Glass Windows

When you choose new glass windows, appearance is often the first consideration. Initial cost is the next issue: Which window within the favored style costs the least? But liking a window's appearance is a fuzzy proposition, and cost really depends on durability and on the energy dollars pumped through the windows each year. We are convinced that if we could see energy loss as we see colour and shape, energy performance would top the list of glass window considerations.

Windows are thermal holes. An 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.

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

Conduction is the movement of heat through a solid material. Touch a hot skillet, and you feel heat conducted from the stove through the pan. Heat flows through a window much the same way. With a less conductive material, you impede heat flow. Multiple-glazed windows trap low-conductance gas such as argon between panes of glass. Thermally resistant edge spacers and window frames reduce conduction, too.

Convection is another way heat moves through glass windows. In a cold climate, heated indoor air rubs against the interior surface of window glass. The air cools, becomes denser and drops toward the floor. As the stream of air drops, warm air rushes in to take its place at the glass surface. The cycle, a convective loop, is self-perpetuating. You recognize this movement as a cold draft and turn up the heat. Unfortunately, each 1°F increase in thermostat setting increases energy use 2%. Multiple panes of glass separated by low-conductance gas fillings and warm edge spacers, combined with thermally resistant frames, raise inboard glass temperatures, slow convection and improve comfort.

Radiant transfer is the movement of heat as long-wave heat energy from a warmer body to a cooler body. Radiant transfer is the warm feeling on your face when you stand near a wood stove. Conversely, your face feels cool when it radiates its heat to a cold sheet of window glass. But radiant-heat loss is more than a perception. 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.

Air leakage siphons about half of an average home's heating and cooling energy to the outdoors. Air leakage through windows is responsible for much 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. But the difference is slight; well-made double hung are acceptable. How well the individual pieces of the window unit are joined together also affects air leakage. Glass-to-frame, frame-to-frame and sash-to-frame connections must be tight. The technical specifications for windows list values for air leakage as cubic feet per minute per square foot of window. We look for windows with certified air-leakage rates of less than 0.30 cfm/ft2. Lowest values are best.

Letting in the right amount of sun:

In a cold climate we welcome the sun's heat and light most of the time. And once we capture the heat, we don't want to give it up. In a warm climate, we don't want the heat, but we do want the light. Advances in window technology let us have it both ways.

Less than half of the sun's energy is visible. Longer wavelengths--beyond the red part of the visible spectrum--are infrared, which is felt as heat. Shorter wavelengths, beyond purple, are ultraviolet (UV). 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.

Enter low-E glass coatings, transparent metallic oxides that reflect up to 90% of long-wave heat energy, while passing shorter wave, visible light. In hot climates, they reflect the sun's long-wave heat energy while admitting visible light, thereby keeping the house cooler in the summer. And in cold climates, they reflect long-wave radiant heat back into the house, again while admitting visible light. Floors, walls and furniture absorb this shorter wavelength visible light. It reradiates from them as long-wave heat energy that the reflective, low-E coating keeps inside. Low-E coatings work best in warm climates when applied to the internal, or interpane, surface of the interior pane. Conversely, in cold climates, low-E coatings work best applied to the interpane surface of the exterior pane.

Low-E coatings improve the insulating value of a window roughly as much as adding an additional pane of glass does. And combining low-E coatings with low-conductance gas fillings, such as argon or krypton, boost energy efficiency by nearly 100% over clear glass. Argon and krypton are safe, inert gases, and they will leak from the window over time. Studies suggest a 10% loss over the course of 20 years, but that will reduce the U-value of the unit by only a few percent. The added cost for low-E coatings and low-conductance gas fillings is only about 15% of the window's overall cost. It's a no-brainer.

Taking in the view

Windows with high visible transmittance are easy to see through and admit natural daylight. Besides giving you a nice view, high-VT windows can save energy because you need less artificial light. Some tints and coatings that block heat also reduce visible transmission, so be careful. Manufacturers list the VTs of windows as comparisons with the amount of visible light that would pass through an open hole in the wall the same size as the window. VT is sometimes expressed as a "whole-window" value including the effect of the frame. What is important is the ability to see through the glass, not the frame, so be sure you get the VT of the glass, not of the entire unit.

The VT in residential windows extends from a shady 15% for some tinted glass up to 90% for clear glass. To most people, glass with VT values above 60% looks clear. Any value below 50% begins to look dark and/or reflective. People have very different perceptions of what is clear and what has a tint of color, especially when they look through glass at an angle." Look at a sample of glass outdoors and judge for yourself before you decide to order the window.

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.

Window manufacturers sometimes boast R-8 (U-0.125) values. Be careful. This may be only the value at the center of the glass, which is always artificially higher than the whole-unit value. Look for whole-unit values of U-0.33 or better. Some manufacturers stretch low-E coated plastic film within the gas-filled airspace of double-glazed units to provide an effective third or fourth "pane." The weight of these windows is comparable to double glazing, and the true overall window performance is boosted to levels of U-0.17 or better for some. These units are pricey, but they can be more energy efficient in cold climates. The R-value is lower than a typical wall, but if triple-glazed units are designed with a high SHGC and are placed in a sunny wall, they can be net energy gainers.

Keeping warm around the edges

If you've lived in a cold climate, you've seen condensation and even frost on windows. When warm indoor air cools below its dewpoint, liquid water condenses on the glass. Condensation typically develops around the edges of window glass. No surprise. The edge is where most multiple-pane glazing is held apart by conductive spacers.

Warm edge-spacers reduce the chance of condensation forming.

The material the spacer is made from affects the rate that heat travels through a window's edge. Many window makers now offer warm edge spacers as standard fare. Aluminum spacers are not acceptable. The best windows use less conductive materials such as thin stainless steel, plastic, foam and rubber. Warm edge spacers can improve the U-value of a window by 10% and boost the edge temperature by around 5°F, thereby reducing condensation.

Vinyl windows have been around for 35 years. Vinyl is energy efficient, durable, rotproof, insectproof and weather resistant. It's made with chemicals that inhibit UV-degradation. Vinyl is colored throughout and requires no painting. The knock on vinyl is that it fades, can't be painted, becomes brittle with age and is thermally unstable (especially dark colors). Temperature changes cause it to contract and expand more than wood, aluminum and even the glass it holds. If you choose vinyl frames, specify light colors and heat-welded corners. Heat-welded corners hold up best over time.

The pigments that are used in paint are almost identical to those used in vinyl, but vinyl's color goes all the way through. A little rubdown with Soft Scrub or one of recommended cleaners will bring vinyl back to its original brilliance.

Fiberglass-frame windows are showing up in a few product lines. Fiberglass is extremely strong, and because it is made of glass fibers, the frames and the glass expand at the same rate. Fiberglass must be painted and is more expensive than vinyl. Owens Corning, Andersen and Marvin are three major manufacturers that produce fiberglass windows. Owens Corning is the only manufacturer that makes fiberglass windows with insulated frames. But before you get too excited, the whole-window U-value for a low-E argon-filled casement window carries the same 0.32 rating for both an uninsulated vinyl and an insulated fiberglass unit.

Aluminum-frame windows are durable, requiring little maintenance. However, they are energy siphons and shouldn't be used where energy efficiency is a consideration.

The range of window options available today is staggering. But a working knowledge of the terms and these few guidelines should make choosing windows a little less intimidating.