The Complete Professional's Guide to Window Film Solar Performance

Have you ever been put on the spot by a customer asking a follow-up question on solar performance numbers? Did you feel uncomfortable or unsure about your answer? Do you think it may have been the reason you lost a particular sale? As a skilled window film professional, you understand that customers rely on a combination of intellectual and emotional responses when making a purchase.

Although there are literally thousands of consultants and self-help books that will make you more effective with the emotional trigger points to close the sale, you will not even get to step up to the plate without being able to describe how the product performs.

This comprehensive guide is written to better prepare you with the technical knowledge to handle all types of potential customers, all type of window tinting films, even the dreaded retired engineer who has the lethal combination of technical training and spare time!

Part 1: Solar Energy Fundamentals and Performance Metrics

How Solar Energy Travels Through Windows

The sun's energy travels those millions of miles to reach Earth the same way it passes through your customer's windows: radiation. Simply put, this is energy movement by electromagnetic waves. Just like the waves from a tossed pebble ripple out in a pond, the sun's energy moves via waves to get from one place to another.

This solar energy doesn't travel in just one big wave, but instead in many waves that differ from each other by wavelength. Here wavelength means the spacing, or length, between the waves. Back to our pond analogy, ripples that are close to each other have a short wavelength while those that are spread further apart have a longer wavelength.

Why is this important? Because different solar wavelengths give us different challenges to address with window film:

• Ultraviolet radiation (UV): The shortest wavelengths from the sun

• Visible light: The middle wavelengths from the sun

• Infrared heat/energy (IR): The longest wavelengths

Solar Energy Distribution Across Wavelengths

The amount of energy changes dramatically across the solar spectrum. Think about it in terms of your radio. When you turn the dial, you are moving from one wavelength to another in the radio portion of the electromagnetic spectrum. Some stations come in very clearly (have a lot of energy behind the broadcast), while others are very faint.

The same principle applies with the solar spectrum. Some wavelengths are not produced with as much energy from the sun or are absorbed in the atmosphere (like portions of the UV), while other wavelengths come through quite strong. The wavelength is measured in nanometers (nm), which is one billionth of a meter. Pretty small indeed!

Total Solar Energy Rejected (TSER) - The Foundation Metric

One of the most important performance numbers is Total Solar Energy Rejected (TSER). This metric describes the total amount of solar energy (UV + visible + IR) that is blocked, or rejected, from passing through the window.

Looking at the solar spectrum as piles of white sand, a TSER of 50% would mean that you shoveled away, or eliminated, half of the sand. This doesn't tell you which pile of sand you shoveled, but simply the total amount of sand that is now gone. Same with TSER—it doesn't tell you which wavelengths are being rejected, but it does tell you the total solar energy performance.

Solar Heat Gain Coefficient (SHGC)

Another way to describe total solar energy performance is with the Solar Heat Gain Coefficient (SHGC), defined in the NFRC 200-2004 standard. This term is common in the glass business and will be used more and more with window film in the future.

The connection between SHGC and TSER is very simple: TSER = (1 - SHGC) × 100%

So if you have a TSER = 40%, the SHGC = 0.6. The TSER can be thought of as the percentage of solar energy rejected (higher the better) while the SHGC is the fractional amount of solar energy that passes through (lower the better).

The Critical Importance of Spectral Selectivity

A film with a higher TSER is NOT always the better performing film. This is crucial to understand because you can always increase the TSER number by simply making a film darker or more reflective. Remember that TSER looks at UV, visible and IR together.

To make an apples-to-apples comparison, you need to look at films with the same visible light performance. Here's a powerful example: two glazing systems both have TSER = 50%. However, one system has a visible light transmittance (VLT) of 80% while the other has a VLT of 20%. These look identical from TSER perspective, but the 80% VLT film is a much higher performing film because it is spectrally selective—it blocks out more IR than visible light.

If we took the 80% VLT system and tinted it down to the same 20% level, without rejecting any additional UV or IR energy, the TSER number would increase to approximately 80%.

