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Heat gain and loss through windows accounts for up to 50 percent of a home’s heating and cooling needs. Not only do high-efficiency windows save energy, but they can also save homeowners up to 15 percent on energy bills. If all residential windows in the United States were replaced with ENERGY STAR models, Americans could save over seven billion dollars in energy costs over the next 15 years.

High-efficiency windows were initially much more expensive than the market standard, but the cost difference has continued to decrease as options expand. Multiple glazings, low-e coatings, warm-edge spacers, insulating sashes, and inert gas fills have all come down in price in recent years and today, it is more cost-effective then ever before to choose energy-efficient windows. While some energy-efficient measures can be postponed when first costs are an issue, those embedded in the infrastructure of a house – such as windows, must be included from the start. However, unless buyers understand the technology and the long-term benefits, they may not approve the higher costs.

Changing Market Conditions.

Believe it or not, we only have to go back to 1997 to find market share for efficient residential windows at a mere 12 percent. The ENERGY STAR program was not yet labeling windows and there was little information in the marketplace regarding what makes a window truly ‘efficient.’

Using their core strategies of marketing, sales training, technical assistance and incentives to raise awareness and affect behaviors of key players such as window manufacturers, builders, retailers and distributors the Northwest Energy Efficiency Alliance (NEEA) surpassed their goal of 54 percent market share one year early in 2000, and by 2002 it had reached 75 percent, increasing window efficiency awareness to it’s  highest level ever in the U.S. This market transformation project titled the Northwest ENERGY STAR Residential Windows Program resulted in the installation of energy efficient windows that saved 3 aMW per year, and are expected to save 35 aMW by 2010.

Read The Label.

Window labels are an important information source. However, since windows are made up of a variety of materials – framing, glazing, and insulating layers of air or inert gases, a rating of the entire window gives a better indication of actual performance than a rating of the glass area only. If you are checking specifications, be sure you know whether they apply to the whole unit or just the glazing.

As discussed in my previous post, The NFRC Window Label makes it easier to compare the performance of windows as a whole system, including the frame. Remember, it is a voluntary label, so it does not appear on all window products, but NFRC certification is required in many states.

A reasonable selection criterion should be that the products have at least an NFRC and Energy Star label. Homeowners should understand that building codes specify minimum levels of energy-efficiency and it often makes sense to exceed the code in order to reduce long-term energy costs and increase occupant comfort.

ENERGY STAR Windows, Doors & Skylights.

ENERGY STAR is a joint program of the U.S. Department of Energy and the Environmental Protection Agency. It labels products that are more efficient than equivalent, standard efficiency products. Windows, appliances, heating and cooling products, compact fluorescent light bulbs, and even whole houses are labeled with the ENERGY STAR logo.

The ENERGY STAR Windows program is designed to help consumers identify efficient windows, doors, and skylights for their climates. The program partners with manufacturers, utilities, retailers, and other organizations to leverage market incentives.

Product recommendations and qualification criteria are given for Four Climate Zones: a mostly heating zone (Northern), two heating and cooling zones (North-Central and South-Central) and a mostly cooling zone (Southern).

ENERGY STAR Framing Materials.

· Fiberglass frames are strong, durable, low maintenance, and provide good insulation. Fiberglass frames can be either hollow or filled with foam insulation.

· Vinyl frames are low maintenance and provide good thermal insulation. Sections may be hollow or filled with foam insulation. Wide vinyl sills may be reinforced with metal or wood.

· Aluminum frames are durable, low maintenance, recyclable, and typically have at least 15 percent recycled content. Frame design typically includes thermal breaks to reduce conductive heat loss through the metal.

· Wood frames are strong, provide good insulation, and are generally favored in historical neighborhoods. The exterior surfaces of many wood windows are clad (or covered) with aluminum or vinyl to reduce maintenance.

· Combination frames use different materials separately throughout the frame and sash to provide optimal performance. For example, the exterior half of a frame could be vinyl while the interior half could be wood.

· Composite frames are made of various materials that have been blended together through manufacturing processes to create durable, low maintenance, well-insulated windows.

Window Review.

