operated, they can be controlled by a switch, a smart-home system, or a
sunlight sensor, which does not require people to be inside the building. The amount of
electricity required to switch 100 windows from light to dark equates to the same amount of
energy needed for a single incandescent lamp (Woodford, 2017). According to scientists at the
US Department of Energy’s National Renewable Energy Laboratory (NREL), these windows can
save up to 8% of the building’s total energy consumption. Aside from this, smart glass also
provides privacy almost instantly compared to using curtains or blinds. This privacy is a
convenience not available with standard glass. Additionally, smart glass is highly reliable due to
their simplistic design. This capstone, will be proposing the use of electrochromic glass or “smart
glass” as a more practical and efficient substitute to traditional windows. To do so this study will
analyze the overall cost of electrochromic glass to determine how much energy and money can
be saved through their installation in the Tucson area. Then, it will compare the results to other
types of glass to determine how it ranks in comparison.
What is Smart Glass?
Smart glass, or switchable glass, is glass whose transparent properties can be altered with
the introduction of a voltage charge (Wong and Chan, 2014). By being able to change from being
transparent to opaque, this means that the windows themselves can reduce the amount of light
and heat entering a building without the need for curtains or blinds. The reason this is significant
is that this reduces the cost in heating, air-conditioning, and lighting while maintaining an
optimal human thermal comfort zone. By introducing smart glass, a building can minimize glare,
maximize daylighting, and protect interior furnishings and artwork. Smart glass comes in
different forms which are electrochromic glass, suspended particle glass, and polymer dispersed
liquid crystal (PDLC) glass.
Three Types of Smart Glass
Electrochromic Glass
Electrochromic glass is the most common type of smart glass seen on the market. Its
manufacturing process is relatively simple due to its layered structure and the materials
necessary are not too expensive. While producing high performance electrochromic glass can
have a higher cost due to its high-performance materials such as In2O3 (ITO). Electrochromic
glass is a glass pane with electrochromic materials within it. These materials are able to change
optical properties due to electrochemical redox reactions with the introduction of electric current
across it (Wong and Chan, 2014). By being introduced to glass, the reflectivity, emissivity,
transmissivity, and absorptivity of light that passes through the glass can now be controlled with
a voltaic charge. Due to these properties, electrochromic glass is the more prominent form of
smart glass seen on the market (Wong and Chan, 2014).
Electrochromic glass consists of five layers which are placed on glass substrate or within
two glass substrates as seen in Fig. 1. The center layer is composed of an ion conductor that
allows the flow of small mobile ions such as H+ and Li+ ions. This center layer is placed
between an electrochromic film and an ion storage film. Tungsten oxide is typically used for the
electrochromic film layer. The reasoning behind this is tungsten oxide is a transition metal,
meaning that it has electrochromic properties which allows it to easily undergo redox reactions
(Wong and Chan, 2014). When voltage is applied, ions are introduced and expand the
geometrical arrangement of the tungsten oxide while electrons modify the electronic structure,
overall changing the optical properties. On both sides of these three layers are transparent
conductors. In today’s industry, tungsten oxide film is being used as a standard for
electrochromic film while In2O3 (ITO) is used for the transparent conducting film for high-end
purposes. For more general purposes such as window glass, tin oxide is used due to its lower cost
(Wong and Chan, 2014).
To alter the optical properties
of the glass, a voltage is applied
across the transparent conductors. In
this process ions are transferred
between the ion storage and the
electrochromic film. By controlling
the amount of voltage applied to the
transparent conductors, the degree to
which the properties are altered is
also controlled. In simply removing
Figure 1 Electrochromic Glass
Source: Emaze
any voltage to the glass, it will return to its original transparent state. The applied voltage also
allows for the oxidation reduction of the metal oxide electrochromic film. The voltage changes
the redox state of the glass by introducing the energy needed for the change to occur. Each redox
state having its own electronic absorption spectra. This change in spectra then changes the
optical properties of the entire electrochromic glass.
Suspended Particle Glass
Suspended particle glass, or electrophoretic glass, uses a device that also changes its
optical properties similarly to electrochromic glass. Much like electrochromic glass, suspended
particle glass requires voltage to alter its optical properties (Wong and Chan, 2014). Yet the
reason why it is not as popular as electrochromic glass is that it requires a voltage source to
maintain transparent and is opaque without.
