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Glass Windows - Important Points

When choosing new glass windows, appearance is often the first consideration. However, there are a number of additional considerations which are equally important. Let us assist you in identifying the windows that best suit your lifestyle and taste. Additional considerations include cost, alternate style, material and texture options, and energy and noise reduction efficiency. Cost is related to both appearance, durability and energy efficiency. Energy efficiency in windows is an important concern.

Windows are weak points in house construction where a great amount of energy can be lost. Attention should be paid to ensuring that the loss of energy is minimized. The average home may lose 30% of its heat or air-conditioning energy through its windows. Energy-efficient windows save money each and every month. There are even some cases where new windows can be net energy gainers. The payback period for selecting energy-efficient units ranges from two years to ten years. In new construction, their higher initial cost can be offset because you'll probably need a smaller, less expensive heating and cooling system. And more-durable windows may cost less in the long haul because of lowered maintenance and replacement costs. Plus, you'll be more comfortable the whole while you live with them.

Window Energy Efficiency

Windows can gain and lose heat through air leakage, conduction, convection and radiation.

Air leakage allows approximately half of an average home's heating and cooling energy to escape to the outside, in particular, windows account for the majority of this loss. Well-designed windows have durable weather-stripping and high-quality closing devices that effectively block air leakage. Hinged windows such as casements and awnings clamp more tightly against weather-stripping than do double-hung windows, providing a small difference in air leakage. How well the individual pieces of the window unit are joined together also affects air leakage. When choosing efficient windows, look for products with air-leakage rates of less than 0.30 cfm/ft2.

Conduction is the movement of heat or cold through solid material and conduction can effect how heat or cold flows through a window. Using a less conductive material impedes heat flow; multiple-glazed windows trap low-conductance gas such as argon between panes of glass, while thermally resistant edge spacers and window frames reduce conduction.

Convection is another way that heat and cold can move through glass windows. In a cold climate, heated indoor air touches the interior surface of window glass, cooling the air which becomes denser and drops toward the floor allowing warm air to rush in to take its place at the glass surface, this is a convective loop which is self-perpetuating. You recognize this movement as a cold draft and turn up the heat. In order to slow convection and improve interior house comfort, the use of multiple panes of glass separated by low-conductance gas fillings and warm edge spacers, combined with thermally resistant frames, raise inboard glass temperatures.

Radiant transfer is the movement of heat energy from a warmer body to a cooler body and is recognized as the warm feeling when you stand near a stove. Conversely, the reverse occurs with cold. Clear glass absorbs heat and reradiates it outdoors. Radiant-heat loss through windows can be greatly reduced by placing low-E coatings on glass that reflect specific wavelengths of energy. In the same way, low-E coatings keep the summer heat out.

Low-E glass reflects heat energy while admitting visible light. This keeps heat out during the summer and during the winter. In the winter, low-angle visible light passes into the house and is absorbed by the home's interior.

Sunlight - The sun's light and heat is welcome in the right quantities and current window technology makes this more available to us.

Less than half of the sun's energy is visible, others such as infrared are not. When the sun's energy strikes a window, visible light, heat and UV are either reflected, absorbed or transmitted into the building. Only a fraction of the sun's energy is visible. There are windows that selectively block fabric-fading UV, visible light or infrared, which is felt as heat. Windows that block most UV and infrared while admitting visible light work well in cool climates. Low-E glass coatings, which are transparent metallic oxides reflect up to 90% of long-wave heat energy, and pass visible light. In hot climates they reflect the sun's heat energy, admit visible light and keeping a house cool, while in cold climates the radiant heat passes into the house as well as the visible light. These low-E coating work best in warm climates when applied to the internal, interpane, interior pane surface; and cold climates when applied to the interpane surface of the exterior pane. Low-E coatings can improve the insulating value of a window and when combined with low-conductance gas fillings such as argon or krypton, safe inert gases, the energy efficiency is substantially increased in comparison to clear glass. Although Argon and krypton can leak from a window over time, by example 10% loss over 20 years, the U-value will only be reduced by a few percent.

High Visible Transmittance

Windows with high visible transmittance are easy to see through and admit natural daylight. In addition, HVT windows save energy due to the reduced need for artificial light. Manufacturers refer to the VTs of windows in comparison to the amount of visible light that would pass through an open hole in a wall, sometimes referred to as a "whole-window" value including the effect of the frame. When purchasing HVT windows, you should ensure that the VT relates to the glass and does not include the frame. Generally, HVT glass with a value above 60% appears clear, while values below 50% appear dark, it is important that you look at the glass in different light to understand what best suits your needs.

Preventing UV-Damage Windows that block UV-radiation reduce fabric fading. Expect to find windows off the shelf that block more than 75% of the UV-energy. Contrary to conventional wisdom, some visible light fades fabric, too. Some manufacturers use the Krochmann Damage Function to rate a window's ability to limit fabric-fading potential. It expresses the percentage of both UV and of that portion of the visible spectrum that passes through the window and causes fading. Lower numbers are better. These units are expensive, but are more energy efficient in cold climates. The R-value is lower than a typical wall, however using triple-glazed windows and a high SHGC can increase their energy efficiency.

Condensation and Warm Edge Spacers In cold climates condensation and frost can form on windows. To reduce this, use warm edge-spacers affects the rate that heat travels through a window's edge. Many window manufacturers now include warm edge spacers as standard. It should be noted that a better manufactured window uses less conductive materials such as stainless steel, plastic, rubber and foam. Warm edge spacers can improve a window's U-value by 10% while reducing condensation and increasing the edge temperature 2-3 degrees celsius.

Vinyl Windows are energy efficient, and durable, as well as rot proof, insect proof and weather resistant. Vinyl windows are made with chemicals that inhibit UV-degradation and as the colour goes all the way through the vinyl, it does not require painting. Unfortunately the downside is that the vinyl can fade over time, it can't be painted, and it becomes brittle over time. Changes in temperature can cause the vinyl to expand and contract greater than aluminum or wood. For the best durability, choose vinyl frames that are light in colour and have heat-welded corners.

Fibreglass Frame Windows are very strong and the glass fibres expand at the same rate as the glass. The down side to fibreglass is that it must be painted and is more expensive than vinyl. Unfortunately, an insulated fibreglass window is not as energy efficient as a simular vinyl window.

Aluminum Frame Windows are durable and require very little maintenance, however, they are not energy efficient.

In Conclusion there is a wide range of window styles, types and options available today, all of which require consideration in the decision process. We hope that the above outline will assist you the consumer in making an informed decision. Call us today.

Let us assist you in choosing and installing windows and window accessories that are suited to your lifestyle and taste. This may be either as replacement doors or as part of a renovation or new build.