During the heating season, there is a large temperature difference between the inside air and the outside air. Cool air descends and hot air rises. If a window is cold while the interior air is warm, there is a tendency for the window glass to cool the air adjacent to it. As warm air hits a window surface that is cool relative to the adjacent wall, the warm air is cooled and drops toward the floor, causing drafts. During the cooling season, although the outside air temperature feels hot at 90°F and above, the differences between outside air temperature, inside air temperature, and body temperature are much smaller. This slows air movement, so drafts are much less noticeable. Windows with a higher U-factor reduce these temperature differences, slowing the heat transfer process and improving occupant comfort. (U-factor indicates the flow of nonsolar heat through windows, expressed in Btu/per hour/ft2/°F. U-factor is the inverse of R-value. The greater the heat loss, the higher the U-factor.) What many people perceive as a draft is a combination of the cool air falling and the heat loss by thermal radiation from their warm skin to the cold glass surface. (Remember that for single-pane windows, the inside surface of the glass is closer to the outdoor temperature than to the indoor temperature).

Windows that face the sun heat the house with direct gain. Older windows, though they let in solar heat, also lose a lot of heat through air leaks and poor insulation. Today, windows sold for cold climates let in solar heat yet are well insulated and airtight.

Condensation can also be a problem on single-pane windows when the outside air temperature is low and indoor temperature and humidity are high. Windows with a lower U-factor allow the inside pane of glass to remain warmer, and the probability for condensation goes down. Condensation appearing between the panes of insulated glass indicates a failed seal and requires replacement of the insulated glazing, or the whole window, depending on the window design.

Insulation reduces heat transfer by creating dead air space. Still air is a poor conductor of temperature, so the more still air that is trapped, the slower the transfer. While a thin layer of still air on the surface of a single-pane window provides insulating value, multiple-pane windows trap relatively large volumes of still air, significantly increasing the insulating value.

The solar heat gain coefficient (SHGC) is a fraction of incident solar radiant energy that enters through the glass as interior heat gain. The higher the SHGC, the greater the heat gain. SHGC includes both frame and glass elements. SHGC is composed of two parts. The first part is the directly transmitted solar radiation, and the second part is the fraction of the absorbed radiation that enters as additional heat. If direct solar transmittance is low, but the inward flowing portion of the absorbed radiation is high, SHGC will be fairly high while the visible transmittance is low, just the opposite of what is generally desired in hot climates.

The shading coefficient (SC) is an older indicator of solar impact on glazing. ‘Shading coefficient’ is a bit of a misnomer, since the higher the value, the more heat is transferred, not shaded. It’s a measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8″ double- strength, single glass. It is equal to the SHGC multiplied by 1.15 and is expressed as a number without units between 0 and 1.

Options.

Energy-efficient windows use a variety of technologies to reduce conduction, convection, and radiation, and to absorb or reflect the sunlight, which is part of the radiation. The most effective technologies available for residential windows include multiple glazings, insulated frame materials and designs, spacer technology, low-e and solar-control coatings, insulating spaces filled with inert gases, and reduced air leakage.

Multiple Glazing, Frame Design, and Spacers.

Compared to single panes, the first double-pane windows in aluminum frames saved around 25 percent of the energy used to heat a home. Because the indoor glass temperature itself was warmer and convection currents were reduced, the probability of discomfort near the windows dropped from slightly more than 60 percent to less than 40 percent. Further improvements have brought even more savings. Unfortunately, aluminum frames are also excellent conductors of heat. The edge-effect of the conductivity of the frame contributes to increased heat loss around the window perimeter in a band about 2 1/2 inches wide; hence the smaller the window, the larger the relative problem.

In early versions of thermal-pane (or double-pane) windows, the temperature difference often resulted in condensation. Moisture collected where the frame met the wallboard, and it stained and deteriorated the drywall and surrounding areas, or worse. A substantial amount of infiltration still entered the home through and around the window framing and closures. The simple double-pane, aluminum-framed windows sold today typically have a thermal break to reduce condensation woes. Moving away from metal frames to wood, vinyl, fiberglass, and composites, and adding insulation has greatly reduced the heat transfer through the frame, as well as improving the windows’ appearance.

The spacer is the small piece of material separating the panes of glass. It affects how much heat is transmitted between the panes. Improved spacer technology and better construction and sealing methods are also factors in the higher performance of modern windows.