Suspended particle glass consists of three layers, although ones with five layers have also
been made. As seen in Fig. 2 the center layer of this glass is an organic fluid or gel that suspends
needle shaped particles. These needle shaped
particles are usually made of polyiodides
(dihyrocinchonidine bisulfite polyiodide) or
paraphathite (Wong and Chan, 2014). Holding
the central layer together is two transparent
conductors which act as electrodes. Two glass
substrates are then placed on both sides of these
three layers, forming the suspended particle
glass.
Figure 2 Suspended Particle Glass Source: VisualMechanical
When voltage is applied to this glass, the needle shaped particles rotate and align to form
a minimum energy state which allows light to pass through the layers and glass. Once this
voltage is removed the particles will then randomly relocate and absorb the light instead. With
applied voltage, the allowed transmission may range from about 20-60%. While the amount
voltage required varies depending on the thickness of the glass but will generally be around 20150 V (Wong and Chan, 2014).
Polymer Dispersed Liquid Crystal (PDLC) Glass
Similar to electrochromic and suspended particle glass, PDLC glass is designed to change
its transparency when voltage is applied. As seen in suspended particle glass, PDLC glass uses
suspended particles which rotate when voltage is applied, changing the optical properties of the
PDLC glass. Another similarity to suspended particle glass is that it requires voltage in order to
remain in its transparent state. Thus, the reasoning why it is not as commonly used as
electrochromic glass. PDLC glass also has a long transition period from either the opaque or
transparent state, which may have a duration of up to 15 min (Wong and Chan, 2014). Putting it
behind even suspended particle glass on the market.
PDLC glass consists of three layers. The center layer of which being composed of liquid
crystal droplets that are applied to a polymer matrix which is then cured. A transparent electrical
conductor film is placed on both sides of this layer, acting as electrodes. Glass substrates are then
placed on both sides of these three layers, forming the PDLC glass. An illustration of PDLC
glass can be seen in Fig. 3.
Much like suspended particle glass, when voltage is applied to PDLC glass the liquid
crystal molecules become aligned to allow light to pass through the glass. Then when the voltage
is gone, they will randomly relocate
themselves making the glass opaque.
The amount of voltage required is
dependent on the thickness of glass
and type of crystal polymer used.
When using a film with around 50%
liquid crystal droplets, the PDLC glass
will appear to be a milky white (Wong
and Chan, 2014).
Figure 3 Polymer Dispersed Liquid Crystal (PDLC) Glass Source: Glazette
Methodology
This paper, proposes the use of electrochromic glass as a more practical and efficient
substitute to traditional glass. Traditional glass being single pane and double pane glass
windows. To do so this study will be analyzing the overall cost of electrochromic glass,
including cost of purchasing, installation, maintenance, and money saved. Using data on smart
glass production company SAGE. Then comparing that cost to traditional glass cost to determine
if electrochromic glass is viable in the Tucson area. There are various ways to produce smart
glass, so each method will be analyzed in order to determine the most effective glass. Once these
values are determined Energy-10, a powerful energy simulation tool for buildings and homes,
will be used to determine if electrochromic glass is more efficient than single or double pane
glass.
Literary Review
Smart Glass and Its Potential Energy Savings by Wong and Chan was published in 2014
and is a comprehensive analysis of the various forms of smart glass and their real-world
application. In this paper, Wong and Chan discuss how smart glass differentiates itself from
other glass alternatives in the way that they can alter their physical properties to become more
efficient where they are utilized. This change is properties is engineered to alter the amount of
heat energy that can penetrate the glass, providing heating, cooling, and lighting cost savings as
only a desired amount of sunlight enters the building (Wong and Chan, 2014). Smart glass can be
classified under 3 types; electrochromic, suspended particle, and polymer dispersed liquid crystal
(PDLC). Each have their own mechanisms, advantages, and disadvantages.
Control criteria of electrochromic glasses for energy savings in Mediterranean buildings
refurbishment was written by authors Tavares, Bernardo, Gaspar, and Martins (2016). Revised
and accepted in 2016, the main focus of this publication is to determine the most suited control
strategy for electrochromic glass and their influence on internal loads in the Mediterranean
climate, while also taking into account influences on facade orientation (Tavares et al., 2016). To
do so they compare electrochromic glass to other glass alternatives using the ESP-r building
energy simulation software. This differentiates this study from others, as the goal of some
publications is to maximize the thermal comfort and the use of daylighting without regard to
reducing the energy consumption. While others are to maximize daylighting, and minimize the
electrical energy consumption for lighting.
