November, 2007

What's Green About Windows?

What’s Green About Windows? When you deliberately punch large openings in perfectly good walls, you’d better have quality window units to install.

By John Wagner

Household energy costs in most U.S. homes could be reduced by up to 15% by installing more energy-efficient windows. In fact, an average household spends more than 40% of its annual energy budget on heating and cooling costs. That 15% energy-savings equates to real cash, to say nothing of the environmental costs incurred when generating the energy required for heating and cooling America’s homes.

Today, windows account for 3.5 quadrillion BTUs in U.S. energy consumption, at a cost of $20 billion per year. If all residential windows in the U.S. were replaced with the efficient Energy Star qualifying models, we’d save $7 billion in energy costs over the next 15 years.

Since reducing energy consumption is such a worthy goal for new and remodeled homes, a key green indicator of any home is the ability of windows to block the transfer of heat in any form, either streaming in during summer or out during winter. To put this in perspective, think about the role windows play: When building or remodeling, contractors go to great lengths to build walls that create a high-integrity thermal envelope. But then they go and punch large holes in those walls for the windows. They’d better have quality units to insert in those openings, or all the work done on the wall is wasted.

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Installing windows is a fairly simple task. With basic building skills, a can of spray foam, and a tube of silicone, it’s easy to get a quality thermal connection between the house frame and the window frame. So most often, the quality of the window is the single greatest variable in the wall’s overall performance. So, let’s take a look at what makes a great window in the hope that this information prepares you with good answers and recommendations any time a customer comes into your yard saying, "I need to buy a green window.”

What Makes a Great Window?

The Frame. Aluminum window frames transfer heat and cold very efficiently, so—especially in cold climates—you should steer buyers to wood frames, but look for products that are clad with aluminum or vinyl. Even fiberglass, which is very durable, is better than aluminum.

Selling tips:

• When selling wood frames, avoid finger-jointed wood frames, unless they are clad. For unclad windows, clear stock is best, because there are no finger joints or knots to admit moisture.
• Fiberglass offers good thermal resistance, and fiberglass is highly resistant to the weather. Plus, there is almost no differential between the expansion and contraction rates of fiberglass and glass. This drives down the chance of broken air seals because they move at the same rate when heating up or cooling down.
• On vinyl frames, search for the gold AAMA label, which certifies the window is a quality vinyl product that has met pre-set tolerances for resistance to wind and water.
• Aluminum is a tough sell as a green window. Though aluminum is light and strong, it is so conductive that it’s hard to get a good thermal-resistant product.

The Cladding. Cladding is usually aluminum or vinyl that covers wood. Since the wood beneath cladding is usually fingerjointed, it’s important that the cladding not admit wind or water. There should also be a strong connection between the cladding and the frame, as this will extend the life of the window.

Selling tips:

• Cladding is essential for casement windows, which can get lots of weather, especially if there is no overhang to protect the window when it rains.
• Aluminum cladding usually comes in white, off-white, and brown, but it can be custom colored.

Edge Spacers and Insulated glass. The air between the panes is the main insulator in insulated glass unit (IGU) multipane windows, the most common window type sold today. But good thermal resistance also depends heavily on the edge spacers that separate the panes, which are often made of aluminum.

Selling tips:

• Look for "warm-edge technology,” where a stainless-edge spacer is embedded in polyisobutylene. Polyisobutylene is a synthetic rubber, or elastomer, that is impermeable to any gas. Since it is elastic, it flexes with the temperature yet doesn’t break the seal with the window.

Gas fills and Low-E Coatings. Windows that are resistant to heat transfer (low U-factor) have inert gasses such as argon or krypton between the panes. Low-emittance (Low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers on a window surface that reduce the U-factor by suppressing radiant heat. Windows manufactured with low-E films typically cost about 10% to 15% more than windows without the films, but they can reduce energy loss by up to 50%. (Low-E coated glass can also reduce harmful UV rays by up to 75%, and that can protect items inside a home from fading.)

Selling tips:

• Any window that has an Energy Star rating or that wants to qualify as a green window must have gas fills and low-E coatings. They are must-haves.

How to Read a Window Label

The National Fenestration Rating Council (NFRC)—an industry group that reviews standardized information about window performance—has created a performance label for windows to guide window choices. The label rates the windows for energy efficiency and provides an apples-to-apples system for comparing one window to another. Depending on your location and your local weather, you will want to adjust your window’s performance to your environment and surroundings, especially heat gain through sunlight in warmer areas and heat loss through the window in the winter. Each NFRC label contains ratings that are easy to read once you have some guidance. First, the presence of the NFRC Certified label indicates that the NFRC has licensed the manufacturer and certified the window’s performance. In the upper right part of every label, you will find the manufacturer, model, style, and the materials used in its construction (such as "aluminum-clad”) to indicate the outward-facing and inward-facing components that make up the window. How the window opens is also indicated, such as "vertical slider.” However, that information isn’t as important as the characteristics that can affect performance, so let’s take a close look at those.

Performance Ratings. The NFRC label (which will always be found on Energy Star-qualified windows) provides performance ratings in a number of categories:

• U-Factor measures the rate of heat transfer and tells you how well the window insulates. The U-factor is like the R-rating of insulation, but with windows, the lower the U-Factor, the better the window insulates. U-Factor values generally range from 0.25 to 1.25. In areas where you spend most of your energy dollars on heat, the U-factor is the number you should pay the most attention to.
• Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy admitted and tells you how well the product blocks heat caused by sunlight. The lower the SHGC, the less solar heat the window transmits. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. In areas where you spend most of your energy dollars on cooling, the SHGC number is the one that is most important.
• Visible Transmittance (VT) measures the amount of light a window lets through. The higher the VT, the more light you see. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80.
• For target numbers, a quality window will have a U-factor of around 0.32, a SHGC of around 0.27, and a VT of around 0.46. Look for a 20-year warranty (or more) on the window. Anything less is a signal that the window isn’t of the highest quality.

Here are some other factors to look at:

• Air Leakage (AL) measures the rate at which air passes through cracks in the window. AL is measured in cubic feet of air passing through 1 sq. ft. of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cfm/ft2.
• Condensation Resistance measures how well the window resists water build-up. The higher the condensation resistance factor, the less build-up the window allows. Condensation resistance is scored on a scale from 0 to 100. The Condensation Resistance rating is useful when comparing different products, but Energy Star qualification is based only on U-Factor and SHGC.
• DP. DP (design pressure) is a rating of the window’s ability to resist wind. The higher the DP, the more resistant the window is to wind and wind-driven rain. DP is also a good indication of the overall quality of the window, whether you expect a hurricane or not.
• Impact-Resistant Glass. Just like the glass in your car windshield, impact-resistant glass is made by placing plastic film between pieces of glass. That same film can also block UV light.

When selling windows, make note of the material used in the frame, the cladding, the quality of the cladding-to-window connection, and look for the Energy Star label. For more information about windows, visit NFRC.org or search "windows” at EPA.gov.