Triple-pane windows create two layers of air, but as the number of panes increases, either the total thickness must also increase or the amount of dead air space is decreased. Triple-pane windows were developed before low-e coatings came out, and performance of the two is often very similar. Increasing the amount of materials in window units increases the cost, and in the case of triple-pane windows, the increase is substantial. In extremely cold climates, however, the extra cost may be justified.

Low-E Technology & Insulating Gases.

Low-e technology is a big improvement over single-pane, double-pane, or conventionally tinted windows, and the extra cost is now minimal. Low-e glass contains a microscopically thin, transparent layer of metal or metallic oxide applied to the interior face of a multiglazed window, or a thin plastic film placed between the panes. The best of these coatings consist of a soft layer that must be protected from handling and abrasion.

These coatings act as radiant barriers and are placed on different surfaces to achieve different effects. A noble gas (typically argon, krypton, or xenon) is often used instead of air between the panes to further improve the insulating value. These gasses have a higher density compared to air but have higher costs. Argon has a thermal conductivity 67 percent that of air and is relatively inexpensive, krypton has about half the conductivity of argon but significantly more expensive, or xenon, which although is most effective has found very little application due to cost. These gasses are used because they are non-toxic, clear, odorless, chemically inert, and commercially available because of their widespread application in industry.

Engineering allows low-e windows to be spectrally selective – cold-climate (high solar heat gain) systems or hot – climate (low solar heat gain) systems in order to control the amount of shortwave infrared radiation from the sun that passes through the glass. This means that not only is the U-factor improved, but the spectrum of light is also slowed. Some light frequencies are blocked with different technologies, and the SHGC is controlled. Windows are available with high, medium, and low SHGC. Windows with the lowest heat gain (because of reflective gas) also have the lowest visible transmittance (VT) – the percentage or fraction of visible light transmitted by a window or skylight and have a more coloured outdoor reflection.

Microclimates.

Rare is the house that fits neatly into only one category, be it heating or cooling. A mixed climate requires consideration of both heat loss control and solar heat gain protection.

Microclimates may be created by a highly reflective surface that is close to the house, such as water, snow or sand, by an adjacent structure, by tree canopies, or by localized weather conditions, and can be very small. Indeed, the orientation of houses in the same neighborhood can greatly impact the comfort and economy of a home.

Many people in heating or mixed climates find that reducing the solar gain year round makes the house more comfortable. Choosing an SHGC lower than that suggested by ENERGY STAR may help to achieve this. Unless a heating system is zoned or controlled room by room, a bright sunny day could overheat some spaces without significantly reducing the heating load on the central system; and in areas where air conditioning is not typically used, overheating is quite likely in the summer months.

Air Leakage.

Operable windows are popular for ventilation, but unintentional air leaks may account for up to 10% of the energy use in a home. For example, wind pressure can flex the window unit and increase airflow, and sliding windows with weather stripping do not provide as tight a closure as windows with compressing seals.

The industry standard for airflow leakage is 0.30 CFM/ft2. In areas of harsh weather, or on the stormy side of the house, a window with an even lower value can reduce infiltration and noise. Check the rating on the NFRC label. Installation instructions should be followed carefully for good performance.

UV Light.

Spectrally selective low-e glass insulated panels reduce the light coming through with a fairly colour-neutral effect on the view, but placed next to uncoated glass, they appear darker. The VT rating may seem low when it is close to 0.6, but keep in mind that like SHGC, this rating includes both frame and glazing. The higher the VT rating, the more light comes through. Where glare is a problem, a lower VT rating may be preferable. In warm climates, where single-pane windows are still common, tinted glass or add-on films and other shading options may offer the best solar control available.

Ultraviolet light from the sun is an important cause of sun-damage and fading to interior furnishings and finishes. Glass alone absorbs a large amount of the UV, and low-e glass further reduces it. While windows do not eliminate sun damage, high-tech windows can slow it considerably, extending the life of furnishings and floor, and wall coverings.

Curb Appeal.

Replacing the windows in your home will be one of the most important upgrades a homeowner can make. Not only do windows have a great impact on a home’s energy efficiency and indoor comfort, they make a statement about you and your personal style (both indoors and out), and play a significant role in your home’s overall curb appeal.

They say a picture is worth 1,000 words, so before you leave be sure to visit Von Löwen Designs to view an assortment of refreshing examples in kitchen and bath design concepts, refined palette and interior finishes, and sustainable yet chic, green remodeling ideas that may encourage and inspire your next remodel or home improvement project.