Room temperature processing for solid-state electrochromic devices on single substrate:
From glass to flexible plastic is an article written by authors Cossari, Cannavale, Gambino, and
Gigli in 2016. These authors propose an alternative method of creating electrochromic glass that
is adopting a low-cost, eco-friendly, and has an effortless fabrication process. This new
electrochromic glass is fabricated on a single substrate and is composed of glass along with
flexible plastic (Cossari et al., 2016). That is how this article differentiates itself from others,
where instead of discussing existing types of smart glass, it proposes a new means of production.
While this article states how more efficient this new single substrate electrochromic glass is, it
does not state the actual difference in cost in fabrication or utilization compared to standard
electrochromic glass.
Service Life
Currently it is unknown what the estimated service life of smart glass is. According to the
smart glass manufacturer, The LTI Group, they have been testing the glass by turning it on and
off over 7 million times in the time span of ten years and still counting (LTI, 2017). Generally,
the life span depends on the amount of cycles, on and off, that the glass will go through, although
recent advances in productions technology have increased this shelf life to almost infinite (Wong
and Chan, 2014).
Maintenance
Maintaining smart glass is very similar to maintaining conventional glass except it
requires more care. It has a transparent coating on the external surface that keeps it cleaner for a
longer duration of time (Clayton Glass). It is crucial when cleaning this glass to use a nonabrasive sponge, non-metal window squeegee, a clean lint-free cloth, or chamois leather. If there
is any dirt or abrasive particles used when cleaning, the glass may scratch or there may be
damage on the coating. For cleaning products, soft water or mild glass-cleaning products should
be used. In cases where there are tough marks on the glass, vinegar is recommended, although
vinegar should not be used as a regular cleaning method (Clayton Glass). It is important to not
use chemical products, products with silicones or abrasive particles, and to always apply water
first before cleaning.
Drawbacks
As good as smart glass seems, it also comes with its drawbacks. Its main handicap being
its need for a power or voltage source. This alone has prevented the large-scale integration of
smart glass as a replacement to traditional glasses. What makes the replacement process difficult
is the need for additional wiring and circuitry to be added. Where without an energy source, the
overall effectiveness of smart glass is reduced significantly.
Smart glass is designed to reduce the transmittance of light in a designated area. When
used on a building, the cost of heating and air conditioning can be reduced based on the
utilization which is largest motive of its implementation (Wong and Chan, 2014). The amount of
voltage required to power smart glass is relatively low, although can be ‘on’ for long durations of
time depending on the location. Smart glass also has a shelf life, where it will fade over time.
This shelf life is dependent on how frequent it is used, although in recent years, new technology
has increased the shelf life to be almost infinite (Wong and Chan, 2014).
Being a new product, manufacturing and installation of smart glass is still picking up.
With only a few companies that specialize in smart glass, more companies in this technology
need to be developed in order to create a large-scale market possible. With more companies
producing smart glass, the overall cost of manufacturing and implementation will decrease.
Data
Building Model
To compare the different type of glass to determine which one is the most efficient, this
study will be using a residential house as the building model. The information gathered on this
house is from an energy audit report (Levya, 2016). This house is in Organ Pipe Cactus National
Monument and is a single-family home. This house lies in the same 2009 IECC Climate Zone as
Tucson which is 2B. The front faces north-east with a grass lawn and mesquite and palo verde
trees covering a patio. While the back has an attached patio, covered by a wooden awning,
protecting the house from direct sunlight. On the north side of this house there is an attached
garage which provides shade to the driveway.
Below in Fig. 4, the location and surrounding area is showed. This is crucial in
understanding what the surrounding area of the house looks like and any objects that may
obstruct direct sunlight from entering. Fig. 5 and Fig. 6 show a 3D model representation of the
house at different corners of the house. This is simply to help visually illustrate what the house
looks like and where the windows are located. Fig. 7 is the building schedule of the house. The
key elements to take away from this in terms using this house as a standard to residential houses
in Tucson is the house orientation, volume of conditioned space, area of walls and windows on
each
Figure 4 House Location
Source: (Levya, 2016)
side, composition of walls, floor, and roof, and its mechanical components.
