Understand Window Energy Ratings: U-Value, Lo-E Glass, & SHGC

ENERGY EFFICIENT WINDOWS: A GREEN INTRODUCTION

Going GREEN is one of the hottest trends in America and having energy efficient windows in your home is quickly becoming a must-have for many people.

Not only is the "green" movement near the top of many political agendas, its presence is often seen throughout the mainstream and minor media. The bottom line is this: green sells and going green is cool.

I feel the trend is positive. With energy costs reaching new heights, we need to be proactive and intentional. We should not only reduce the amount of energy we use, but also conserve the energy we don't. And since nearly 15-20% of all home energy is lost via windows and doors, having energy efficient windows in your home should be a priority.

The focus of this hub is to help explain (without getting too technical) the current energy ratings scheduled to today's energy efficicient windows. These ratings are crucial in making informed decisions regarding the purchase of energy efficient windows for new home construction or replacement.

With the green trend gaining momentum, an informed decision concerning one of the most critical areas of energy loss (or conservation) in your home is paramount.

I'll begin by explaining a few terms needing definition.

INSULATED GLASS: (IG)

Two or more individual panes of glass separated by a specified spacer bar system and then sealed to be air and water tight. The "captured" airspace between the panes of glass forms the insulating barrier. The majority of modern energy efficient window systems utilize some type of insulated glass (IG) application.

EMISSIVITY:

Emissivity is the capability of a surface to emit heat radiation. A black surface is often used as a constant in measuring other surfaces against it.

For example, in measuring the emissivity of a particular IG unit, the IG unit is placed next to a solid black surface and subjected to an identical heat source. Measurements of heat radiated from each surface are then taken. The lower the number results in better heat-reflecting capablity.

With relation to energy efficient window systems, lower emissivities are desired.

U-VALUE: (AND ITS RELATION TO R-VALUE)

U-Value is the measure of a window's ability to reduce heat loss during indirect radiation exposure; such as during the winter months in moderating climates. Lower U-values translate into less indirect heat lost from the interior of the home resulting in lower heating costs.

U-value is the inverse of R-value (a more common term used in the insulation business). To find a correlating R-value from a given U-value, simply divide the number 1 by the U-value. Lower U-values correlate to higher R-values. For example: 1 divided by a .50 U-value gives us an R-value of 2.00.

Lower U-values are important because many municipalities are adopting the 2006 version of the International Residential Code (IRC 06 for short). This code mandates all energy efficient window and exterior door units with IG to carry a minimum U-Value of .40, translating to an R-value of 2.5.

This may sound like a low insulating value but even the finest energy efficient windows today carry U-values hovering in the .22 through .30 ranges; thus correlating to R-values of 4.55 through 3.33.

And given the fact most exterior wall cavities are a minimum R-13 (with standard 2 x 4 framing) to R-19 (with 2 x 6 framing), the most energy efficient windows are 3 to 4 times less efficient than the wall they're installed in; thus the significance to upgrade building codes with regard to window glazing applications.

So, even with the large disparity between the wall vs. window insulating factor, improving U-values greatly increases the energy efficiency of the home.

SOLAR HEAT GAIN COEFFICIENT: (SHGC)

Solar Heat Gain Coefficient (SHGC) is a measure of a window's ability to reduce heat gain during direct radiation exposure; such as during the summer months in warmer climates. A lower SHGC translates into less direct heat being pulled into the home resulting in lower cooling costs. SHGC and U-value are closely linked since the lowering of one directly affects the other.

LOW EMISSIVITY: (LO-E)

Lo-E refers to the ability of an IG unit to suppress direct heat radiation and absorb indirect heat radiation. By placing a Lo-E coating, which usually consists of a microscopically thin layer of metallic oxides (primarily silver), on a glass surface, the ability to transfer heat radiation is lowered. The heat remains on the side of glass where it originated.

In a nutshell, Lo-E coatings reflect direct heat radiation and absorb indirect heat radiation.

FOR EXAMPLE:

I witnessed a demonstration where a standard IG unit 4 x 4 inches square was placed next to an identically sized IG unit with a low emissivity (Lo-E) layer applied. Both IG units were placed equal distance from an identical heat source with an air thermometer placed on the opposite (or inside) side; thereby simulating a warm-climate condition. The result was staggering.

On the standard IG unit without the Lo-E coating, the inside glass temperature was 7 degrees warmer than the inside glass temperature of the Lo-E coated unit. Imagine what a 7 degree difference would make over the entire glass square footage in your home; especially when you're trying to heat in the winter. -- Take into consideration this was an extreme demonstration. In reality, temperature differences for Lo-E coated IG units are nearer to 4-5 degrees; still a great amount on the thermostat.

Keep in mind the opposite of this demonstation is also true. Heat sources from the inside of your home during winter months will keep the inside of your glass warmer.

HOW DOES LO-E WORK?

Lo-E glass works based on the angle of direct solar radiation.

Due to the sun's differing angles at various times of the year, Lo-E coatings work well in all seasons. In summer, when the angle is more direct, or "a high sky" they reflect heat. In winter, when the sun's angle is less direct, "a lower sky" they absorb the indirect heat.

Referring back to the previous demonstration, the non Lo-E coated glass allowed the direct heat to pass through the glass thereby warming your interior glass.

The Lo-E coated glass, on the other hand allowed some of the indirect heat in but blocked the direct heat thereby keeping your interior surface of glass cooler.

And being cool in the summer and warm in the winter is a good thing.

I'd be remiss if I didn't add one detail concerning Lo-E applications in overall warmer climates; since they can be optimized with additional coatings. Let me first begin by briefly explaining what a glass surface is.

GLASS SURFACES:

All energy efficient windows with insulated glass are broken down into 4 surfaces:

Surface 1 (S1): The exterior surface of the exterior pane of glass.

Surface 2 (S2): The interior surface of the exterior pane of glass.

Surface 3 (S3): The exterior surface of the interior pane of glass.

Surface 4 (S4): The interior surface of the interior pane of glass.

These designations are important because the optimum effectiveness of the Lo-E coating is determined by which surface it is applied to.

As mentioned earlier, with Lo-E applications, the heat radiation remains on the side of glass where it originated.

For warmer climates, Lo-E coatings are sometimes applied on S3 and a secondary tint applied to S2 to reduce the initial heat radiation.

Such applications don't work as well in cooler climates since you want some of the initial heat to absorb in order to remain trapped once it tries to leave.

The ideal surface for a Lo-E coating in cooler climates is S2.

IN CONCLUSION:

Hopefully, I've shed a little light (no pun intended), on what those silly acronyms and numbers signify on today's energy efficient windows. No matter what climate you live in it's always a smart thing to lower your energy costs. And becoming knowledgeable concerning energy efficient window ratings is a great start. I look forward to any questions and comments.

A great resource to find the energy ratings of your windows can be found at the following website.

http://www.nfrc.org/getratings.aspx