The Anatomy Of An Energy Efficient Window

PART 1: WINDOW GLASS

Every year, the United States loses $50 billion to energy losses through windows. That’s equivalent to about 4 to 5 percent of the

country’s total annual energy consumption.

You might think that that doesn’t amount to much on a per-household level, but you’ll be surprised at the impact switching to more energy-efficient win-dows can have on your energy consumption and carbon footprint. Check out these figures from ENERGY STAR:

How much can you save each year by switching toENERGY STAR qualified windows?

PART 1: WINDOW GLASS

PART 1: WINDOW GLASS

That’s per household, per year.

But what makes a window energy-efficient? The short answer is EVERYTHING. That is, every part of the window pitches in. But of all the different window com-ponents, window glass or glazing contributes the most to energy efficiency.

Window glass can be grouped into different types: toned, low-emissivity, laminated, and insulated. The techniques used to increase the energy effi-ciency of ordinary window glass can also be combined for better performance.

TONED

Color additives are used during the manufacturing process to impart dif-ferent tones to window glass. This affects the glazing’s solar heat gain coefficient and how much light can be transmitted through it. Different tones are available, but blue, green, and bronze are the most common.

If a heavier pigment is used, this “supertones” the window glass. Supertoned glazing filters out near-infrared types of light, meaning it can lower solar heat gain without reducing the amount of visible light that enters your living spaces.

PART 1: WINDOW GLASS

LOW-EMISSIVITY

Low-e glass is window glass coated with a protective film set within an insu-lating cavity. It functions like supertoned glass in that it reduces the amount of UV light that passes through a window to lower solar heat gain without impeding light transmission. However, low-e glass achieves this effect with-out the use of pigments, meaning the glazing remains colorless.

Low-e glass is known to increase the cost of a window by 10 to 15 percent, but it gets the Department of Energy’s vote because it can cut energy loss by a whopping 30 to 50 percent.

LAMINATED

A plastic glazing layer known as an interlayer is used to impart protective and energy-efficient properties to this type of window glass. The window’s impact resistance, energy efficiency, and overall performance in this case are functions of the kind of interlayer used.

PART 1: WINDOW GLASS

INSULATED

Insulated glass units are more commonly referred to as double-glazed or triple-glazed windows. In the case of insulated glass, it is the space between the glass panes and the gas sealed within the cavity that primarily contrib-utes to the energy efficiency of the window, unless of course the glass itself was treated (e.g. given a low-e coating) prior to assembly.

. . . . . .

Gas fills, the gases trapped between the panes in insulated glass systems, are used to improve windows’ thermal efficiency. Thick and slow-moving, these gases reduce air convection within the window cavity, lowering heat transfer. Different types of gases have been used as gas fills (e.g. carbon di-oxide, sulfur hexafluoride, xenon), but today the most common options are krypton and argon.

Which one is better?

Both are colorless, odorless, and non-toxic, but krypton, which has a con-ductivity of 0.0051, has better insulating properties, making it more expen-sive than argon, which has a conductivity of 0.0092. Generally speaking, there is a $100 price difference between a krypton-filled window and an argon-filled window with similar features.

The plus side to using argon is that it is more readily available and is ob-tained on the cheap as a byproduct of atmospheric oxygen extraction.

Some manufacturers combine the two gases to create a balance between performance and cost.

PART 2: GAS FILL AND SPACER

What happens when the gas fill is gone?

According to the National Glass Association, gas fill depressurization or leakage is slow but inevitable. Still, as long as at least 80% of the gas fill re-mains, you can expect a window to stay energy-efficient. If, say, a window’s gas fill is leaking at a rate of 1% per year (the predicted maximum rate), that window will still be energy-efficient for up to 20 years.

The exception will be if you start to see condensation between your window panes. By the time you spot condensation, your window will no longer have an effective thermal barrier inside it, meaning its energy efficiency rating will go down. A window can still perform well to some extent, but it just won’t be as effective as it was before its gas fill was compromised.

What about spacers?

Spacers separate the glass panes that serve as boundaries for the gas-filled cavity in the middle.

They can be made out of fiberglass, steel, foam, aluminum, or a combination of materials.

Apart from separating window glazing, spacers are designed to seal in the gas fill, reduce condensation, and provide extra protection against heat flow. Low-conductance foam spacers have the best thermal performance, but even spacers made out of conductive metals can deliver excellent insu-lation benefits, provided they are well-designed.

PART 2: GAS FILL AND SPACER

PART 3: WINDOW FRAME AND OPERATION

While the main function of a window frame is to provide structural support for the other components, they also contribute to the overall energy effi-ciency of a window. How much they contribute depends on the frame ma-terial’s level of thermal resistance.

Generally, composite, fiberglass, wood, and vinyl make better choices if your goal is to maximize energy efficiency. Metal frames conduct heat much too rapidly, which can negate the effects of insulating glazings, gas fills, and spacers. Some metal frames, however, feature a thermal break, a plastic in-sulating strip placed between the frame and the sash, which disrupts heat flow and increases the frame’s energy efficiency.

So which frame should you choose?

The choice of material is highly subjective, but if energy efficiency is your main goal, make sure the frame you choose contributes the least to the win-dow’s overall U-factor, which is the rate at which non-solar heat is transmit-ted. The lower the U-factor is, the better your window will be at blocking unwanted heat transfer.

PART 3: WINDOW FRAME AND OPERATION

But did you know that how you use a window alsocontributes to its energy efficiency?

A window must be properly sealed to keep it from becoming an energy drain. It needs to effectively keep outside air out and indoor air in to keep your HVAC system from getting overworked. This means that a window op-eration type that allows for the lowest possible air leakage rate is your ideal option.

Window Operation Types: A Quick Guide

Air leakage is lowest when a window is fixed, such as the case with a picture window, and highest in single-hung, double-hung, and sliding windows. Awning, casement, and hopper windows are in the middle ground.

Final Thoughts

Whatever kind of glazing, gas fill, spacer, frame, and operation you choose, remember that a window is always greater than the sum of its parts. And however energy-efficient a window may be when fresh from the factory floor, it still has to be installed properly by a licensed professional.

Renewal by Andersen of Denverand Colorado Springs

1401 W Bayaud Ave #5 Denver, CO 80223

Denver(303) 968-3287

Colorado Springs(719) 313-5403

www.LoveYourWindows.com