Figure 5 3D Model of House, North-East corner
Figure 6 3D Model of House, South-West corner
Source: (Levya, 2016)
Source: (Levya, 2016)
House Building Schedule
Building: Name: House 39 Address: 39 Organ Pipe Dr, Ajo, AZ 85321
City: Location: Organ Pipe Cactus National Monument
Degree Days: HDD65=1,678 & CDD50=6,921
Utility Rates: Electric=0.06$/KWh
3
Climate Zone: 2009 IECC Climate Zone 2B
4
45°East of South
Orientation
5
Volume
Conditioned Space
17062.5ft^3
6
Areas
Roof North
812.5ft^2
Roof South
812.5ft^2
Conditioned Floor Area
1625ft^2
Exposed Concrete Area
379ft^2
Carpeted Area
520ft^2
Tiled Area
733ft^2
Walls
South
526ft^2
West
215ft^2
North
526ft^2
East
215ft^2
Total Walls
1482ft^2
Windows
South
87ft^2
West
11ft^2
North
105ft^2
East
30ft^2
Total Windows
233ft^2
Doors
146ft^2
Perimeter
186ft 4in
7
Ratios
Total Glass to Floor Area
0.14
South Glass to Floor Area
0.05
Surface to Volume Area
0.18
8
Roof: tin sheet, 2x4 wood frame, 16” o.c., built up, R-11 continuous
Insulation
insulation, no rad. barrier
Walls (Exterior): 1: 2x4, 16” o.c., 2: red brick, 3: slump block, insulated
Walls (Interior): 8in brick, dry wall
Slab: 6” earth contact slab on grade, no perimeter insulation (carpeted
and no carpet)
Doors: R-1.69 (U-value=0.59)
Windows: dbl clear, fiberglass w/o brk, U-value=0.55 all, S.C.=0.68,
VT=0.61, frame width=1.3”, SHGC=0.6 (all)
9
Roof: medium abs=0.56, Walls: medium abs=0.5
Shortwave
Reflectance
10 Infiltration
1 Air Change per Hour
11 Mechanical
Air conditioner + Gas Furnace + Evaporative Cooler
Efficiency: Cooling: SEER=14.0, Heating: AFUE=80%
Evaporative Cooler: CFM=7000
Thermostat: Occupied: Cool=77F, Heat=70F Setback: Cool=80F,
Heat=68F
Occupancy: 3 Residents
Figure 7 House Building Schedule
Source: (Levya, 2016)
1
2
Comparison
Using the physical properties of the different types of glasses in Fig. 8, a simulation was
run using Energy-10, a powerful energy simulation tool for buildings and homes. The goal is to
determine how efficient electrochromic glass is compared to single and double pane glass. This
can be done by knowing the U-factor, solar heat gain coefficient (SHGC), and the visual
transmittance of the different glasses.
Properties
Single
Clear
Double
Clear
Double EC SAGE (Smart Glass)
Clear
Coloured
U(Btu/hr-ft^2-F)
1.03
0.48
0.29
0.29
SHGC
0.82
0.76
0.48
0.09
T-vis
0.89
0.81
0.62
0.035
Figure 8 Glass Physical Properties
Source: (Levya, 2016)
After running the simulation over a timespan of a year, there is a clear deviation in
energy use (kBtu) and energy cost ($) for the different types of glass. Compared to using single
pane clear glass, there is a 5.52% percent decrease in energy use (kBtu) if double pane clear
glass is used. While there is an 8.78% decrease if Double EC SAGE glass is used. For energy
cost
($)
Energy Use (kBtu)
there
86000
is a
84000
82000
80000
78000
76000
74000
72000
Figure 9 EnergySingle
UseClear
(kBtu)
Double Clear
Double EC SAGE
7.21% decrease in using double pane clear glass instead of single pane clear glass. While there is
a 9.81% decrease if Double EC SAGE glass is used. The total savings in using double pane clear
glass is $93 annually and the total savings in using Double EC SAGE glass is $126 annually.
Energy Cost ($)
1300
1280
1260
1240
1220
1200
1180
1160
1140
1120
1100
1080
Single Clear
Double Clear
Double EC SAGE
Figure 10 Energy Cost ($)
Discussion
The goal of this paper was to determine if using electrochromic glass was more efficient
than conventional single and double pane glass in the Tucson area. In order to do so, this study
first identified what smart glass was and the various forms it comes in. Once obtaining the
physical properties of electrochromic glass, single pane, and double pane glass, the building
simulation software Energy-10 was used to obtain the efficiency of each glass. Using a building
model from Organ Pipe National Monument as my residential house model since it lies in the
same 2009 IECC Climate Zone as Tucson, which is 2B.