Luminous Efficacy - The Comparison Tool

Luminous Efficacy was created to help people make fair comparisons because it is defined as the VLT divided by the Solar Heat Gain Coefficient. (Note: Luminous Efficacy is sometimes incorrectly defined as VLT divided by Shading Coefficient, so make sure to double check how it is being used).

The higher the Luminous Efficacy, the better you are doing at rejecting the IR heat and letting the visible light through. This is particularly important since studies show that up to 30-40% of the total electricity bill for commercial buildings is interior lighting. Daylighting will become increasingly important in the future.

Visible Light Transmittance (VLT) - Understanding Tint Level

You can get a pretty good idea of the VLT simply by looking at how dark the film makes the window. Visible light transmittance is defined within NFRC 200-2004. Simply put, VLT is the total amount of the visible energy (sunlight from 380-780 nm) that makes it through your filmed window.

Glare Reduction - The Trade-off Reality

You cannot make a window film with high VLT and good glare reduction without a breakthrough invention, since these two performance metrics are exactly opposite each other. Improving one always lowers the other with today's technology.

Glare reduction is defined as the percentage of VLT reduced when film is added to the window: Glare Reduction = 100% × [VLT(glass) - VLT(filmed glass)] / VLT(glass)

It simply tells you the percentage of tint your film adds to the window.

Heat Gain Reduction - Measuring Actual Improvement

When customers ask how much difference window film makes in improving their window's solar performance, simply looking at TSER or SHGC can be misleading because an average performing film on a great window can get a better number than a great performing film on a poor window. They really don't tell you how much of an improvement the window film made.

Shading coefficient helps somewhat because it compares the SHGC for your filmed window to the case of 1/8" clear glass (SHGC = 0.87). But what if your window isn't 1/8" clear glass?

Heat Gain Reduction answers this question directly because, analogous to glare reduction, it tells you the percent improvement of solar energy rejection when window film is added: Heat Gain Reduction = 100% × [SHGC(glass) - SHGC(filmed glass)] / SHGC(glass)

A Heat Gain Reduction of 20% means you are reducing 20% more of the solar energy when film is added. It's that simple.

Why We Don't Publish Certain Numbers

You'll see other companies publish numbers for Total Solar Energy Absorbed, Total Solar Energy Reflected and Total Solar Energy Transmitted. Here's why these numbers can be misleading:

Total Solar Energy Reflected plus Total Solar Energy Absorbed doesn't equal the TSER, even though you would think so since energy can only be reflected or absorbed, because again some of the energy that is absorbed re-radiates into the room.

The only number that tells you something more about the film is Total Solar Energy Absorbed because it helps you understand how much the glass may be heating up from the film absorbing energy. This gives you an indication of potential glass breakage. However, we calculate the exact center of glass temperature increase with film added to the glass and use this number as part of glass warranty programs.

Reflectivity - Understanding the Mirror Effect

Clear glass by itself has a reflection of visible light around 8%. Whether or not this means you can see a reflection off the window depends on if the light is brighter outside or inside. Traditionally, adding window film increased the visible reflectivity of the glass.

The definition is straightforward:

• Visible light reflection of 10% from the interior (Rint) means the glass is slightly more reflective inside and therefore you will see a little bit more of a reflection at night

• Visible light reflection of 50% from the exterior (Rext) means the glass will really have a commercial mirror-like appearance during the day

Notice that at night, however, this goes away. Again, it all depends on whether it is brighter inside or out.

UV Rejection - The Straightforward Metric

Part 2: Advanced Heat Transfer Concepts

The Critical Distinction: Infrared vs. Heat Rejection

This is a very important point to remember. Although it is true that human skin senses IR wavelengths as heat, these IR wavelengths do not all come directly from the sun. Infrared energy comes in two forms:

1. Near infrared: The solar infrared wavelengths

2. Far infrared: The wavelengths emitted inside a building or car as energy is absorbed within

The key point here is that

Bottom line: Near infrared rejection is an indicator of solar comfort, but you must look at the total solar energy (UV + visible + IR) when making claims about heat rejection. The most direct way to do so is with Total Solar Energy Rejected for overall glazing performance or Heat Gain Reduction to capture the added benefit of the window film.

