Windows and what you expect
You expect a lot from your windows and so do we.
You want a window to provide fresh air and cooling breezes at times, but at other times expect them to be completely airtight and provide good thermal insulation. You expect windows to be durable in every way: resistant to condensation, wind, driving rain and ice. Windows must operate easily and accommodate attachments like curtains, awnings, and other devices. You want windows that are quick to install, that integrate with the rest of the building, and that won’t break the bank. Given that they are a big investment, they should last a long time. The good news for anyone buying windows today and in the next ten years is the on-going technical and market progress. “Over the last 20 years or so, we’ve been through a generation of double-glazed low-e [low-emissivity] windows penetrating the market.” Said Neil Hammond of Prior Products Ltd, codes, and other factors are moving us toward the next generation of higher performance with innovations in glazing, coatings, frames, and more.”
Choosing a Frame Material
Window frame materials are chosen first for structural characteristics, but the material should ideally provide good thermal insulation. Durability, maintenance requirements, and cost also factor in to frame selection.
The share of the window frame and sash market owned by PVC, or vinyl, was only 5% in 1984 but now dominates with 66%. In replacement windows, vinyl has an even bigger edge with 70% of the market.
Vinyl’s advantages are
• Its low cost and minimal maintenance requirements.
• Vinyl windows generally do not need painting
• Is available in aesthetically pleasing colours and grain effects
Major downsides include
• Their aesthetics, which many people consider inferior
• The environmental costs of PVC production.
However, both in terms of structural qualities and energy performance, vinyl are comparable to wood. When the cavities in extruded vinyl are filled with polyurethane foam, energy performance exceeds that of wood.
Vinyl’s Achilles’ heel in practice is over time, expansion and contraction from temperature changes can loosen seals, and cause cracks at corners and on flanges.
Wood was the first material used in window frame and sash construction, and it remains popular for homes, although it continues to lose market share to vinyl.
Plus points are
• Wood is attractive
• It is a natural and renewable product
• It has a warm feel, and it is relatively energy-efficient.
With proper care it should be durable, although paints or stains will need to be reapplied over the lifetime of the windows. The need for strength, dimensional stability, and durability means that only top-quality; knot-free wood is used for windows.
• Maintenance required either yearly or bi yearly
• Expensive (can be almost twice the cost of Vinyl)
Due to its structural strength in window frames, aluminium (along with steel) has been used in great quantity but is gradually losing market share to vinyl. Aluminium is also used as a cladding for wood windows. Aluminium can have high recycled content and is readily recyclable, but manufacturing of the raw materials is very energy-intensive. Metal window frames are very thermally conductive, and so they impose a big energy penalty on whole-window performance and can cause condensation.
In most cases, energy performance will determine the environmental impact of windows over their lifetime, and with most current windows on the market, that will be determined by glazing choices.
Double- and triple-glazing
With storm windows dating back 200 years and sealed double-glazing units dating to the 1930s, adding a second layer of glazing has long been the first step for window manufacturers toward improving energy performance. A second layer of glazing—or a third in the case of triple-glazed windows—improves window insulation by trapping dead air. Double-glazing has become ubiquitous over the last couple of decades, and triple-glazing is now becoming more common in both residential and commercial windows. Triple-glazing is provided through a third layer.
Low-conductivity gas fills
Because heat conduction across the air space in a sealed unit contributes to heat loss, we can improve performance by replacing the air with a lower-conductivity gas. The most commonly used gas fill is 90% argon, which is plentiful, inexpensive, and inert. Although safe, this inert “noble” gas is very buoyant and difficult to contain, and will leak over time. Tests suggest that after years of service, most of the argon will remain and that a loss of 1% of the gas per year is expected. While the benefits of gas fills may not be permanent, they are substantial and long-lasting, and the incremental price premium is easily justified.
First introduced in 1979, low-e glazing has grown tremendously in popularity. A thin transparent coating of silver or tin oxide on the glass surface or on a suspended plastic film with such a coating allows short-wavelength sunlight to pass through but blocks long-wavelength heat radiation.
Warm-edge glazing spacers
Glass units are sealed around the perimeter by spacers that maintain the distance between the panes of glass and help seal in any gas fills being used. Aluminium has been the most common material for glazing spacers, but it is very thermally conductive. Warm-edge spacers using rubber, foam, silicone, thermally broken steel, and other materials have become common in high-performance windows, and drastically reduce heat loss or gain at the edges of glass units. Warm-edge spacers with integrated desiccant beads also reduce the risk of fogging within the unit.
Fixed windows that don’t open are less expensive and, due to simpler construction, more durable than operable units. They are also more airtight, offering an important energy benefit. While operable windows provide ventilation and a better connection to the outdoors, it’s worth considering for each window location whether it should be fixed or operable. Operable units use weather-stripping to ensure airtightness. As a general rule, hinged casement or awning windows that open out use a compression-type, synthetic weather-stripping gasket that offers a tighter, more durable seal than a seal used on a sliding-sash window. However, there are significant differences among manufacturers and products, so it always makes sense to examine product labels carefully.
Glazing dimensions and lights
Because high-performance glazing’s generally lose more heat at the edges, the larger the glazing-area-to-perimeter ratio the better the overall window energy performance. Divided lights, in which the sash is divided into multiple individual panes separated by muntins, offer a more traditional look, but they result in reduced glazing area and thus lower overall (unit) insulation value. (These windows also tend to cost more due to a more complicated assembly.)
But in the end………
There are a lot of window options; making a choice should include environmental and energy considerations, but in the end can come down to subjective factors such as aesthetics or how well the sales rep sells the durability of the showroom model. Whatever you choose shouldn’t be too cheap, said Neil. “People make most of the decisions about a home based on preferences and amenities.” The same homeowner who doesn’t think twice about granite countertops might balk at the cost of a higher-performing window, but Neil said it’s an important investment that offers more “payback” than the countertops ever will.