As shown in Fig. 9 and Fig. 10, upon running the simulation, it was found that Double
EC SAGE glass was the most efficient out of the three with an 8.78% decrease in energy use
(kBtu) and a 9.81% decrease in energy cost ($) compared to single pane clear glass. This comes
out to an annual savings of $126 if electrochromic glass is used in place of single pane glass. The
general cost of electrochromic glass is $100 for an 1x1 sq. ft^2 area. For the model used, this
comes out to a total of $3,849. Which for the model used, this would mean a return investment in
roughly 30 and a half years, excluding installation cost. The general rule of thumb when for
green building retrofit projects to save on energy is that they must have a return investment
within 7 years (USGBC). The current lifespan of electrochromic glass is unknown considering it
is a new technology but the smart glass manufacturer, The LTI Group, has stated that they have
been testing the glass by turning it on and off over 7 million times in the time span of ten years
and still counting (LTI, 2017).
Conclusion
Electrochromic glass, a type of smart glass, is a new technology that can be used in place
of conventional glass in order to save on energy usage and overall cost in electricity. This is
significant to the Tucson area as it has limited cloud coverage and long summer months. After
running the building simulation software Energy-10, it was found that electrochromic glass had a
9.81% decrease in energy cost compared to single pane glass. Resulting in a savings of $126
annually. While this seems good, the return investment is roughly 30 and a half years. With the
general rule of thumb for return investment being within 7 years, it is not recommended to
implement electrochromic glass in the Tucson area. Being a new technology, there is still more
room for improvement as more companies enter this field and new discoveries are being made.
With that bringing the cost of production to decline and its efficiency to increase. Meaning that
with time this new technology may become the new standard for conventional glass.
Limitations
The purpose of this study was to compile information on electrochromic glass and
determine if it is a viable alternative to current traditional windows. There is currently few
companies which produce smart glass such as SAGE Glass, meaning there is limited amount of
data available on electrochromic glass efficiency. In order to obtain the cost of installation a
quote from a smart glass company is needed, upon contacting various companies to obtain a
quote for this paper I was denied considering I am not a home owner actually looking to install
smart glass. Being a new technology, there is no proven track record of electrochromic glass,
making it hard to prove cost benefits (Malins, 2014).
Recommendations for Future Study
1) Electrochromic Glass Installation Quote
a. What is the cost of Installation?
b. New construction vs. conventional glass replacement?
c. Residential vs. commercial building?
2) Lifespan
a. Once it is discovered, what is the estimate lifespan?
b. What is the associated cost of replacing?
c. End cost comparison to conventional glass.
d. Durability in Tucson climate compared to different climate zones.
3) Cost Reduction
a. Change in the number of electrochromic glass production companies.
b. Is there a change in cost with more competition?
c. Any new methods of producing a more efficient and cheaper smart glass?
4) Case Studies
a. Obtain change in energy usage and cost from buildings that have implemented
electrochromic glass.
b. Due a comparison from different locations to determine if some electrochromic
glass is more efficient in other climate zones.
References
Clayton Glass. (2011). Smart Glass Self-cleaning glass. Tanfield Lea Industrial Estate.
Retrieved: https://claytonglass.files.wordpress.com/2011/09/sgg-smartglass-b1.pdf
Cossari, P., Cannavale, A., Gambino, S., Gigli, G. (2016) Room temperature processing for
solid-state electrochromic devices on single substrate: From glass to flexible plastic. Solar
Energy Materials and Solar Cells.
Levya, A., Eda, J., Lagunas, A., Wang, Y., Welch, M. (2016) Organ Pipe Cactus National
Monument Energy Audit.
LTI Smart Glass, Inc. (2017) The LTI Group: World Leader in Security, Architectural and
Decorative Glass.
Malins, L. (2014) Glass of the Future. Burnham-Moores Center for Real Estate. University of
San Diego.
SAGE Electrochromics, Inc. (2010) Performance Assessment of Sage Glass Electrochromic
Coatings and Control Scenarios. Paladino and Company.
Tavares, P., Bernardo, H., Gaspar, A., Martins, A. (2016) Control criteria of electrochromic
glasses for energy savings in Mediterranean buildings refurbishment. Solar Energy.
Wong, K., Chan, R. (2014) Smart Glass and Its Potential Energy Savings. Journal of Energy
Resources Technology.
Woodford, C. (2017) “Smart” windows (electrochromic glass). Retrieved from
http://www.explainthatstuff.com/electrochromic-windows.html.
Additional Resources
https://climate.nasa.gov/effects/
http://www.usgbc.org/articles/green-building-facts
http://www.glazette.com/
https://www.emaze.com/
http://www.visualmechanical.com/
https://modernize.com/home-ideas/32437/smart-windows-cost?v2