Understanding Far Infrared Heat

The far infrared wavelengths carry the emitted energy from a heated building or car. The solar wavelengths include UV, visible and near infrared and span the range from about 300 nm to 2500 nm. The far infrared wavelengths are out beyond that point and from a practical standpoint go from about 5000 nm (or the equivalent 5 microns) to around 50 microns.

When one refers to a Low-E window or window film, this simply means that the far infrared wavelengths are partially blocked from passing through the glazing. This is most important in cold weather climates where you want to prevent heat from escaping through the windows by radiation.

Emissivity is the parameter used to describe this performance. The lower the emissivity, the better the glazing system is at blocking the far infrared wavelengths.

Three Methods of Heat Transport

Heat passes through a window in three ways:

1. Radiation: The movement of heat by waves, such as the sun's energy. Window film is very effective at blocking this solar radiation—this is why we emphasized this area in Part 1 of this review.

2. Convection: The transport of heat by fluid (e.g. air) movement. This tends to be a minor effect through the window due to the tight seals in most windows today, but it is important for areas like HVAC design.

3. Conduction: The movement of heat through a material due to the temperature difference between a hot and cold region. This is an important element in the thermal performance of a window and is measured by the U Value.

U-Value and R-Value Explained

U-Value represents the flow of heat so the lower the number, the better the glazing system is at insulating against heat loss (or gain) due to conduction. It is important to note that the U-Value is primarily determined by the design of the window.

Performance comparison:

• The best window films for improving cold weather insulation reduce the U-Value by about 20%

• Most window films change this value by only a few percent

• Going from a single glass unit to an IGU reduces the U-Value by over 50%

  
  

R-Value is another term that you hear more frequently for describing insulation performance. The big pink fiberglass batts that you see at home improvement stores are rated in R values. Although not commonly used for window films, one can compute the equivalent R Value with the equation: R Value = 1 / U Value

Heat Loss Reduction - Measuring Conductive Improvement

Just like the benefit of window film on blocking heat is clearly identified with Heat Gain Reduction, Heat Loss Reduction measures the percent improvement in reducing heat loss when window film is added: Heat Loss Reduction = 100% × [U Value(glass) - U Value(filmed glass)] / U Value(glass)

A Heat Loss Reduction of 20% means you are reducing 20% more of the building's energy from escaping through the glass when film is added.

Year-Round Sales Strategy

If you haven't done so previously, make sure you look into decorative film lines like 3M Fasara. These products move year round because they primarily address privacy, daylighting or design issues with glass. There is no seasonality to this market and the films offer very strong benefits over the alternative, etched glass. Architects and interior designers absolutely love these products when they become aware of them, so make sure to build your network in this area and you will see definite results.

Part 3: Low-E Technology and Advanced Applications

Understanding Low-E Coatings

Low-emittance coatings, or Low-E for short, are microscopically thin metal or metallic oxide coatings deposited on glass to reduce the transport of heat through the window from regions of hot to cold.

In climates that require heating, this means you can save energy by partially reflecting the room heat back into the building versus losing it to the cold exterior. In climates that require cooling, this means that you can save energy by partially reflecting the outdoor heat back outside versus having it enter the air conditioned interior of the building.

How Low-E Coatings Work

Low-E coatings are designed to reflect far infrared energy. Remember that infrared energy comes in two forms: (1) near infrared, or the solar infrared wavelengths, and (2) far infrared, or the wavelengths emitted inside a building or car as energy is absorbed within.

The solar wavelengths include UV, visible and near infrared and span the range from about 300 nm to 2500 nm. The far infrared wavelengths are out beyond that point and from a practical standpoint go from about 5000 nm, or the equivalent 5 microns, to around 50 microns. So if you reflect emitted energy, you reduce the amount of heat transfer and increase the energy efficiency.

Low-E Performance Reality Check

Does a Low-E window mean that you are getting the best energy-performing window, i.e. adding window film gives no benefit?

No. Low-E is simply referring to a window that has a coating designed to partially block the far infrared wavelengths. Emissivity is the parameter used to describe this performance—the lower the emissivity, the better the glazing system is at blocking the far infrared wavelengths.