The National Fenestration Rating Council (NFRC) is a nonprofit organization created by the window, door, and skylight industry. Membership includes manufacturers, suppliers, builders, architects, designers, specifiers, code officials, utilities and government agencies. The NFRC label provides the only accurate and concise method to ascertain fenestration energy performance features that will help establish how well a product will aid with cooling your home in the summer, warming it through the winter, keeping out wind, and resisting condensation. By utilizing the information included on the label, designers, homeowners and builders alike can reliably compare one product with another, and make informed decisions about the windows, doors, and skylights they buy.

The NFRC label appears on all products certified to the NFRC standards and on all window, door, and skylight products, which are part of the ENERGY STAR program. At this time, NFRC labels on fenestration units give ratings for U-factor, solar heat gain coefficient (SHGC), visible transmittance (VT), and condensation resistance (CR).

All energy performance values on the NFRC label represent the rating of windows and doors as a whole system, including glazing and frame.

U-Factor.

U-factor measures how well a product prevents heat from escaping a home or building. U-factor ratings generally fall between 0.20 and 1.20. The lower the U-factor, the better a product is at keeping heat in. U-factor is particularly important during the winter heating season. This label displays U-factor in U.S. units. Labels on products sold in markets outside the United States may display U-factor in metric units.

Northern Climate: Select windows with a U-factor of 0.35 or less. If air conditioning loads are minimal, windows with U-factors as high as 0.40 are also energy-efficient if the Solar Heat Gain Coefficient is 0.50 or higher. Some double-glazed low-e products have U-factors below 0.30. Some three-layer products have U-factors as low as 0.15.

North/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

South/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

Southern Climate: A low U-factor is useful during cold days when heating is needed. A low U-factor is also helpful during hot days when it is important to keep the heat out, but it is less important than SHGC in warm climates. Select windows with a U-factor lower than 0.75 and preferably lower than 0.60.

Solar Heat Gain Coefficient (SHGC).

The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. In other words – SHGC measures how well a product blocks heat from the sun. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better a product is at blocking unwanted heat gain. Blocking solar heat gain is particularly important during the summer cooling season.

Northern Climate: To reduce heating, select the highest SHGC you can find (usually 0.30-0.60 for the U-factor ranges required in colder climates) so that winter solar gains can offset a portion of the heating energy need. If cooling is a significant concern, select windows with a SHGC less than 0.55. Use a computer program such as RESFEN to understand heating and cooling trade-offs.

North/Central Climate: If you have significant air conditioning costs or summer overheating problems, look for SHGC values of 0.40 or less. If you have moderate air conditioning requirements, select windows with a SHGC of 0.55 or less. While windows with lower SHGC values reduce summer cooling and overheating, they also reduce free winter solar heat gain. Use RESFEN to understand heating and cooling trade-offs.

South/Central Climate: A low SHGC is the most important window property in warm climates. Select windows with a SHGC less than 0.40.

Southern Climate: A low SHGC is the most important window property in warm climates. Select windows with a SHGC less than 0.40.

Visible Transmittance (VT).

The visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted. The NFRC’s VT is a whole window rating and includes the impact of the frame, which does not transmit any visible light. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight. Select windows with a higher VT to maximize daylight and view.

Air Leakage (AL).

Heat loss and gain occur by infiltration through cracks in the window assembly. AL measures how much outside air comes into a home or building through a product. AL rates typically fall in a range between 0.1 and 0.3. The lower the AL, the better a product is at keeping air out. AL is an optional rating, and manufacturers can choose not to include it on their labels. This label displays AL in U.S. units. Labels on products sold in markets outside the United States may display AL in metric units. Select windows with an AL of 0.30 or less (units are cfm/sq ft).

Condensation Resistance (CR).

Condensation resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. CR is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation. CR is an optional rating, and manufacturers can choose not to include it on their NFRC labels. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation resistance and formation.

They say a picture is worth 1,000 words, so before you leave be sure to visit Von Löwen Designs to view an assortment of refreshing examples in kitchen and bath design concepts, refined palette and interior finishes, and sustainable yet chic, green remodeling ideas that may encourage and inspire your next remodel or home improvement project.