Catalogue No.: M-92-156/2001E ISBN: 0-662-27162-9

Section 7 –
High-Performance Windows

The window industry has been quick to develop alternative window technologies to address most of the performance shortcomings of conventional glazing systems. Its efforts over the past decade have been nothing short of revolutionary, and the end-result is the high-performance window, which is several times better than the windows of just a few years ago.

The list of high-performance window improvements currently available–low-E coatings, inert gas fills and insulated frame and edge components–are indicative of the recent advances.

High-performance windows come in a wide variety of window types and applications. It can be very confusing for the uninitiated to sort through the new improvements. understanding this new technology, as well as learning to use the ER system, are important steps to making informed decisions about new window purchases.

7.1 Low-E Coatings

Standard window glass easily allows the sun's energy to pass through it. However, at night, it is equally effective at emitting infrared heat energy back through it to the exterior through the process known as radiative heat loss (Fig. 33). This high-emissivity characteristic of conventional glazing has led researchers to develop low-emissivity (low-E) coatings.

A low-E coating is a thin, invisible metallic layer–only several atoms in thickness–applied directly to glazing surfaces. In a typical double-pane application, the low-E coating is normally applied to the exterior face of the interior glazing (Fig. 33).

A low-E coating works in an ingenious way: while it is transparent to short-wave solar energy, it is opaque to long-wave infrared energy. What this means is that a low-E coating allows most of the sun's solar spectrum (including visible light) to pass through the window to the interior. But the coating reflects most heat energy (from room temperature objects) back to its source, which is a benefit both in the winter, because it keeps the heat in (Fig. 34), and in the summer, because it keeps out the heat radiated from warm objects outside (Fig. 35).

A low-E coating on one pane in a double-glazed window can give the window an insulating value about the same as a standard triple-glazed unit, without the added weight of a third glazing (Fig. 36). The lower weight reduces wear and tear on the window's hinges, casement cranks, etc.–making it easier to operate and giving the window longer life. It also reduces transportation costs, which means lower prices.

There is usually some loss of solar contribution due to the low-E coating (Fig. 37). But while this reduces the benefits of passive solar heat gains somewhat, it is more than offset by the improved insulative value of the low-E window at night. An added bonus is that fewer UV rays make it through, which can mean less fading of carpets and fabric.

There are now many different types of low-E coatings with different performance characteristics. Northern low-E coatings are probably your best compromise in a heating climate like Canada's. They maximize solar heat gains and reduce heat loss at night. Solar control low-E coatings might be justified on west-facing windows when no other means of solar control is possible. These reduce solar heat gain as well as visibility, and are often tinted.

In most cases, the consumer has little control over window location, especially in an existing home. However, if you're designing a new home you may wish to use the ERS rating to compare different glazing options in different orientations.

7.2 Gas Fills

The other big advance in window technology has been the introduction of inert gas fills into the space between glazings (Fig. 38). The term inert refers to a class of chemically stable, non-reactive (safe) gases. Argon and krypton are the usual choice, with argon being the most common and cheapest.

Filling the space between glazing layers with argon gas does two things: 1) it reduces conduction heat loss, because argon has a lower conductivity than air; and, 2) it reduces convection losses, because it is heavier than air and suppresses gas movement between the glazings (Fig. 38).

Krypton gives slightly better performance than argon and permits a smaller optimal spacing between panes (about 8 mm or a third of an inch). A narrow pane space requires less of this much-more-expensive gas, and allows multiple-pane systems with less chance of stress breakage. Since argon is more cost-effective, an increasing number of manufacturers offer it either as a standard feature or as an inexpensive upgrade.

7.3 Special Films

Low-E coatings are also applied to thin sheets of transparent polyester, and suspended in the cavity between glazings (Fig. 39) or directly on the glass surface. This combines a high solar transmission with a low emissivity. Some films are designed to combine low emissivity with reduced solar transmission, making them ideal for southern climates or west-facing windows if solar gains are a severe problem during the summer.

While these films are effective in certain applications, you need to be sure that both you and the window supplier or manufacturer select the right film for the right application.

Researchers are working on exciting new categories of smart windows–electrochromic, thermochromic and photochromic–referred to as "switchable" glazing.

The most promising are electrochromic films that allow the amount of sunlight passing through windows to be controlled by means of a small current running through a transparent electrolite layer in the window. The biggest application for these films in the residential sector will be in buildings with large amounts of west glazing, where overheating in the summer is a problem.

Be careful about the pressure-sensitive after-market films which can be applied directly to existing windows. They are normally designed for the commercial building market. While some of these solar control films do have low-E coatings, they also have very low solar transmission factors. In other words, the energy saved in heat retention may be more than offset by the large reductions in solar gains. Use of these films are recommended for residential applications in only very specific cases such as a sunroom which tends to overheat in the summer. Caution should also be advised as the use of after-market films may void the warranty.

7.4 Low-Conductivity Spacers

Once radiation losses have been reduced through low-E films, and convection and conduction losses through the glazing have been reduced by gas fills, the spacer at the perimeter of the window becomes the weak thermal link in the window unit. Most spacers have traditionally been made out of hollow aluminum, although lightweight and durable, this metal is, unfortunately, very effective at conducting heat.

From an energy efficiency point of view, the new low-conductivity spacer is a major improvement. Many different approaches and materials are appearing in the marketplace, but performance varies considerably. Generally speaking, these spacers can improve the energy performance of a low-E, gas-filled window by as much as 20 percent (Fig. 40). Use the ER number to compare spacer effectiveness.

These better spacers also keep the inside glass warmer at the perimeter, which reduces thermal stresses on the glass and reduces the likelihood of condensation in cold weather.

7.5 Better Frame Materials

Window frames are another weak link in the overall window unit, as mentioned in Section 3.5. Recognizing that up to one third of the overall window may be frame materials–and that high-performance glazing is better insulated than most conventional frame materials–manufacturers have moved quickly to develop more efficient alternatives.

Window frames that combine different materials and take advantage of the strengths of each are available from a growing number of manufacturers. The best energy performance in window frames has been achieved using a fibreglass frame with foam insulation in the frame cavities.

The bottom line on frames is that if you are investing in windows with low-E coatings, gas fills and low-conductivity spacers, then select a frame material which minimizes conductive heat losses.

Remember also that frames can have a significant effect on solar gains (and the ER number). Stronger materials that allow narrow frames and sashes, such as thermally broken aluminum or fibreglass, allow more glass area and solar gain. These are called low profile frames. Again, frame thermal efficiency will be reflected in a higher ER number.

7.6 Design Summary

In the majority of cases, if you are replacing all the windows in your home, you will probably select the same glazing on all sides. In this case, make your selection on the basis of ER numbers. In exceptional cases, such as passive solar homes or sun spaces, more detailed comparisons may be required.

Keep the following principles in mind as you decide on your window approach:

• A few large windows are better than many small ones. Larger windows reduce the proportion of frame to glazing, and maximize overall performance.

• The thermally weakest areas of a high-performance window are frames and edges; once the centre of glazing is efficient, the frame and edge losses will be proportionally higher, so look for insulated (non-metal) spacers and thermally broken, low-profile frames.

• Avoid large areas of west-facing glass. The sunsets may be beautiful, but your air conditioning bills won't be.

• Operable windows should be limited to locations where ventilation or emergency exits are required by codes.

• Reduced frame and sash areas contribute to better overall window performance.

• For passive solar designs, the ERS rating system may be used.

• In all other cases, select windows based on ER numbers.

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