However, emissivity says nothing about the ability of the window to block the sun's heat (total solar energy rejected), and does not even fully describe the insulating performance of the window (the U-value). Remember that the U-value represents the flow of heat due to the temperature difference between a hot and cold region, so this insulating performance is also affected by other factors such as the design of the window (e.g. single vs. double pane) and the type of gas contained within the insulated glass unit (e.g. air vs. krypton).

Low-E Solar Energy Performance

Low-E windows do provide some rejection of solar energy, the actual amount depending on the types of materials and the number of coating layers. One needs to refer to the published solar heat rejection number (SHGC or TSER) to determine the actual performance.

The two major categories of Low-E technologies are:

1. Pyrolytic (hard coatings): More durable

2. Sputtered (soft coatings): More fragile but block more solar energy

   
   

Identifying Low-E Coatings

Simple Method: Place a lit match or small flashlight near the window, with the room darkened, and look at the multiple reflected images. The color of the image will change on the surface that has the Low-E coating. The type of color change will indicate the type of coating for an experienced viewer.

Professional Method: Utilize a handheld Low-E detector and perform the measurement on the window directly. This is more accurate and error-proof.

Window Film and Low-E Combinations

Window films will do little to nothing to change the insulative performance of Low-E windows (U-value) but can make a noticeable difference on the solar heat gain (SHGC). This solar heat gain benefit is most prevalent with pyrolytic coatings and decreases based on the number of silver layers used for sputtered glass.

However, even in the case of a high performance Low-E window, the application of window film can offer other customer benefits such as glare control or UV blockage even though there is little heat rejection gain.

Part 4: NFRC Certification and Standards

Why NFRC Certification Matters

The NFRC is a non-profit organization developed in 1989. The purpose of the organization is to standardize energy performance requirements on glazing such that consumers can receive accurate and reliable information. NFRC certification information for window films can be found in the certification products directory on the NFRC website at www.nfrc.org.

Understanding Different Performance Numbers

The NFRC website lists certified solar performance numbers for window films by type of glass. This is important because the NFRC values are not the same as what is published in traditional sample cards. The reason for this is an apples-to-oranges comparison.

NFRC Values: NFRC has always certified completely assembled products, such as a window or a door. Thus the specifications that the NFRC prints are "whole window" values that include a specific size of window and frame type. Since light does not transmit through the frame, just the fraction of the window that is glass, the VLT number will be lower for NFRC. The same reasoning applies for SHGC.

Traditional Film Values: Window film companies traditionally publish "center-of-glass" values that are not dependent on the size of the window or frame type.

The Importance of Whole Window vs. Center-of-Glass

Energy savings is quickly becoming the biggest focus in the world today. With the continued push for energy savings, the federal government, state governments, and utility companies have begun to offer more and more energy savings rebates. There are many rebates available for upgrading window performance, of which window film plays a vital role.

Typically energy rebates for windows are based on two specifications: the SHGC and Luminous Efficacy.

Why is this important? The specifications for building codes and some energy rebates are given in whole window values versus center-of-glass values. But remember that whole window values will have a lower SHGC, and a lower VLT, than center-of-glass values. Thus it is possible for a window film to qualify for a rebate even though it misses the target at first blush based on sample card data. This same consideration needs to be applied to certification procedures, such as LEED.

Getting Accurate Values for Your Projects

In order to help navigate through this issue more clearly, whole window values have been calculated for six different "standard" (as defined by the NFRC) windows with all NFRC certified films. These values can be obtained on the NFRC website under the certified products directory.

To get exact whole window values for your specific windows of interest, contact your technical service representative for the necessary calculation. The information needed to perform this analysis will be:

• Size of window

• Frame type

• Glass type

Summary

Armed with this comprehensive understanding of solar performance metrics, you can confidently address any technical question from customers, make accurate product recommendations, and identify opportunities for energy rebates and building code compliance. Whether you're dealing with basic residential applications or complex commercial projects involving Low-E glass and NFRC requirements, this knowledge foundation will help you succeed in even the most challenging sales situations.

Remember: the key to successful technical sales is not just knowing the numbers, but understanding what they mean and how they relate to your customer's specific needs and goals.